Evolution of Salmonella-Host Cell Interactions through a Dynamic Bacterial Genome

Enhancing the effectiveness of Polymyxin E with a Fisetin Nanoemulsion against a Colistin-resistant Salmonella typhimurium infection

BACKGROUND

Polymyxin E is widely recognized as a last resort for treating multidrug-resistant gram-negative bacteria. Unfortunately, the effectiveness of polymyxin E is significantly reduced when treating life-threatening bacterial infections due to plasmid-mediated polymyxin E resistance. The synergistic effect of applying a polymyxin E adjuvant is a promising strategy for overcoming the growing threat of antibiotic-resistant pathogens.

PURPOSE

To evaluate the synergistic effect of fisetin and polymyxin E on S. typhimurium infections in vivo and further elucidate the underlying mechanism of this effect.

METHODS

The effect of combining fisetin and polymyxin E to treat mobilized colistin resistance-1-positive (MCR-1-positive) gram-negative bacteria in vitro was examined using various methods, such as checkerboard assays, growth curves and time‒kill curves. To elucidate the mechanism by which fisetin affects MCR-1, we employed ultraviolet (UV) absorption spectroscopy, thin layer chromatography (TLC), and western blot analysis to investigate its effect at the protein level. Subsequently, molecular dynamics simulations (MDS) and metabolomics analysis were utilized to determine the site of interaction between fisetin and MCR-1 as well as the potential pathways and mechanisms involved. A new nanoemulsion of fisetin was produced using high-pressure homogenization, and its stability was tested. Finally, two animal models of S. typhimurium HYM2 infection were established to evaluate the synergistic effect of polymyxin E and fisetin in vivo.

RESULTS

Our study revealed that fisetin exhibited a synergistic effect when combined with polymyxin E against MCR-1-positive S. typhimurium. The TLC results demonstrated that fisetin could inhibit the phosphoethanolamine (PEA) transfer of the MCR-1 protein, thereby restoring the activity of polymyxin E in strains against MCR-1. The MDS analysis indicated robust and immediate binding between fisetin and the MCR-1 protein, with both hydrophobic and polar effects playing significant roles in determining the binding energy of the former. Metabolomic studies demonstrated that the addition of fisetin significantly modulated bacterial metabolites. Moreover, it effectively inhibited the activity of ABC transporters in bacteria, thereby mitigating bacterial drug resistance and enhancing the therapeutic efficacy of polymyxin E. Furthermore, in mouse and chick models of infection, intragastric administration of the fisetin nanoemulsion together with polymyxin E resulted in significant therapeutic benefits, including increased survival rates, reduced bacterial colonization, and decreased levels of inflammatory factors.

CONCLUSION

Fisetin, an MCR-1 inhibitor and a promising synergistic partner of polymyxin E, has significant potential for clinical application in the treatment of S. typhimurium infections, particularly those resulting extensively from drug-resistant MCR-1-positive strains.

Phage-host co-evolution has led to distinct generalized transduction strategies

Generalized transduction is pivotal in bacterial evolution but lacks comprehensive understanding regarding the facilitating features and variations among phages. We addressed this gap by sequencing and comparing the transducing particle content of three different Salmonella Typhimurium phages (i.e. Det7, ES18 and P22) that share a headful packaging mechanism that is typically initiated from a cognate pac site within the phage chromosome. This revealed substantial disparities in both the extent and content of transducing particles among these phages. While Det7 outperformed ES18 in terms of relative number of transducing particles, both phages contrasted with P22 in terms of content. In fact, we found evidence for the presence of conserved P22 pac-like sequences in the host chromosome that direct tremendously increased packaging and transduction frequencies of downstream regions by P22. More specifically, a ca. 561 kb host region between oppositely oriented pac-like sequences in the purF and minE loci was identified as highly packaged and transduced during both P22 prophage induction and lytic infection. Our findings underscore the evolution of phage transducing capacity towards attenuation, promiscuity or directionality, and suggest that pac-like sequences in the host chromosome could become selected as sites directing high frequency of transduction.

Multi-locus sequence typing, antimicrobials resistance and virulence profiles of Salmonella enterica isolated from bovine carcasses in Minas Gerais state, Brazil

The aim of this study was to identify virulence and antimicrobial resistance profiles and determine the sequence type (ST) by multilocus sequence typing (MLST) of Salmonella enterica isolates from bovine carcasses from slaughterhouse located in Minas Gerais state, Brazil, and its relationship with bovine isolates obtained on the American continent based on sequence type profile. The MLST results were compared with all Salmonella STs associated with cattle on American continent, and a multi-locus sequence tree (MS tree) was built. Among the 17 S. enterica isolates, five ST profiles identified, and ST10 were the most frequent, grouping seven (41.2%) isolates. The isolates presented 11 different profiles of virulence genes, and six different antibiotics resistance profiles. The survey on Enterobase platform showed 333 Salmonella STs from American continent, grouped into four different clusters. Most of the isolates in the present study (13/17), were concentrated in a single cluster (L4) composed by 74 STs. As a conclusion, five different STs were identified, with ST10 being the most common. The isolates showed great diversity of virulence genes and antibiotics resistance profiles. Most of the isolates of this study were grouped into a single cluster composed by 74 STs formed by bovine isolates obtained on the American continent.

Detection of Salmonella Pathogenicity Islands and Antimicrobial-Resistant Genes in Salmonella enterica Serovars Enteritidis and Typhimurium Isolated from Broiler Chickens

Rapid growth in commercial poultry production is one of the major sources of Salmonella infections that leads to human salmonellosis. The two main Salmonella enterica serovars associated with human salmonellosis are enteritidis and typhimurium. The aim of this study was to determine the prevalence of S. enterica serovars Enteritidis and S. Typhimurium as well as their Salmonella pathogenicity islands (SPI) and antibiotic resistance profiles in broiler chicken feces from slaughterhouses. A total of 480 fecal samples from broiler chickens that were grouped into 96 pooled samples were identified to have Salmonella spp. using the invA gene, whilst the Spy and sdfI genes were used to screen for the presence of S. Enteritidis and S. Typhimurium serovars, respectively, by polymerase chain reaction (PCR) assays. The isolates were also screened for the presence of Salmonella pathogenicity islands (SPIs) using PCR. The disc diffusion assay was performed to determine the antibiotic resistance profiles of the isolates. A total of 36 isolates were confirmed as Salmonella spp. through amplification of the invA gene. Out of 36 confirmed Salmonella spp. a total of 22 isolates were classified as S. Enteritidis (n = 8) and were S. Typhimurium (n = 14) serovars. All (n = 22) S. Enteritidis and S. Typhimurium isolates possessed the hilA (SPI-1), ssrB (SPI-2) and pagC (SPI-11) pathogenicity islands genes. Amongst these serovars, 50% of the isolates (n = 11/22) were resistant to tetracycline and nalidixic acid. Only 22% of the isolates, S. Typhimurium (13.6%) and S. Enteritidis (9.1%) demonstrated resistance against three or more antibiotic classes. The most detected antibiotic resistance genes were tet(K), mcr-1, sulI and strA with 13 (59.1%), 9 (40.9%), 9 (40.9%) and 7 (31.8%), respectively. The findings of this study revealed that S. Typhimurium is the most prevalent serotype detected in chicken feces. To reduce the risk to human health posed by salmonellosis, a stringent public health and food safety policy is required.

The Yin and Yang of pathogens and probiotics: interplay between Salmonella enterica sv. Typhimurium and Bifidobacterium infantis during co-infection

Probiotic bacteria have been proposed as an alternative to antibiotics for the control of antimicrobial resistant enteric pathogens. The mechanistic details of this approach remain unclear, in part because pathogen reduction appears to be both strain and ecology dependent. Here we tested the ability of five probiotic strains, including some from common probiotic genera Lactobacillus and Bifidobacterium, to reduce binding of Salmonella enterica sv. Typhimurium to epithelial cells in vitro. Bifidobacterium longum subsp. infantis emerged as a promising strain; however, S. Typhimurium infection outcome in epithelial cells was dependent on inoculation order, with B. infantis unable to rescue host cells from preceding or concurrent infection. We further investigated the complex mechanisms underlying this interaction between B. infantis, S. Typhimurium, and epithelial cells using a multi-omics approach that included gene expression and altered metabolism via metabolomics. Incubation with B. infantis repressed apoptotic pathways and induced anti-inflammatory cascades in epithelial cells. In contrast, co-incubation with B. infantis increased in S. Typhimurium the expression of virulence factors, induced anaerobic metabolism, and repressed components of arginine metabolism as well as altering the metabolic profile. Concurrent application of the probiotic and pathogen notably generated metabolic profiles more similar to that of the probiotic alone than to the pathogen, indicating a central role for metabolism in modulating probiotic-pathogen-host interactions. Together these data imply crosstalk via small molecules between the epithelial cells, pathogen and probiotic that consistently demonstrated unique molecular mechanisms specific probiotic/pathogen the individual associations.

Diverse Prophage Elements of Salmonella enterica Serovars Show Potential Roles in Bacterial Pathogenicity

Nontyphoidal salmonellosis is an important foodborne and zoonotic infection that causes significant global public health concern. Diverse serovars are multidrug-resistant and encode several virulence indicators; however, little is known on the role prophages play in driving these traits. Here, we extracted prophages from seventy-five Salmonella genomes which represent the fifteen important serovars in the United Kingdom. We analyzed the intact prophages for the presence of virulence genes and established their genomic relationships. We identified 615 prophages from the Salmonella strains, from which 195 prophages are intact, 332 are incomplete, while 88 are questionable. The average prophage carriage was found to be 'extreme' in S. Heidelberg, S. Inverness, and S. Newport (10.2-11.6 prophages/strain), 'high' in S. Infantis, S. Stanley, S. Typhimurium, and S. Virchow (8.2-9.0 prophages/strain), 'moderate' in S. Agona, S. Braenderup, S. Bovismorbificans, S. Choleraesuis, S. Dublin, and S. Java (6.0-7.8 prophages/strain), and 'low' in S. Javiana and S. Enteritidis (5.8 prophages/strain). Cumulatively, 61 virulence genes (1500 gene copies) were detected from representative intact prophages and linked to Salmonella delivery/secretion system (42.62%), adherence (32.7%), magnesium uptake (3.88%), regulation (5%), stress/survival (1.6%), toxins (10%), and antivirulence (1.6%). Diverse clusters were formed among the intact prophages and with bacteriophages of other enterobacteria, suggesting different lineages and associations. Our work provides a strong body of data to support the contributions diverse prophages make to the pathogenicity of Salmonella, including thirteen previously unexplored serovars.

Antibiotic resistance and virulence genes profile of Non typhodial Salmonella species isolated from poultry enteritis in India

Salmonella species (spp) is the most important gastrointestinal pathogen present ubiquitously. Non typhoidal Salmonella (NTS) is commonly associated with gastroenteritis in humans. Layer birds once get infection with NTS, can become persistently infected with Salmonella Typhimurium and intermittently shed the bacteria. It results in a high risk of potential exposure of eggs to the bacteria. The current study was conducted to determine the serotype diversity, presence of virulence genes, antibiotic resistance pattern, and genes of NTS from poultry enteritis. Out of 151 intestinal swabs from poultry total 118 NTS were isolated, which were characterized serologically as S. Typhimurium (51 strains), S. Weltevreden (57 strains) and untypable (10 strains). Most effective antibiotics were amikacin, gentamycin and ceftriaxone (33.05%) followed by ampicillin, azithromycin and ciprofloxacin (16.69%), co-trimoxazole (13.55%), and tetracycline (6.78%). Multidrug resistance recorded in 17.70% (N = 21/118) strains. Antimicrobial-resistant genes i.e. blaTEM, blaSHV, blaCTX-M, tet(A), tet(B), tet(C), sul1, sul2, sul3. blaTEM and tet(A) were present in 95% (20/21). Eleven virulence genes i.e. invA, hilA, sivH, tolC, agfA, lpfA, spaN, pagC, spiA, iroN and fliC 2 were present in all the 30 isolates. While, sopE was present in only 2 isolates, NTS strains with characteristics of pathogenicity and multidrug resistance from poultry enteritis were detected. Multidrug resistance showed the necessity of prudent use of antibiotics in the poultry industry.

Evolution and genomic profile of Salmonella enterica serovar Gallinarum biovar Pullorum isolates from Brazil

Salmonella enterica subspecies enterica serovar Gallinarum biovar Pullorum (S. Pullorum) is a pathogenic bacterium that causes Pullorum disease (PD). PD is an acute systemic disease that affects young chickens, causing white diarrhea and high mortality. Although many sanitary programs have been carried out to eradicate S. Pullorum, PD outbreaks have been reported in different types of birds (layers, broilers, breeders) worldwide. This study aimed to evaluate the evolution and genetic characteristics of S. Pullorum isolated from PD in Brazil. Phylogenetic analysis of S. Pullorum genomes sequenced in this study and available genomic databases demonstrated that all isolates from Brazil are from sequence type 92 (ST92) and cluster into two lineages (III and IV). ColpVC, IncFIC(FII), and IncFII(S) were plasmid replicons frequently found in the Brazilian lineages. Two resistance genes (aac(6')-Iaa, conferring resistance to aminoglycoside, disinfecting agents, and antiseptics (mdf(A)) and tetracycline (mdf(A)) were detected frequently. Altogether, these results are important to understand the circulation of S. Pullorum and, consequently, to develop strategies to reduce losses due to PD.

Unravelling the role of anaerobic metabolism (pta-ackA) and virulence (misL and ssa) genes in Salmonella Heidelberg shedding using chicken infection model

The mechanism of colonisation of the chicken intestine by Salmonella remains poorly understood, while the severity of infections vary enormously depending on the serovar and the age of the bird. Several metabolism and virulence genes have been identified in Salmonella Heidelberg; however, information on their roles in infection, particularly in the chicken infection model, remains scarce. In the present publication, we investigated three Salmonella Heidelberg mutants containing deletions in misL, ssa, and pta-ackA genes by using signature-tagged mutagenesis. We found that mutations in these genes of S. Heidelberg result in an increase in fitness in the chicken model. The exception was perhaps the pta-ackA mutant where colonisation was slightly reduced (2, 7, 14, and 21 days post-infection) although some birds were still excreting at the end of the experiment. Our results suggest that for intestinal colonisation of the chicken caecum, substrate-level phosphorylation is likely to be more important than the MisL outer membrane protein or even the secretion system apparatus. These findings validate previous work that demonstrated the contribution of ackA and pta mutants to virulence in chickens, suggesting that the anaerobic metabolism genes such as pta-ackA could be a promising mitigation strategy to reduce S. Heidelberg virulence.

STnc1280, a trans-coding sRNA is involved in virulence modulation via targeting gldA mRNA in Salmonella Typhimurium

Introduction. Salmonella Typhimurium (STM) is a food-borne Gram-negative bacterium, which can infect humans and a wide range of livestock and poultry, causing a variety of diseases such as septicaemia, enteritis and abortion.Hypothesis/Gap Statement. We will decipher the impacts of sRNA STnc1280 on STM virulence and provide a theoretical basis to reveal the regulatory role and molecular mechanism of STnc1280.Aim. The main objective of this study was to clarify whether sRNA STnc1280 exerts regulatory roles on STM pathogenicity.Methodology. The STnc1280 gene was amplified and its molecular characteristics were analysed in this study. Then, STnc1280 gene deletion strain (STM-ΔSTnc1280) and the complementary strain (ΔSTnc1280/STnc1280) were constructed by λ-Red homologous recombination method, respectively, to analyse of adhesion and invasive ability and pathogenicity of different strains. Subsequently, the potential target gene regulated by STnc1280 was predicted using target RNA2 software, followed by the verification of the interaction between STnc1280 and target mRNA using the dual plasmid reporter system (DPRS). Furthermore, the mRNA and protein level of target gene was determined using qRT-PCR and Western blot, respectively.Results. The results revealed that the cell adhesion and invasive ability and pathogenicity of STM-ΔSTnc1280 were significantly reduced compared to STM-SL1344 strain, indicating that the deficiency of STnc1280 gene significantly influenced STM pathogenicity. The DPRS results showed that STnc1280 can interact with the mRNA of target gene gldA, thus suppressing the expression of lacZ gene. Furthermore, the level of gldA mRNA was not influenced in STM-ΔSTnc1280, but the expression of GldA protein decreased significantly.Conclusion. Combining the bioinformatic analysis, these findings suggested that STnc1280 may bind to the SD sequence of gldA mRNA, hindering the binding of ribosomes to gldA mRNA, thereby inhibiting the expression of GldA protein to modulate the virulence of STM.

Infection biology of Salmonella enterica

Salmonella enterica is the leading cause of bacterial foodborne illness in the USA, with an estimated 95% of salmonellosis cases due to the consumption of contaminated food products. Salmonella can cause several different disease syndromes, with the most common being gastroenteritis, followed by bacteremia and typhoid fever. Among the over 2,600 currently identified serotypes/serovars, some are mostly host-restricted and host-adapted, while the majority of serotypes can infect a broader range of host species and are associated with causing both livestock and human disease. Salmonella serotypes and strains within serovars can vary considerably in the severity of disease that may result from infection, with some serovars that are more highly associated with invasive disease in humans, while others predominantly cause mild gastroenteritis. These observed clinical differences may be caused by the genetic make-up and diversity of the serovars. Salmonella virulence systems are very complex containing several virulence-associated genes with different functions that contribute to its pathogenicity. The different clinical syndromes are associated with unique groups of virulence genes, and strains often differ in the array of virulence traits they display. On the chromosome, virulence genes are often clustered in regions known as Salmonella pathogenicity islands (SPIs), which are scattered throughout different Salmonella genomes and encode factors essential for adhesion, invasion, survival, and replication within the host. Plasmids can also carry various genes that contribute to Salmonella pathogenicity. For example, strains from several serovars associated with significant human disease, including Choleraesuis, Dublin, Enteritidis, Newport, and Typhimurium, can carry virulence plasmids with genes contributing to attachment, immune system evasion, and other roles. The goal of this comprehensive review is to provide key information on the Salmonella virulence, including the contributions of genes encoded in SPIs and plasmids during Salmonella pathogenesis.

Identification and bioinformatic analysis of invA gene of Salmonella in free range chicken

Abstract Salmonella is a serious cause of the health issues in human and animal worldwide. Salmonella has been isolated from different biological samples and it considers as the key role in induction of inflammation of gastrointestinal tract which in turn cause diarrhoea in different species. To further understand the involvement of Salmonella in contaminating and infecting fresh eggs and meat of free-range chicken. This study aimed to establish the microbiological and molecular detections of Salmonella in the cloaca of the free-range chicken and to identify predicted biological functions using Kyoto Encyclopedia of Gene and Genomic (KEGG) pathways and protein-protein interaction. Cloacal swabs were collected from free range chicken raised in the local farm in Duhok city. The isolates were cultured and biochemical test performed using XLD and TSI, respectively. Molecular detection and functional annotation of invA gene was carried out using Conventional PCR and bioinformatics approaches. The present study found that Salmonella was detected in 36 out of 86 samples using microbiological methods. To confirm these findings, invA gene was utilised and 9 out of 36 Salmonella isolates have shown a positive signal of invA by agarose gel. In addition, bioinformatic analysis revealed that invA gene was mainly associated with bacterial secretion processes as well as their KEGG terms and Protein-Protein Interaction were involved in bacterial invasion and secretion pathways. These findings suggested that invA gene plays important role in regulating colonization and invasion processes of Salmonella within the gut host in the free range chicken.

Expression of accessory genes in Salmonella requires the presence of the Gre factors

Genomic studies with Salmonella enterica serovar Typhimurium reveal a crucial role of horizontal gene transfer (HGT) in the acquisition of accessory cellular functions involved in host-interaction. Many virulence genes are located in genomic islands, plasmids and prophages. GreA and GreB proteins, Gre factors, interact transiently with the RNA polymerase alleviating backtracked complexes during transcription elongation. The overall effect of Gre factors depletion in Salmonella expression profile was studied. Both proteins are functionally redundant since only when both Gre factors were depleted a major effect in gene expression was detected. Remarkably, the accessory gene pool is particularly sensitive to the lack of Gre factors, with 18.6% of accessory genes stimulated by the Gre factors versus 4.4% of core genome genes. Gre factors involvement is particularly relevant for the expression of genes located in genomic islands. Our data reveal that Gre factors are required for the expression of accessory genes.

HilE represses the activity of the Salmonella virulence regulator HilD via a mechanism distinct from that of intestinal long-chain fatty acids

The expression of virulence factors essential for the invasion of host cells by Salmonella enterica is tightly controlled by a network of transcription regulators. The AraC/XylS transcription factor HilD is the main integration point of environmental signals into this regulatory network, with many factors affecting HilD activity. Long-chain fatty acids, which are highly abundant throughout the host intestine, directly bind to and repress HilD, acting as environmental cues to coordinate virulence gene expression. The regulatory protein HilE also negatively regulates HilD activity, through a protein-protein interaction. Both of these regulators inhibit HilD dimerization, preventing HilD from binding to target DNA. We investigated the structural basis of these mechanisms of HilD repression. Long-chain fatty acids bind to a conserved pocket in HilD, in a comparable manner to that reported for other AraC/XylS regulators, whereas HilE forms a stable heterodimer with HilD by binding to the HilD dimerization interface. Our results highlight two distinct, mutually exclusive mechanisms by which HilD activity is repressed, which could be exploited for the development of new antivirulence leads.

Genetic characterization of a multidrug-resistant Salmonella enterica serovar Agona isolated from a dietary supplement in Germany

Salmonella enterica subsp. enterica serovar Agona has a history of causing food-borne outbreaks and any emergence of multidrug-resistant (MDR) isolates in novel food products is of concern. Particularly, in food products frequently consumed without sufficient heating prior to consumption. Here, we report about the MDR isolate, 18-SA00377, which had been isolated from a dietary supplement in Germany in 2018 and submitted to the German National Reference Laboratory for Salmonella. WGS-based comparative genetic analyses were conducted to find a potential reservoir of the isolate itself or mobile genetic elements associated with MDR. As a phylogenetic analysis did not yield any closely related S. Agona isolates, either globally or from Germany, a detailed analysis of the largest plasmid (295,499 bp) was performed as it is the main carrier of resistances. A combined approach of long-read and short-read sequencing enabled the assembly of the isolate's chromosome and its four plasmids. Their characterization revealed the presence of 23 different antibiotic resistance genes (ARGs), conferring resistance to 12 different antibiotic drug classes, as well as genes conferring resistance to six different heavy metals. The largest plasmid, pSE18-SA00377-1, belongs to the IncHI2 plasmid family and carries 16 ARGs, that are organized as two distinct clusters, with each ARG associated with putative composite transposons. Through a two-pronged approach, highly similar plasmids to pSE18-SA00377-1 were identified in the NCBI database and a search for Salmonella isolates with a highly similar ARG resistance profile was conducted. Mapping and structural comparisons between pSE18-SA00377-1 and these plasmids and Salmonella isolates showed that both the plasmid backbone and identical or similar ARG clusters can be found not only in Salmonella isolates, originating mostly from a wide variety of livestock, but also in a diverse range of bacterial genera of varying geographical origins and isolation sources. Thus, it can be speculated that the host range of pSE18-SA00377-1 is not restricted to Salmonella and its spread already occurred in different bacterial populations. Overall, this hints at a complex history for pSE18-SA00377-1 and highlights the importance of surveilling multidrug-resistant S. enterica isolates, especially in novel food items that are not yet heavily regulated.

Ill Communication: Host Metabolites as Virulence-Regulating Signals for Plant-Pathogenic Bacteria

Plant bacterial pathogens rely on host-derived signals to coordinate the deployment of virulence factors required for infection. In this review, I describe how diverse plant-pathogenic bacteria detect and respond to plant-derived metabolic signals for the purpose of virulence gene regulation. I highlight examples of how pathogens perceive host metabolites through membrane-localized receptors as well as intracellular response mechanisms. Furthermore, I describe how individual strains may coordinate their virulence using multiple distinct host metabolic signals, and how plant signals may positively or negatively regulate virulence responses. I also describe how plant defenses may interfere with the perception of host metabolites as a means to dampen pathogen virulence. The emerging picture is that recognition of host metabolic signals for the purpose of virulence gene regulation represents an important primary layer of interaction between pathogenic bacteria and host plants that shapes infection outcomes.

From Eberthella typhi to Salmonella Typhi: The Fascinating Journey of the Virulence and Pathogenicity of Salmonella Typhi

Salmonella Typhi (S. Typhi), the invasive typhoidal serovar of Salmonella enterica that causes typhoid fever in humans, is a severe threat to global health. It is one of the major causes of high morbidity and mortality in developing countries. According to recent WHO estimates, approximately 11-21 million typhoid fever illnesses occur annually worldwide, accounting for 0.12-0.16 million deaths. Salmonella infection can spread to healthy individuals by the consumption of contaminated food and water. Typhoid fever in humans sometimes is accompanied by several other critical extraintestinal complications related to the central nervous system, cardiovascular system, pulmonary system, and hepatobiliary system. Salmonella Pathogenicity Island-1 and Salmonella Pathogenicity Island-2 are the two genomic segments containing genes encoding virulent factors that regulate its invasion and systemic pathogenesis. This Review aims to shed light on a comparative analysis of the virulence and pathogenesis of the typhoidal and nontyphoidal serovars of S. enterica.

Antimicrobial mechanism of semi-bionic extracts of three traditional medicinal plants-Rheum palmatum L., Scutellaria baicalensis Georgi, and Houttuynia cordata Thunb-That can be used as antibiotic alternatives

The Chinese traditional medicinal plants Rheum palmatum L., Scutellaria baicalensis Georgi, and Houttuynia cordata Thunb in a ratio of 108:65:27 form a compound named Dahuang Qinyu San (DQS), which inhibits and kills Escherichia coli and Salmonella to a certain extent in fish and shrimp aquaculture environments. The active ingredients quercetin, emodin, baicalin, and aloe-emodin are obtained from the semi-biomimetic extract of DQS (SEDQS). However, the antibacterial mechanism of SEDQS against Salmonella is still unclear. This study used the microwell-plate method to determine the Minimum Inhibitory Concentration (MIC) of SEDQS against Salmonella enteritidis (S. enteritidis) isolated from geese. In addition, the effect of SEDQS on the growth curve, respiratory metabolic system, cell wall, soluble protein, and nucleic acid in bacterial liquid of S. enteritidis was detected by spectrophotometer and reagent kit. The effects of SEDQS on S. enteritidis DNA, binding gel blocking, virulence gene expression, and pathogenicity-related proteins were determined by gel electrophoresis, SDS-PAGE, and fluorescence quantitative PCR. The study found that a concentration of 1/4 MIC-2 MIC (2.27-18.2 mg/ml) SEDQS can significantly inhibit the normal growth of S. enteritidis, destroy the cell membrane structure of bacteria resulting in the leak of nucleic acid, protein, and other contents (P < 0.01). It also significantly inhibited the activities of succinate dehydrogenase (SDH) and malate dehydrogenase (MDH; P < 0.01) in a concentration-dependent manner. When the concentration of SEDQS was 1/2 MIC to 2 MIC (4.55-18.2 mg/ml), the expression levels of gyrB, fimA, filC, spvR, Hcp, and vgrG virulence genes (P < 0.01) all decreased by more than 31, 11, 18, 30, 34, and 21% respectively compared with the control group. SEDQS could significantly inhibit the expression of six virulence genes and play an important role in the pathogenicity of the S. enteritidis infected host. The SEDQS could exert antibacterial pharmacological effects by inhibiting the growth and metabolism of S. enteritidis and inhibiting the expression of major virulence factors. It has potential application value as an antibiotic alternative.

Immune evasion and persistence in enteric bacterial pathogens

The major function of the mammalian immune system is to prevent and control infections caused by enteropathogens that collectively have altered human destiny. In fact, as the gastrointestinal tissues are the major interface of mammals with the environment, up to 70% of the human immune system is dedicated to patrolling them The defenses are multi-tiered and include the endogenous microflora that mediate colonization resistance as well as physical barriers intended to compartmentalize infections. The gastrointestinal tract and associated lymphoid tissue are also protected by sophisticated interleaved arrays of active innate and adaptive immune defenses. Remarkably, some bacterial enteropathogens have acquired an arsenal of virulence factors with which they neutralize all these formidable barriers to infection, causing disease ranging from mild self-limiting gastroenteritis to in some cases devastating human disease.

Genotypic virulence profiles and associations in Salmonella isolated from meat samples in wet markets and abattoirs of Metro Manila, Philippines

BACKGROUND

Salmonella are pathogenic foodborne bacteria with complex pathogenicity from numerous virulence genes housed in Salmonella pathogenicity islands (SPIs), plasmids, and other gene cassettes. However, Salmonella virulence gene distributions and mechanisms remain unestablished. In the Philippines, studies mainly report Salmonella incidences and antimicrobial resistance, but little to none on virulence profiles, their associations to animal sources, collection sites and Salmonella serogroups. Hence, a total of 799 Salmonella isolates, previously obtained from pig, cow, and chicken meat samples in wet markets and abattoirs (wet markets: 124 chicken, 151 cow, and 352 pig meat isolates; abattoirs: 172 pig tonsil and jejunum isolates) in Metro Manila, Philippines, were revived and confirmed as Salmonella through invA gene polymerase chain reaction (PCR). Isolates were then screened for eight virulence genes, namely avrA, hilA, sseC, mgtC, spi4R, pipB, spvC and spvR, by optimized multiplex PCR and significant pair associations between virulence genes were determined through Fisher's exact test. Gene frequency patterns were also determined. Salmonella serogroups in addition to animal sources and location types were also used to predict virulence genes prevalence using binary logistic regression.

RESULTS

High frequencies (64 to 98%) of SPI virulence genes were detected among 799 Salmonella isolates namely mgtC, pipB, avrA, hilA, spi4R and sseC, from most to least. However, only one isolate was positive for plasmid-borne virulence genes, spvC and spvR. Diversity in virulence genes across Salmonella serogroups for 587 Salmonella isolates (O:3 = 250, O:4 = 133, O:6,7 = 99, O:8 = 93, O:9 = 12) was also demonstrated through statistical predictions, particularly for avrA, hilA, sseC, and mgtC. mgtC, the most frequent virulence gene, was predicted by serogroup O:9, while sseC, the least frequent, was predicted by serogroup O:4 and chicken animal source. The highest virulence gene pattern involved SPIs 1-5 genes which suggests the wide distribution and high pathogenic potential of Salmonella. Statistical analyses showed five virulence gene pair associations, namely avrA and hilA, avrA and spi4R, hilA and spi4R, sseC and spi4R, and mgtC and pipB. The animal sources predicted the presence of virulence genes, sseC and pipB, whereas location type for hilA and spi4R, suggesting that these factors may contribute to the type and pathogenicity of Salmonella present.

CONCLUSION

The high prevalence of virulence genes among Salmonella in the study suggests the high pathogenic potential of Salmonella from abattoirs and wet markets of Metro Manila, Philippines which poses food safety and public health concerns and threatens the Philippine food animal industry. Statistical associations between virulence genes and prediction analyses across Salmonella serogroups and external factors such as animal source and location type and presence of virulence genes suggest the diversity of Salmonella virulence and illustrate determining factors to Salmonella pathogenicity. This study recommends relevant agencies in the Philippines to improve standards in food animal industries and increase efforts in monitoring of foodborne pathogens.

Effects of colonization-associated gene yqiC on global transcriptome, cellular respiration, and oxidative stress in Salmonella Typhimurium

BACKGROUND

yqiC is required for colonizing the Salmonella enterica serovar Typhimurium (S. Typhimurium) in human cells; however, how yqiC regulates nontyphoidal Salmonella (NTS) genes to influence bacteria-host interactions remains unclear.

METHODS

The global transcriptomes of S. Typhimurium yqiC-deleted mutant (ΔyqiC) and its wild-type strain SL1344 after 2 h of in vitro infection with Caco-2 cells were obtained through RNA sequencing to conduct comparisons and identify major yqiC-regulated genes, particularly those involved in Salmonella pathogenicity islands (SPIs), ubiquinone and menaquinone biosynthesis, electron transportation chains (ETCs), and carbohydrate/energy metabolism. A Seahorse XFp Analyzer and assays of NADH/NAD⁺ and H₂O₂ were used to compare oxygen consumption and extracellular acidification, glycolysis parameters, adenosine triphosphate (ATP) generation, NADH/NAD⁺ ratios, and H₂O₂ production between ΔyqiC and SL1344.

RESULTS

After S. Typhimurium interacts with Caco-2 cells, yqiC represses gene upregulation in aspartate carbamoyl transferase, type 1 fimbriae, and iron-sulfur assembly, and it is required for expressing ilvB operon, flagellin, tdcABCD, and dmsAB. Furthermore, yqiC is required for expressing mainly SPI-1 genes and specific SPI-4, SPI-5, and SPI-6 genes; however, it diversely regulates SPI-2 and SPI-3 gene expression. yqiC significantly contributes to menD expression in menaquinone biosynthesis. A Kyoto Encyclopedia of Genes and Genomes analysis revealed the extensive association of yqiC with carbohydrate and energy metabolism. yqiC contributes to ATP generation, and the analyzer results demonstrate that yqiC is required for maintaining cellular respiration and metabolic potential under energy stress and for achieving glycolysis, glycolytic capacity, and glycolytic reserve. yqiC is also required for expressing ndh, cydA, nuoE, and sdhB but suppresses cyoC upregulation in the ETC of aerobically and anaerobically grown S. Typhimurium; priming with Caco-2 cells caused a reversed regulation of yiqC toward upregulation in these ETC complex genes. Furthermore, yqiC is required for maintaining NADH/NAD⁺ redox status and H₂O₂ production.

CONCLUSIONS

Specific unreported genes that were considerably regulated by the colonization-associated gene yqiC in NTS were identified, and the key role and tentative mechanisms of yqiC in the extensive modulation of virulence factors, SPIs, ubiquinone and menaquinone biosynthesis, ETCs, glycolysis, and oxidative stress were discovered.

Salmonella-Based Biorodenticides: Past Applications and Current Contradictions

The idea of using pathogens to control pests has existed since the end of the 19th century. Enterobacteria from the genus Salmonella, discovered at that time, are the causative agents of many serious diseases in mammals often leading to death. Mostly, the strains of Salmonella are able to infect a wide spectrum of hosts belonging to vertebrates, but some of them show host restriction. Several strains of these bacteria have been used as biorodenticides due to the host restriction until they were banned in many countries in the second part of the 20th century. The main reason for the ban was their potential pathogenicity for some domestic animals and poultry and the outbreaks of gastroenteritis in humans. Since that time, a lot of data regarding the host specificity and host restriction of different strains of Salmonella have been accumulated, and the complexity of the molecular mechanisms affecting it has been uncovered. In this review, we summarize the data regarding the history of studying and application of Salmonella-based rodenticides, discuss molecular systems controlling the specificity of Salmonella interactions within its multicellular hosts at different stages of infection, and attempt to reconstruct the network of genes and their allelic variants which might affect the host-restriction mechanisms.

Salmonella spp. in low water activity food: Occurrence, survival mechanisms, and thermoresistance

The occurrence of disease outbreaks involving low-water-activity (a_w ) foods has gained increased prominence due in part to the fact that reducing free water in these foods is normally a measure that controls the growth and multiplication of pathogenic microorganisms. Salmonella, one of the main bacteria involved in these outbreaks, represents a major public health problem worldwide and in Brazil, which highlights the importance of good manufacturing and handling practices for food quality. The virulence of this pathogen, associated with its high ability to persist in the environment, makes Salmonella one of the main challenges for the food industry. The objectives of this article are to present the general characteristics, virulence, thermoresistance, control, and relevance of Salmonella in foodborne diseases, and describe the so-called low-water-activity foods and the salmonellosis outbreaks involving them.

Detection of Antimicrobial Resistance, Pathogenicity, and Virulence Potentials of Non-Typhoidal Salmonella Isolates at the Yaounde Abattoir Using Whole-Genome Sequencing Technique

One of the crucial public health problems today is the emerging and re-emerging of multidrug-resistant (MDR) bacteria coupled with a decline in the development of new antimicrobials. Non-typhoidal Salmonella (NTS) is classified among the MDR pathogens of international concern. To predict their MDR potentials, 23 assembled genomes of NTS from live cattle (n = 1), beef carcass (n = 19), butchers’ hands (n = 1) and beef processing environments (n = 2) isolated from 830 wet swabs at the Yaounde abattoir between December 2014 and November 2015 were explored using whole-genome sequencing. Phenotypically, while 22% (n = 5) of Salmonella isolates were streptomycin-resistant, 13% (n = 3) were MDR. Genotypically, all the Salmonella isolates possessed high MDR potentials against several classes of antibiotics including critically important drugs (carbapenems, third-generation cephalosporin and fluoroquinolone). Moreover, >31% of NTS exhibited resistance potentials to polymyxin, considered as the last resort drug. Additionally, ≤80% of isolates harbored “silent resistant genes” as a potential reservoir of drug resistance. Our isolates showed a high degree of pathogenicity and possessed key virulence factors to establish infection even in humans. Whole-genome sequencing unveiled both broader antimicrobial resistance (AMR) profiles and inference of pathogen characteristics. This study calls for the prudent use of antibiotics and constant monitoring of AMR of NTS.

Regulatory Evolution of the phoH Ancestral Gene in Salmonella enterica Serovar Typhimurium

One important event for the divergence of Salmonella from Escherichia coli was the acquisition by horizontal transfer of the Salmonella pathogenicity island 1 (SPI-1), containing genes required for the invasion of host cells by Salmonella. HilD is an AraC-like transcriptional regulator in SPI-1 that induces the expression of the SPI-1 and many other acquired virulence genes located in other genomic regions of Salmonella. Additionally, HilD has been shown to positively control the expression of some ancestral genes (also present in E. coli and other bacteria), including phoH. In this study, we determined that both the gain of HilD and cis-regulatory evolution led to the integration of the phoH gene into the HilD regulon. Our results indicate that a HilD-binding sequence was generated in the regulatory region of the S. enterica serovar Typhimurium phoH gene, which mediates the activation of promoter 1 of this gene under SPI-1-inducing conditions. Furthermore, we found that repression by H-NS, a histone-like protein, was also adapted on the S. Typhimurium phoH gene and that HilD activates the expression of this gene in part by antagonizing H-NS. Additionally, our results revealed that the expression of the S. Typhmurium phoH gene is also activated in response to low phosphate but independently of the PhoB/R two-component system, known to regulate the E. coli phoH gene in response to low phosphate. Thus, our results indicate that cis-regulatory evolution has played a role in the expansion of the HilD regulon and illustrate the phenomenon of differential regulation of ortholog genes. IMPORTANCE Two mechanisms mediating differentiation of bacteria are well known: acquisition of genes by horizontal transfer events and mutations in coding DNA sequences. In this study, we found that the phoH ancestral gene is differentially regulated between Salmonella Typhimurium and Escherichia coli, two closely related bacterial species. Our results indicate that this differential regulation was generated by mutations in the regulatory sequence of the S. Typhimurium phoH gene and by the acquisition by S. Typhimurium of foreign DNA encoding the transcriptional regulator HilD. Thus, our results, together with those from an increasing number of studies, indicate that cis-regulatory evolution can lead to the rewiring and reprogramming of transcriptional regulation, which also plays an important role in the divergence of bacteria through time.

Virulence Factors and Antimicrobial Resistance in Salmonella Species Isolated from Retail Beef in Selected KwaZulu-Natal Municipality Areas, South Africa

Salmonellosis and antimicrobial resistance caused by non-typhoidal Salmonella are public health concerns. This study aimed at determining prevalence, serovars, virulence factors and antimicrobial resistance of Salmonella from beef products. Four-hundred beef samples from 25 retail outlets in KwaZulu-Natal, South Africa were analyzed for Salmonella using standard methods, confirmation with matrix-assisted laser desorption ionization–time of flight and serotyping according to the White–Kauffmann–Le Minor scheme. The Kirby Bauer disk diffusion method was used to determine antimicrobial resistance against Cefotaxime, Kanamycin, Ampicillin, Amoxicillin, Trimethoprim Sulfamethoxazole, Ciprofloxacin, Chloramphenicol, Gentamicin Cefoxitin and Tetracycline. A polymerase chain reaction was performed to detect invA, agfA, lpfA, hilA, sivH, sefA, sopE, and spvC virulence genes. Salmonella was observed in 1.25% (5/400) of the samples. Four serovars (Enteritidis, Hadar, Heidelberg, Stanley) were identified. Almost all Salmonella were susceptible to all antimicrobials except S. Enteritidis isolate that was resistant to Tetracycline, Ampicillin and Amoxicillin. All Salmonella isolates carried at least two virulence factors. The findings indicate low Salmonella prevalence in meat from selected KZN retail beef; however, routine surveillance to monitor risk associated with virulence factors is required to mitigate potential outbreaks. The resistant S. Enteritidis highlights a need to routinely monitor antimicrobial resistance in order to enhance human health.

The ancestral stringent response potentiator, DksA has been adapted throughout Salmonella evolution to orchestrate the expression of metabolic, motility, and virulence pathways

DksA is a conserved RNA polymerase-binding protein known to play a key role in the stringent response of proteobacteria species, including many gastrointestinal pathogens. Here, we used RNA-sequencing of Escherichia coli, Salmonella bongori and Salmonella enterica serovar Typhimurium, together with phenotypic comparison to study changes in the DksA regulon, during Salmonella evolution. Comparative RNA-sequencing showed that under non-starved conditions, DksA controls the expression of 25%, 15%, and 20% of the E. coli, S. bongori, and S. enterica genes, respectively, indicating that DksA is a pleiotropic regulator, expanding its role beyond the canonical stringent response. We demonstrate that DksA is required for the growth of these three enteric bacteria species in minimal medium and controls the expression of the TCA cycle, glycolysis, pyrimidine biosynthesis, and quorum sensing. Interestingly, at multiple steps during Salmonella evolution, the type I fimbriae and various virulence genes encoded within SPIs 1, 2, 4, 5, and 11 have been transcriptionally integrated under the ancestral DksA regulon. Consequently, we show that DksA is necessary for host cells invasion by S. Typhimurium and S. bongori and for intracellular survival of S. Typhimurium in bone marrow-derived macrophages (BMDM). Moreover, we demonstrate regulatory inversion of the conserved motility-chemotaxis regulon by DksA, which acts as a negative regulator in E. coli, but activates this pathway in S. bongori and S. enterica. Overall, this study demonstrates the regulatory assimilation of multiple horizontally acquired virulence genes under the DksA regulon and provides new insights into the evolution of virulence genes regulation in Salmonella spp.

Development of a Genomics-Based Approach To Identify Putative Hypervirulent Nontyphoidal Salmonella Isolates: Salmonella enterica Serovar Saintpaul as a Model

While differences in human virulence have been reported across nontyphoidal Salmonella (NTS) serovars and associated subtypes, a rational and scalable approach to identify Salmonella subtypes with differential ability to cause human diseases is not available. Here, we used NTS serovar Saintpaul (S. Saintpaul) as a model to determine if metadata and associated whole-genome sequence (WGS) data in the NCBI Pathogen Detection (PD) database can be used to identify (i) subtypes with differential likelihoods of causing human diseases and (ii) genes and single nucleotide polymorphisms (SNPs) potentially responsible for such differences. S. Saintpaul SNP clusters (n = 211) were assigned different epidemiology types (epi-types) based on statistically significant over- or underrepresentation of human clinical isolates, including human associated (HA; n = 29), non-human associated (NHA; n = 23), and other (n = 159). Comparative genomic analyses identified 384 and 619 genes overrepresented among isolates in 5 HA and 4 NHA SNP clusters most significantly associated with the respective isolation source. These genes included 5 HA-associated virulence genes previously reported to be present on Gifsy-1/Gifsy-2 prophages. Additionally, premature stop codons in 3 and 7 genes were overrepresented among the selected HA and NHA SNP clusters, respectively. Tissue culture experiments with strains representing 4 HA and 3 NHA SNP clusters did not reveal evidence for enhanced invasion or intracellular survival for HA strains. However, the presence of sodCI (encoding a superoxide dismutase), found in 4 HA and 1 NHA SNP clusters, was positively correlated with intracellular survival in macrophage-like cells. Post hoc analyses also suggested a possible difference in intracellular survival among S. Saintpaul lineages. IMPORTANCE Not all Salmonella isolates are equally likely to cause human disease, and Salmonella control strategies may unintentionally focus on serovars and subtypes with high prevalence in source populations but are rarely associated with human clinical illness. We describe a framework leveraging WGS data in the NCBI PD database to identify Salmonella subtypes over- and underrepresented among human clinical cases. While we identified genomic signatures associated with HA/NHA SNP clusters, tissue culture experiments failed to identify consistent phenotypic characteristics indicative of enhanced human virulence of HA strains. Our findings illustrate the challenges of defining hypo- and hypervirulent S. Saintpaul and potential limitations of phenotypic assays when evaluating human virulence, for which in vivo experiments are essential. Identification of sodCI, an HA-associated virulence gene associated with enhanced intracellular survival, however, illustrates the potential of the framework and is consistent with prior work identifying specific genomic features responsible for enhanced or reduced virulence of nontyphoidal Salmonella.

Salmonella Genomics in Public Health and Food Safety

The species Salmonella enterica comprises over 2,600 serovars, many of which are known to be intracellular pathogens of mammals, birds, and reptiles. It is now apparent that Salmonella is a highly adapted environmental microbe and can readily persist in a number of environmental niches, including water, soil, and various plant (including produce) species. Much of what is known about the evolution and diversity of nontyphoidal Salmonella serovars (NTS) in the environment is the result of the rise of the genomics era in enteric microbiology. There are over 340,000 Salmonella genomes available in public databases. This extraordinary breadth of genomic diversity now available for the species, coupled with widespread availability and affordability of whole-genome sequencing (WGS) instrumentation, has transformed the way in which we detect, differentiate, and characterize Salmonella enterica strains in a timely way. Not only have WGS data afforded a detailed and global examination of the molecular epidemiological movement of Salmonella from diverse environmental reservoirs into human and animal hosts, but they have also allowed considerable consolidation of the diagnostic effort required to test for various phenotypes important to the characterization of Salmonella. For example, drug resistance, serovar, virulence determinants, and other genome-based attributes can all be discerned using a genome sequence. Finally, genomic analysis, in conjunction with functional and phenotypic approaches, is beginning to provide new insights into the precise adaptive changes that permit persistence of NTS in so many diverse and challenging environmental niches.

Salmonella in Swine: Prevalence, Multidrug Resistance, and Vaccination Strategies

An estimated 1.3 million Salmonella infections and 420 deaths occur annually in the United States, with an estimated economic burden of $3.7 billion. More than 50% of US swine operations test positive for Salmonella according to the National Animal Health Monitoring System, and 20% of Salmonella from swine are multidrug resistant (resistant to ≥3 antimicrobial classes) as reported by the National Antimicrobial Resistance Monitoring System. This review on Salmonella in swine addresses the current status of these topics by discussing antimicrobial resistance and metal tolerance in Salmonella and the contribution of horizontal gene transfer. A major challenge in controlling Salmonella is that Salmonella is a foodborne pathogen in humans but is often a commensal in food animals and thereby establishes an asymptomatic reservoir state in such animals, including swine. As food animal production systems continue to expand and antimicrobial usage becomes more limited, the need for Salmonella interventions has intensified. A promising mitigation strategy is vaccination against Salmonella in swine to limit animal, environmental, and food contamination. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

sRNA STnc150 is involved in virulence regulation of Salmonella Typhimurium by targeting fimA mRNA

Small RNAs (sRNAs) are essential virulent regulators in Salmonella typhimurium (STM). To explore the role of sRNA STnc150 in regulating STM virulence, we constructed a STnc150 deletion strain (ΔSTnc150) and its complementary strain (ΔSTnc150/C). Then, we compared their characteristics to their original parent strain experimentally, identified the target genes of STnc150 and determined the expression levels of target genes. The results showed that the ΔSTnc150 strain exhibited delayed biofilm formation, enhanced adhesion to macrophages, significantly reduced LD50, increased liver and spleen viral loads and more vital pathological damaging ability than its parent and complementary strains. Further, bioinformatics combined with the bacterial dual plasmid reporter system confirmed that the bases 72-88 of STnc150 locating at the secondary stem-loop structure of the STnc150 are complementary with the bases 1-19 in the 5'-terminal of fimA mRNA of the type 1 fimbriae subunit. Western blot analysis showed that fimA protein level was increased in STnc150 strain compared with its parent and complementary strains. Together, this study suggested that STnc150 can down-regulate STM fimA expression at the translation level, which provided insights into the regulatory mechanisms of sRNAs in virulence of STM.

Comparative genome analysis of Salmonella enterica serovar Gallinarum biovars Pullorum and Gallinarum decodes strain specific genes

Salmonella enterica serovar Gallinarum biovar Pullorum (bvP) and biovar Gallinarum (bvG) are the etiological agents of pullorum disease (PD) and fowl typhoid (FT) respectively, which cause huge economic losses to poultry industry especially in developing countries including India. Vaccination and biosecurity measures are currently being employed to control and reduce the S. Gallinarum infections. High endemicity, poor implementation of hygiene and lack of effective vaccines pose challenges in prevention and control of disease in intensively maintained poultry flocks. Comparative genome analysis unravels similarities and dissimilarities thus facilitating identification of genomic features that aids in pathogenesis, niche adaptation and in tracing of evolutionary history. The present investigation was carried out to assess the genotypic differences amongst S.enterica serovar Gallinarum strains including Indian strain S. Gallinarum Sal40 VTCCBAA614. The comparative genome analysis revealed an open pan-genome consisting of 5091 coding sequence (CDS) with 3270 CDS belonging to core-genome, 1254 CDS to dispensable genome and strain specific genes i.e. singletons ranging from 3 to 102 amongst the analyzed strains. Moreover, the investigated strains exhibited diversity in genomic features such as virulence factors, genomic islands, prophage regions, toxin-antitoxin cassettes, and acquired antimicrobial resistance genes. Core genome identified in the study can give important leads in the direction of design of rapid and reliable diagnostics, and vaccine design for effective infection control as well as eradication. Additionally, the identified genetic differences among the S. enterica serovar Gallinarum strains could be used for bacterial typing, structure based inhibitor development by future experimental investigations on the data generated.

Molecular determinants of peaceful coexistence versus invasiveness of non-Typhoidal Salmonella: Implications in long-term side-effects

The genus Salmonella represents a wide range of strains including Typhoidal and Non-Typhoidal Salmonella (NTS) isolates that exhibit illnesses of varied pathophysiologies. The more frequent NTS ensues a self-limiting enterocolitis with rare occasions of bacteremia or systemic infections. These self-limiting Salmonella strains are capable of subverting and dampening the host immune system to achieve a more prolonged survival inside the host system thus leading to chronic manifestations. Notably, emergence of new invasive NTS isolates known as invasive Non-Typhoidal Salmonella (iNTS) have worsened the disease burden significantly in some parts of the world. NTS strains adapt to attain persister phenotype intracellularly and cause relapsing infections. These chronic infections, in susceptible hosts, are also capable of causing diseases like IBS, IBD, reactive arthritis, gallbladder cancer and colorectal cancer. The present understanding of molecular mechanism of how these chronic infections are manifested is quite limited. The current work is an effort to review the prevailing knowledge emanating from a large volume of research focusing on various forms of NTS infections including those that cause localized, systemic and persistent disease. The review will further dwell into the understanding of how this pathogen contributes to the associated long term sequelae.

Transcriptional Regulation of the Multiple Resistance Mechanisms in Salmonella-A Review

The widespread use of antibiotics, especially those with a broad spectrum of activity, has resulted in the development of multidrug resistance in many strains of bacteria, including Salmonella. Salmonella is among the most prevalent causes of intoxication due to the consumption of contaminated food and water. Salmonellosis caused by this pathogen is pharmacologically treated using antibiotics such as fluoroquinolones, ceftriaxone, and azithromycin. This foodborne pathogen developed several molecular mechanisms of resistance both on the level of global and local transcription modulators. The increasing rate of antibiotic resistance in Salmonella poses a significant global concern, and an improved understanding of the multidrug resistance mechanisms in Salmonella is essential for choosing the suitable antibiotic for the treatment of infections. In this review, we summarized the current knowledge of molecular mechanisms that control gene expression related to antibiotic resistance of Salmonella strains. We characterized regulators acting as transcription activators and repressors, as well as two-component signal transduction systems. We also discuss the background of the molecular mechanisms of the resistance to metals, regulators of multidrug resistance to antibiotics, global regulators of the LysR family, as well as regulators of histone-like proteins.

Salmonella enterica serovars associated with bacteremia in Canada, 2006-2019

Background

Members of the bacterial genus Salmonella cause salmonellosis, a disease with a spectrum of clinical presentations from a self-limiting gastroenteritis to more severe bacteremia, organ failure and sepsis. The genus consists of over 2,600 serological variants (serovars). Important differences in the pathogenesis of Salmonella serovars have been noted.

Objective

The purpose of this study was to determine which Salmonella serovars were more likely to be associated with bacteremia in Canada.

Methods

Information on the total number of Salmonella infections and blood isolations reported to the National Enteric Surveillance Program (NESP) from 2006 to 2019 was extracted for each serovar. The risk (proportion) and likelihood (odds) of bacteremia were calculated for all serovars.

Results

Of the 96,082 Salmonella cases reported to the NESP during the 14-year study period, 4.4% (95% CI: 4.3%-4.6%) were bacteremic. Twenty nontyphoidal Salmonella (NTS) serovars were associated with lower rates of bacteremia compared to all NTS serovars, and 19 NTS serovars were identified as having higher rates. Heidelberg, Oranienburg, Schwarzengrund, Virchow, Panama and Poona among the top 25 most commonly reported serovars in Canada during the study period.

Conclusion

The identification of serovars associated with Salmonella bacteremia in Canada is a first step towards understanding differences in pathogenesis and disease presentation.

An incoherent feedforward loop formed by SirA/BarA, HilE and HilD is involved in controlling the growth cost of virulence factor expression by Salmonella Typhimurium

An intricate regulatory network controls the expression of Salmonella virulence genes. The transcriptional regulator HilD plays a central role in this network by controlling the expression of tens of genes mainly required for intestinal colonization. Accordingly, the expression/activity of HilD is highly regulated by multiple factors, such as the SirA/BarA two-component system and the Hcp-like protein HilE. SirA/BarA positively regulates translation of hilD mRNA through a regulatory cascade involving the small RNAs CsrB and CsrC, and the RNA-binding protein CsrA, whereas HilE inhibits HilD activity by protein-protein interaction. In this study, we show that SirA/BarA also positively regulates translation of hilE mRNA through the same mentioned regulatory cascade. Thus, our results reveal a paradoxical regulation exerted by SirA/BarA-Csr on HilD, which involves simultaneous opposite effects, direct positive control and indirect negative control through HilE. This kind of regulation is called an incoherent type-1 feedforward loop (I1-FFL), which is a motif present in certain regulatory networks and represents a complex biological problem to decipher. Interestingly, our results, together with those from a previous study, indicate that HilE, the repressor component of the I1-FFL reported here (I1-FFL_{SirA/BarA-HilE-HilD}), is required to reduce the growth cost imposed by the expression of the genes regulated by HilD. Moreover, we and others found that HilE is necessary for successful intestinal colonization by Salmonella. Thus, these findings support that I1-FFL_{SirA/BarA-HilE-HilD} cooperates to control the precise amount and activity of HilD, for an appropriate balance between the growth cost and the virulence benefit generated by the expression of the genes induced by this regulator. I1-FFL_{SirA/BarA-HilE-HilD} represents a complex regulatory I1-FFL that involves multiple regulators acting at distinct levels of gene expression, as well as showing different connections to the rest of the regulatory network governing Salmonella virulence.

To infect the intestine of a broad range of hosts, including humans, Salmonella is required to express a large number of genes encoding different cellular functions, which imposes a growth penalty. Thus, Salmonella has developed complex regulatory mechanisms that control the expression of virulence genes. Here we identified a novel and sophisticated regulatory mechanism that is involved in the fine-tuned control of the expression level and activity of the transcriptional regulator HilD, for the appropriate balance between the growth cost and the virulence benefit generated by the expression of tens of Salmonella genes. This mechanism forms an incoherent type-1 feedforward loop (I1-FFL), which involves paradoxical regulation; that is, a regulatory factor exerting simultaneous opposite control (positive and negative) on another factor. I1-FFLs are present in regulatory networks of diverse organisms, from bacteria to humans, and represent a complex biological problem to decipher. Interestingly, the I1-FFL reported here is integrated by ancestral regulators and by regulators that Salmonella has acquired during evolution. Thus, our findings reveal a novel I1-FFL of bacteria, which is involved in virulence. Moreover, our results illustrate the integration of ancestral and acquired factors into a regulatory motif, which can lead to the expansion of regulatory networks.

Emerging and divergent roles of pyrophosphorylated nucleotides in bacterial physiology and pathogenesis

Bacteria inhabit diverse environmental niches and consequently must modulate their metabolism to adapt to stress. The nucleotide second messengers guanosine tetraphosphate (ppGpp) and guanosine pentaphosphate (pppGpp) (collectively referred to as (p)ppGpp) are essential for survival during nutrient starvation. (p)ppGpp is synthesized by the RelA-SpoT homologue (RSH) protein family and coordinates the control of cellular metabolism through its combined effect on over 50 proteins. While the role of (p)ppGpp has largely been associated with nutrient limitation, recent studies have shown that (p)ppGpp and related nucleotides have a previously underappreciated effect on different aspects of bacterial physiology, such as maintaining cellular homeostasis and regulating bacterial interactions with a host, other bacteria, or phages. (p)ppGpp produced by pathogenic bacteria facilitates the evasion of host defenses such as reactive nitrogen intermediates, acidic pH, and the complement system. Additionally, (p)ppGpp and pyrophosphorylated derivatives of canonical adenosine nucleotides called (p)ppApp are emerging as effectors of bacterial toxin proteins. Here, we review the RSH protein family with a focus on its unconventional roles during host infection and bacterial competition.

Salmonella enters a dormant state within human epithelial cells for persistent infection

Salmonella Typhimurium (S. Typhimurium) is an enteric bacterium capable of invading a wide range of hosts, including rodents and humans. It targets different host cell types showing different intracellular lifestyles. S. Typhimurium colonizes different intracellular niches and is able to either actively divide at various rates or remain dormant to persist. A comprehensive tool to determine these distinct S. Typhimurium lifestyles remains lacking. Here we developed a novel fluorescent reporter, Salmonella INtracellular Analyzer (SINA), compatible for fluorescence microscopy and flow cytometry in single-bacterium level quantification. This identified a S. Typhimurium subpopulation in infected epithelial cells that exhibits a unique phenotype in comparison to the previously documented vacuolar or cytosolic S. Typhimurium. This subpopulation entered a dormant state in a vesicular compartment distinct from the conventional Salmonella-containing vacuoles (SCV) as well as the previously reported niche of dormant S. Typhimurium in macrophages. The dormant S. Typhimurium inside enterocytes were viable and expressed Salmonella Pathogenicity Island 2 (SPI-2) virulence factors at later time points. We found that the formation of these dormant S. Typhimurium is not triggered by the loss of SPI-2 effector secretion but it is regulated by (p)ppGpp-mediated stringent response through RelA and SpoT. We predict that intraepithelial dormant S. Typhimurium represents an important pathogen niche and provides an alternative strategy for S. Typhimurium pathogenicity and its persistence.

Salmonella Typhimurium is a clinically relevant bacterial pathogen that causes Salmonellosis. It can actively or passively invade various host cell types and reside in a Salmonella-containing vacuole (SCV) within host cells. The SCV can be remodeled into a replicative niche with the aid of Salmonella Type III Secretion System 2 (T3SS2) effectors or else, the SCV is ruptured for the access of the nutrient-rich host cytosol. Depending on the infected host cell type, S. Typhimurium undertake different lifestyles that are distinct by their subcellular localization, replication rate and metabolic rate. We present here a novel fluorescent reporter system that rapidly detects S. Typhimurium lifestyles using fluorescence microscopy and flow cytometry. We identified a dormant S. Typhimurium population within enterocyte that displays capacities in host cell persistence, dormancy exit and antibiotic tolerance. We deciphered the (p)ppGpp stringent response pathway that suppresses S. Typhimurium dormancy in enterocytes while promoting dormancy in macrophages, pinpointing a divergent physiological consequence regulated by the same set of S. Typhimurium molecular mediators. Altogether, our work demonstrated the potential of fluorescent reporters in facile bacterial characterization, and revealed a dormant S. Typhimurium population in human enterocytes that are phenotypically distinct from that observed in macrophages and fibroblasts.

Pathoadaptation of the passerine-associated Salmonella enterica serovar Typhimurium lineage to the avian host

Salmonella enterica is a diverse bacterial pathogen and a primary cause of human and animal infections. While many S. enterica serovars present a broad host-specificity, several specialized pathotypes have been adapted to colonize and cause disease in one or limited numbers of host species. The underlying mechanisms defining Salmonella host-specificity are far from understood. Here, we present genetic analysis, phenotypic characterization and virulence profiling of a monophasic S. enterica serovar Typhimurium strain that was isolated from several wild sparrows in Israel. Whole genome sequencing and complete assembly of its genome demonstrate a unique genetic signature that includes the integration of the BTP1 prophage, loss of the virulence plasmid, pSLT and pseudogene accumulation in multiple T3SS-2 effectors (sseJ, steC, gogB, sseK2, and sseK3), catalase (katE), tetrathionate respiration (ttrB) and several adhesion/ colonization factors (lpfD, fimH, bigA, ratB, siiC and siiE) encoded genes. Correspondingly, this strain demonstrates impaired biofilm formation, intolerance to oxidative stress and compromised intracellular replication within non-phagocytic host cells. Moreover, while this strain showed attenuated pathogenicity in the mouse, it was highly virulent and caused an inflammatory disease in an avian host. Overall, our findings demonstrate a unique phenotypic profile and genetic makeup of an overlooked S. Typhimurium sparrow-associated lineage and present distinct genetic signatures that are likely to contribute to its pathoadaptation to passerine birds.

During Salmonella enterica evolution, many different ecological niches have been effectively occupied by this highly diverse bacterial pathogen. While many S. enterica serovars successfully maintained their ability to infect and colonize in a wide-array of host species, a few biotypes have evolved to colonize and cause a disease in only one or a small group of hosts. The evolutionary dynamic and the mechanisms shaping the host-specificity of Salmonella adapted strains are important to better understand Salmonella pathogenicity and its ecology, but still not fully understood. Here, we report genetic and phenotypic characterization of a S. Typhimurium strain that was isolated from several wild sparrows in Israel. This strain presented unique phenotypic profile that included impaired biofilm formation, high sensitivity to oxidative stress and reduced intracellular replication in non-phagocytic cells. In addition, while this strain was able to cause high inflammatory disease in an avian host, it was highly attenuated in the mouse model. Genome analysis identified that specific genetic signatures found in the sparrow strain are more frequently associated with poultry isolates than clinical isolates of S. Typhimurium. These genetic features are expected to accumulatively contribute toward the adaptation of this strain to birds.

Salmonella Effector SpvB Inhibits NF-κB Activity via KEAP1-Mediated Downregulation of IKKβ

Bacterial pathogens have a broad arsenal of genes that are tightly regulated and coordinated to facilitate adaptation to alter host inflammatory response and prolong intracellular bacterial survival. Salmonella enterica serovar Typhimurium utilizes a type III secretion system (T3SS) to deliver effector molecules into host cells and regulate signal transduction pathways such as NF-κB, thereby resulting in salmonellosis. SpvB, a pSLT-encoded cytotoxic protein secreted by Salmonella pathogenicity island-2 T3SS, is associated with enhanced Salmonella survival and intracellular replication. In this report, we characterized the effects of SpvB on NF-κB signaling pathway. We showed that SpvB has a potent and specific ability to prevent NF-κB activation by targeting IκB kinase β (IKKβ). Previous studies from our laboratory showed that SpvB decreases Nrf2 through its C-terminal domain. Here we further demonstrated that KEAP1, a cytoplasmic protein that interacts with Nrf2 and mediates its proteasomal degradation, is involved in SpvB-induced downregulation of IKKβ expression and phosphorylation. Reduction of KEAP1 by small-interfering RNA prevented the suppression of IKKβ and its phosphorylation mediated by SpvB. These findings revealed a novel mechanism by which Salmonella modulates NF-κB activity to ultimately facilitate intracellular bacterial survival and proliferation and delay host immune response to establish infection.

Impact of the Resistance Responses to Stress Conditions Encountered in Food and Food Processing Environments on the Virulence and Growth Fitness of Non-Typhoidal Salmonellae

The success of Salmonella as a foodborne pathogen can probably be attributed to two major features: its remarkable genetic diversity and its extraordinary ability to adapt. Salmonella cells can survive in harsh environments, successfully compete for nutrients, and cause disease once inside the host. Furthermore, they are capable of rapidly reprogramming their metabolism, evolving in a short time from a stress-resistance mode to a growth or virulent mode, or even to express stress resistance and virulence factors at the same time if needed, thanks to a complex and fine-tuned regulatory network. It is nevertheless generally acknowledged that the development of stress resistance usually has a fitness cost for bacterial cells and that induction of stress resistance responses to certain agents can trigger changes in Salmonella virulence. In this review, we summarize and discuss current knowledge concerning the effects that the development of resistance responses to stress conditions encountered in food and food processing environments (including acid, osmotic and oxidative stress, starvation, modified atmospheres, detergents and disinfectants, chilling, heat, and non-thermal technologies) exerts on different aspects of the physiology of non-typhoidal Salmonellae, with special emphasis on virulence and growth fitness.

A Novel Salmonella Periplasmic Protein Controlling Cell Wall Homeostasis and Virulence

Horizontal gene transfer has shaped the evolution of Salmonella enterica as pathogen. Some functions acquired by this mechanism include enzymes involved in peptidoglycan (PG) synthesis and remodeling. Here, we report a novel serovar Typhimurium protein that is absent in non-pathogenic bacteria and bears a LprI functional domain, first reported in a Mycobacterium tuberculosis lipoprotein conferring lysozyme resistance. Based on the presence of such domain, we hypothesized a role of this S. Typhimurium protein in PG metabolism. This protein, which we named ScwA for Salmonellacell wall-related regulator-A, controls positively the levels of the murein lytic transglycosylase MltD. In addition, the levels of other enzymes that cleave bonds in the PG lattice were affected in a mutant lacking ScwA, including a soluble lytic tranglycosylase (Slt), the amidase AmiC, and a few endo- and carboxypeptidases (NlpC, PBP4, and AmpH). The scwA gene has lower G+C content than the genomic average (43.1 vs. 52.2%), supporting acquisition by horizontal transfer. ScwA is located in the periplasm, stabilized by two disulfide bridges, produced preferentially in stationary phase and down-regulated following entry of the pathogen into eukaryotic cells. ScwA deficiency, however, results in a hypervirulent phenotype in the murine typhoid model. Based on these findings, we conclude that ScwA may be exploited by S. Typhimurium to ensure cell envelope homeostasis along the infection and to prevent host overt damage. This role could be accomplished by controlling the production or stability of a reduced number of peptidoglycan hydrolases whose activities result in the release of PG fragments.

Genomic Analysis of Antimicrobial Resistance and Resistance Plasmids in Salmonella Serovars from Poultry in Nigeria

Antimicrobial resistance is a global public health concern, and resistance genes in Salmonella, especially those located on mobile genetic elements, are part of the problem. This study used phenotypic and genomic methods to identify antimicrobial resistance and resistance genes, as well as the plasmids that bear them, in Salmonella isolates obtained from poultry in Nigeria. Seventy-four isolates were tested for susceptibility to eleven commonly used antimicrobials. Plasmid reconstruction and identification of resistance and virulence genes were performed with a draft genome using in silico approaches in parallel with plasmid extraction. Phenotypic resistance to ciprofloxacin (50.0%), gentamicin (48.6%), nalidixic acid (79.7%), sulphonamides (71.6%) and tetracycline (59.5%) was the most observed. Antibiotic resistance genes (ARGs) detected in genomes corresponded well with these observations. Commonly observed ARGs included sul1, sul2, sul3, tet (A), tet (M), qnrS1, qnrB19 and a variety of aminoglycoside-modifying genes, in addition to point mutations in the gyrA and parC genes. Multiple ARGs were predicted to be located on IncN and IncQ1 plasmids of S. Schwarzengrund and S. Muenster, and most qnrB19 genes were carried by Col (pHAD28) plasmids. Seventy-two percent (19/24) of S. Kentucky strains carried multidrug ARGs located in two distinct variants of Salmonella genomic island I. The majority of strains carried full SPI-1 and SPI-2 islands, suggesting full virulence potential.

The phylogenomics of CRISPR-Cas system and revelation of its features in Salmonella

Salmonellae display intricate evolutionary patterns comprising over 2500 serovars having diverse pathogenic profiles. The acquisition and/or exchange of various virulence factors influences the evolutionary framework. To gain insights into evolution of Salmonella in association with the CRISPR-Cas genes we performed phylogenetic surveillance across strains of 22 Salmonella serovars. The strains differed in their CRISPR1-leader and cas operon features assorting into two main clades, CRISPR1-STY/cas-STY and CRISPR1-STM/cas-STM, comprising majorly typhoidal and non-typhoidal Salmonella serovars respectively. Serovars of these two clades displayed better relatedness, concerning CRISPR1-leader and cas operon, across genera than between themselves. This signifies the acquisition of CRISPR1/Cas region could be through a horizontal gene transfer event owing to the presence of mobile genetic elements flanking CRISPR1 array. Comparison of CRISPR and cas phenograms with that of multilocus sequence typing (MLST) suggests differential evolution of CRISPR/Cas system. As opposed to broad-host-range, the host-specific serovars harbor fewer spacers. Mapping of protospacer sources suggested a partial correlation of spacer content with habitat diversity of the serovars. Some serovars like serovar Enteritidis and Typhimurium that inhabit similar environment/infect similar hosts hardly shared their protospacer sources.

Deletion of the lon gene augments expression of Salmonella Pathogenicity Island (SPI)-1 and metal ion uptake genes leading to the accumulation of bactericidal hydroxyl radicals and host pro-inflammatory cytokine-mediated rapid intracellular clearance

In the present study, we characterized the involvement of Lon protease in bacterial virulence and intracellular survival in Salmonella under abiotic stress conditions resembling the conditions of a natural infection. Wild type (JOL401) and the lon mutant (JOL909) Salmonella Typhimurium were exposed to low temperature, pH, osmotic, and oxidative stress conditions and changes in gene expression profiles related to virulence and metal ion uptake were investigated. Expression of candidate genes invF and hilC of Salmonella Pathogenicity Island (SPI)-1 and sifA and sseJ of SPI-2 revealed that Lon protease controls SPI-1 genes and not SPI-2 genes under all stress conditions tested. The lon mutant exhibited increased accumulation of hydroxyl (OH·) ions that lead to cell damage due to oxidative stress. This oxidative damage can also be linked to an unregulated influx of iron due to the upregulation of ion channel genes such as fepA in the lon mutant. The deletion of lon from the Salmonella genome causes oxidative damage and increased expression of virulence genes. It also prompts the secretion of host pro-inflammatory cytokines leading to early clearance of the bacteria from host cells. We conclude that poor bacterial recovery from mice infected with the lon mutant is a result of disrupted bacterial intracellular equilibrium and rapid activation of cytokine expression leading to bacterial lysis.

The battle for iron in enteric infections

Iron is an essential element for almost all living organisms, but can be extremely toxic in high concentrations. All organisms must therefore employ homeostatic mechanisms to finely regulate iron uptake, usage and storage in the face of dynamic environmental conditions. The critical step in mammalian systemic iron homeostasis is the fine regulation of dietary iron absorption. However, as the gastrointestinal system is also home to >1014 bacteria, all of which engage in their own programmes of iron homeostasis, the gut represents an anatomical location where the inter-kingdom fight for iron is never-ending. Here, we explore the molecular mechanisms of, and interactions between, host and bacterial iron homeostasis in the gastrointestinal tract. We first detail how mammalian systemic and cellular iron homeostasis influences gastrointestinal iron availability. We then focus on two important human pathogens, Salmonella and Clostridia; despite their differences, they exemplify how a bacterial pathogen must navigate and exploit this web of iron homeostasis interactions to avoid host nutritional immunity and replicate successfully. We then reciprocally explore how iron availability interacts with the gastrointestinal microbiota, and the consequences of this on mammalian physiology and pathogen iron acquisition. Finally, we address how understanding the battle for iron in the gastrointestinal tract might inform clinical practice and inspire new treatments for important diseases.

The Salmonella Typhimurium InvF-SicA complex is necessary for the transcription of sopB in the absence of the repressor H-NS

Expression of virulence factors in non-typhoidal Salmonella enterica depends on a wide variety of general and specific transcriptional factors that act in response to multiple environmental signals. Expression of genes for cellular invasion located in the Salmonella pathogenicity island 1 (SPI-1) is tightly regulated by several transcriptional regulators arrayed in a cascade, while repression of this system is exerted mainly by H-NS. In SPI-1, H-NS represses the expression mainly by binding to the regulatory region of hilA and derepression is exercised mainly by HilD. However, the possible regulatory role of H-NS in genes downstream from HilD and HilA, such as those regulated by InvF, has not been fully explored. Here the role of H-NS on the expression of sopB, an InvF dependent gene encoded in SPI-5, was evaluated. Our data show that InvF is required for the expression of sopB even in the absence of H-NS. Furthermore, in agreement with previous results on other InvF-regulated genes, we found that the expression of sopB requires the InvF/SicA complex. Our results support that SicA is not required for DNA binding nor for increasing affinity of InvF to DNA in vitro. Moreover, by using a bacterial two-hybrid system we were able to identify interactions between SicA and InvF. Lastly, protein-protein interaction assays suggest that InvF functions as a monomer. Derived from these results we postulate that the InvF/SicA complex does not act on sopB as an anti-H-NS factor; instead, it seems to induce the expression of sopB by acting as a classical transcriptional regulator.

Salmonella secretion systems: Differential roles in pathogen-host interactions

The bacterial genus Salmonella includes a large group of food-borne pathogens that cause a variety of gastrointestinal or systemic diseases in hosts. Salmonella use several secretion devices to inject various effectors targeting eukaryotic hosts, or bacteria. In the past few years, considerable progress has been made towards understanding the structural features and molecular mechanisms of the secretion systems of Salmonella, particularly regarding their roles in host-pathogen interactions. In this review, we summarize the current advances about the main characteristics of the Salmonella secretion systems. Clarifying the roles of the secretion systems in the process of infecting various hosts will broaden our understanding of the importance of microbial interactions in maintaining human health and will provide information for developing novel therapeutic approaches.