Virulence Factors in Salmonella Typhimurium: The Sagacity of a Bacterium

Molecular Characteristics of Salmonella Spp. Responsible for Bloodstream Infections in a Tertiary Hospital in Nanjing, China, 2019-2021

Objective

To investigate the clinical and molecular characteristics of Salmonella spp. causing bloodstream infections (BSIs) in our hospital.

Methods

We studied 22 clinical Salmonella isolates from BSIs and 16 from non-BSIs, performing antimicrobial susceptibility testing (AST) and whole genome sequencing (WGS). The analysis included serovars, antibiotic resistance genes (ARGs), virulence factors (VFs), sequence types (STs), plasmid replicons, and genetic relationships. We also assessed pathogenicity of the isolates causing BSIs through growth, biofilm formation, and anti-serum killing assays.

Results

WGS analysis identified 13 Salmonella serovars, with four responsible for BSIs. S. Enteritidis was the most prevalent serovar, involved in 19 (50.0%) cases. BSIs were caused by 17S. Enteritidis, two S. Typhimurium, two S. Munster and one S. Diguel. Of the 38 isolates, 27 (71.1%) exhibited high resistance to ampicillin, and 24 (63.2%) to ampicillin/sulbactam. Thirty-six types of ARGs were identified, with blaTEM-1B (n = 25, 65.8%) being the most frequent. Ten plasmid replicons were found; the combination of IncFIB(S)-IncFII(S)-IncX1 was the most common in S. Enteritidis (94.7%). Fifteen STs were identified, among which ST11 was the most prevalent and clonally disseminated, primarily responsible for BSIs. A total of 333 different VFs were detected, 177 of which were common across all strains. No significant differences were observed between the BSI and non-BSI isolates in terms of resistance rates, ARGs, plasmid replicons, and VFs, except for seven VFs. No strong pathogenicity was observed in the BSI-causing isolates.

Conclusion

BSIs were predominantly caused by clonally disseminated S. Enteritidis ST11, the majority of which carried multiple ARGs, VFs and plasmid replicons. This study provides the first data on clonally disseminated S. Enteritidis ST11 causing BSIs, highlighting the urgent need for enhanced infection control measures.

Characterization of Antimicrobial Activities of Bifidobacterium lactis BB-12 and Their Inhibitory Effect Against Some Foodborne Pathogens

Bifidobacterium animalis subsp. lactis BB-12, a probiotic, has shown potential to promote health benefits and control pathogens. This study aimed to investigate the effectiveness of BB-12 and its cell-free supernatant (CFS) in inhibiting the growth of Listeria monocytogenes and Salmonella enterica serovar Typhimurium. To assess the antimicrobial activity of BB-12, agar well diffusion, disk diffusion, and minimum inhibitory concentration (MIC) tests were conducted. The bicinchoninic acid (BCA) assay was performed to measure the protein concentration in CFS. The study's results indicated that the BB-12 strain inhibited the pathogens' growth. The disk diffusion test using BB-12 showed inhibitory results ranging from 11 to 14 mm for both bacteria. The agar well diffusion test reported the zone of inhibition ranging from 11.6 to 16 mm for both bacteria. The MIC test was conducted as a confirmatory test, which demonstrated the highest inhibitory zone using 2 McFarland (6 × 108 CFU/mL) concentrations of probiotics on L. monocytogenes (44.98%) and S. Typhimurium (66.41%). The disk diffusion test revealed that the probiotic CFS had a significant inhibitory impact on S. Typhimurium with a 16.6 mm zone of inhibition. The BCA test findings indicated that the 24- and 48-h CFSs exhibited inhibitory properties against infections. Notably, the 24-h CFS, including a protein level of 78.47 μg/mL, demonstrated a more pronounced inhibitory impact on both pathogens. The findings highlight that utilizing the BB-12 strain and its CFS can serve as a viable approach to battle infections, enhancing food safety and public health.

Lipopolysaccharide with long O-antigen is crucial for Salmonella Enteritidis to evade complement activity and to facilitate bacterial survival in vivo in the Galleria mellonella infection model

Bacterial resistance to serum is a key virulence factor for the development of systemic infections. The amount of lipopolysaccharide (LPS) and the O-antigen chain length distribution on the outer membrane, predispose Salmonella to escape complement-mediated killing. In Salmonella enterica serovar Enteritidis (S. Enteritidis) a modal distribution of the LPS O-antigen length can be observed. It is characterized by the presence of distinct fractions: low molecular weight LPS, long LPS and very long LPS. In the present work, we investigated the effect of the O-antigen modal length composition of LPS molecules on the surface of S. Enteritidis cells on its ability to evade host complement responses. Therefore, we examined systematically, by using specific deletion mutants, roles of different O-antigen fractions in complement evasion. We developed a method to analyze the average LPS lengths and investigated the interaction of the bacteria and isolated LPS molecules with complement components. Additionally, we assessed the aspect of LPS O-antigen chain length distribution in S. Enteritidis virulence in vivo in the Galleria mellonella infection model. The obtained results of the measurements of the average LPS length confirmed that the method is suitable for measuring the average LPS length in bacterial cells as well as isolated LPS molecules and allows the comparison between strains. In contrast to earlier studies we have used much more precise methodology to assess the LPS molecules average length and modal distribution, also conducted more subtle analysis of complement system activation by lipopolysaccharides of various molecular mass. Data obtained in the complement activation assays clearly demonstrated that S. Enteritidis bacteria require LPS with long O-antigen to resist the complement system and to survive in the G. mellonella infection model.

Scs system links copper and redox homeostasis in bacterial pathogens

The bacterial envelope is an essential compartment involved in metabolism and metabolites transport, virulence, and stress defense. Its roles become more evident when homeostasis is challenged during host-pathogen interactions. In particular, the presence of free radical groups and excess copper in the periplasm causes noxious reactions, such as sulfhydryl group oxidation leading to enzymatic inactivation and protein denaturation. In response to this, canonical and accessory oxidoreductase systems are induced, performing quality control of thiol groups, and therefore contributing to restoring homeostasis and preserving survival under these conditions. Here, we examine recent advances in the characterization of the Dsb-like, Salmonella-specific Scs system. This system includes the ScsC/ScsB pair of Cu+-binding proteins with thiol-oxidoreductase activity, an alternative ScsB-partner, the membrane-linked ScsD, and a likely associated protein, ScsA, with a role in peroxide resistance. We discuss the acquisition of the scsABCD locus and its integration into a global regulatory pathway directing envelope response to Cu stress during the evolution of pathogens that also harbor the canonical Dsb systems. The evidence suggests that the canonical Dsb systems cannot satisfy the extra demands that the host-pathogen interface imposes to preserve functional thiol groups. This resulted in the acquisition of the Scs system by Salmonella. We propose that the ScsABCD complex evolved to connect Cu and redox stress responses in this pathogen as well as in other bacterial pathogens.

Genomic-wide analysis approach revealed genomic similarity for environmental Mexican S. Oranienburg genomes

As the human population grows, an increase in food trade is needed. This elevates the risk of epidemiological outbreaks. One of the prevalent pathogens associated with food production in Mexico has been Salmonella Oranienburg. Effective surveillance systems require microbial genetic knowledge. The objective of this work is to describe the genetic composition of Mexican S. Oranienburg genomes. For that, 53 strains from different environmental sources were isolated and sequenced. Additionally, 109 S. Oranienburg genomes were downloaded. Bioinformatic analyses were used to explore the clonal complex and genomic relatedness. A major clonal group formed by ST23 was identified comprising four STs. 202 SNPs were found the maximum difference among isolates. Virulence genes for host invasion and colonization as rpoS, fimbria type 1, and, T3SS were found common for all isolates. This study suggests that Mexican S. Oranienburg strains are potential pathogens circulating continuously in the region between host and non-host environments.

Effect of Salmonella pathogenicity island 1 and 2 (SPI-1 and SPI-2) deletion on intestinal colonization and systemic dissemination in chickens

Salmonella's virulence genes are located in two regions known as Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2). SPI-1 allows the bacteria to invade the intestine, while SPI-2 is important for intracellular survival and replication, although it is also necessary for intestinal disease. The aim of this study was to evaluate the effect of the deletion of SPI-1 or SPI-2 genes on the intestinal and systemic salmonellosis using the avian model. Groups of chickens were orally infected with 1010 Colony-Forming Units (CFU) of S. Typhimurium SL1344 WT strain, as well as mutants ∆SPI-1 or ∆SPI-2. At different times post-infection, 5 chickens from each group were euthanized and examined postmortem. Cecum and liver were taken from each chicken for determination of CFU's, histopathological analysis and immunochemistry. Bacterial colonies were recovered from the liver and cecum samples infected with WT strain, while in the cultures from the organs infected with the mutant strains no colonies were recovered or were drastically affected in the ability to survive. In histopathological analysis, the WT strain produced lesions in liver and ceca, and it was detected by immunohistochemistry throughout the course of the infection. On the other hand, organs of chickens infected with ∆SPI-1 or ∆SPI-2 showed attenuated lesions and the immunohistochemistry revealed less bacteria compared to the WT strain. Taken together, our results show the importance of SPI-1 and SPI-2 genes for the complete intestinal and systemic disease in an in vivo avian model.

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.

Interactions between the AraC/XylS-like transcriptional activator InvF of Salmonella Typhimurium, the RNA polymerase alpha subunit and the chaperone SicA

Salmonella enterica serovar Typhimurium causes gastroenteritis and systemic infections in humans. For this bacterium the expression of a type III secretion system (T3SS) and effector proteins encoded in the Salmonella pathogenicity island-1 (SPI-1), is keystone for the virulence of this bacterium. Expression of these is controlled by a regulatory cascade starting with the transcriptional regulators HilD, HilC and RtsA that induce the expression of HilA, which then activates expression of the regulator InvF, a transcriptional regulator of the AraC/XylS family. InvF needs to interact with the chaperone SicA to activate transcription of SPI-1 genes including sicA, sopB, sptP, sopE, sopE2, and STM1239. InvF very likely acts as a classical activator; however, whether InvF interacts with the RNA polymerase alpha subunit RpoA has not been determined. Results from this study confirm the interaction between InvF with SicA and reveal that both proteins interact with the RNAP alpha subunit. Thus, our study further supports that the InvF/SicA complex acts as a classical activator. Additionally, we showed for the first time an interaction between a chaperone of T3SS effectors (SicA) and the RNAP.

Salmonella Bacteremia in an Older Patient With No Specific Entry: A Case Report

In this case report, we describe a rare case of non-typhoidal Salmonella bacteremia in an 87-year-old woman with no apparent history of daily Salmonella exposure. The patient presented with fever, body discomfort, and diarrhea. Medical examinations ruled out usual sources of Salmonella, including raw food consumption and pet contact. Her medical history included postoperative sigmoid colon cancer, left breast cancer, and other ailments. Although Salmonella infection typically stems from oral intake, this case suggests that bacterial translocation from the gastrointestinal tract could be the primary cause, potentially exacerbated by the patient's age and medical history. Another hypothesis is an ascending infection from diarrhea to the urinary tract, which might have led to pyelonephritis and subsequent bacteremia. This case highlights the importance of recognizing the potential for severe infections such as sepsis in older individuals presenting with diverse symptoms. Therefore, this case further underscores the need for heightened clinical vigilance, especially in community hospitals, to ensure timely and appropriate management of such severe conditions in the older population.

Proteomic analysis reveals the global effect of the BarA/SirA-Csr regulatory cascade in Salmonella Typhimurium grown in conditions that favor the expression of invasion genes

In many bacteria, the BarA/SirA and Csr regulatory systems control expression of genes encoding a wide variety of cellular functions. The BarA/SirA two-component system induces the expression of CsrB and CsrC, two small non-coding RNAs that sequester CsrA, a protein that binds to target mRNAs and thus negatively or positively regulates their expression. BarA/SirA and CsrB/C induce expression of the Salmonella Pathogenicity Island 1 (SPI-1) genes required for Salmonella invasion of host cells. To further investigate the regulatory role of the BarA/SirA and Csr systems in Salmonella, we performed LC-MS/MS proteomic analysis using the WT S. Typhimurium strain and its derived ΔsirA and ΔcsrB ΔcsrC mutants grown in SPI-1-inducing conditions. The expression of 164 proteins with a wide diversity, or unknown, functions was significantly affected positively or negatively by the absence of SirA and/or CsrB/C. Interestingly, 19 proteins were identified as new targets for SirA-CsrB/C. Our results support that SirA and CsrB/C act in a cascade fashion to regulate gene expression in S. Typhimurium in the conditions tested. Notably, our results show that SirA-CsrB/C-CsrA controls expression of proteins required for the replication of Salmonella in the intestinal lumen, in an opposite way to its control exerted on the SPI-1 proteins. SIGNIFICANCE: The BarA/SirA and Csr global regulatory systems control a wide range of cellular processes, including the expression of virulence genes. For instance, in Salmonella, BarA/SirA and CsrB/C positively regulate expression of the SPI-1 genes, which are required for Salmonella invasion to host cells. In this study, by performing a proteomic analysis, we identified 164 proteins whose expression was positively or negatively controlled by SirA and CsrB/C in SPI-1-inducing conditions, including 19 new possible targets of these systems. Our results support the action of SirA and CsrB/C in a cascade fashion to control different cellular processes in Salmonella. Interestingly, our data indicate that SirA-CsrB/C-CsrA controls inversely the expression of proteins required for invasion of the intestinal epithelium and for replication in the intestinal lumen, which suggests a role for this regulatory cascade as a molecular switch for Salmonella virulence. Thus, our study further expands the insight into the regulatory mechanisms governing the virulence and physiology of an important pathogen.

CsrA Positively and Directly Regulates the Expression of the pdu, pocR, and eut Genes Required for the Luminal Replication of Salmonella Typhimurium

Enteric pathogens, such as Salmonella, have evolved to thrive in the inflamed gut. Genes located within the Salmonella pathogenicity island 1 (SPI-1) mediate the invasion of cells from the intestinal epithelium and the induction of an intestinal inflammatory response. Alternative electron acceptors become available in the inflamed gut and are utilized by Salmonella for luminal replication through the metabolism of propanediol and ethanolamine, using the enzymes encoded by the pdu and eut genes. The RNA-binding protein CsrA inhibits the expression of HilD, which is the central transcriptional regulator of the SPI-1 genes. Previous studies suggest that CsrA also regulates the expression of the pdu and eut genes, but the mechanism for this regulation is unknown. In this work, we show that CsrA positively regulates the pdu genes by binding to the pocR and pduA transcripts as well as the eut genes by binding to the eutS transcript. Furthermore, our results show that the SirA-CsrB/CsrC-CsrA regulatory cascade controls the expression of the pdu and eut genes mediated by PocR or EutR, which are the positive AraC-like transcriptional regulators for the pdu and eut genes, respectively. By oppositely regulating the expression of genes for invasion and for luminal replication, the SirA-CsrB/CsrC-CsrA regulatory cascade could be involved in the generation of two Salmonella populations that cooperate for intestinal colonization and transmission. Our study provides new insight into the regulatory mechanisms that govern Salmonella virulence. IMPORTANCE The regulatory mechanisms that control the expression of virulence genes are essential for bacteria to infect hosts. Salmonella has developed diverse regulatory mechanisms to colonize the host gut. For instance, the SirA-CsrB/CsrC-CsrA regulatory cascade controls the expression of the SPI-1 genes, which are required for this bacterium to invade intestinal epithelium cells and for the induction of an intestinal inflammatory response. In this study, we determine the mechanisms by which the SirA-CsrB/CsrC-CsrA regulatory cascade controls the expression of the pdu and eut genes, which are necessary for the replication of Salmonella in the intestinal lumen. Thus, our data, together with the results of previous reports, indicate that the SirA-CsrB/CsrC-CsrA regulatory cascade has an important role in the intestinal colonization by Salmonella.

Anti-inflammatory effect of ApoE23 on Salmonella typhimurium-induced sepsis in mice

Two independent experiments were performed with three groups each (sepsis control, sepsis, and sepsis with apoE23 treatment) to investigate the anti-inflammatory effect of apolipoprotein 23 (apoE23) in a mouse model of sepsis induced by S. typhimurium. Survival rates; plasma level variations in tumor necrosis factor (TNF)-α, interleukin (IL)-6, and lipopolysaccharide (LPS); S. typhimurium colony-forming units in the spleen tissue; and mRNA and protein expression levels of low-density lipoprotein receptor (LDLR), LDLR-related protein (LRP), syndecan-1, and scavenger receptor B1 were evaluated in the livers of mice from the three groups. Results found that the survival rate of septic mice treated with apoE23 was 100% within 48 h, while it was only 40% in septic mice without apoE23 treatment (P < 0.001). The plasma LPS, TNF-α, and IL-6 levels and the S. typhimurium load in mice in the apoE23-treated group were significantly lower than those in septic mice (P < 0.05). Moreover, apoE23 restored the downregulated expression of LDLR and LRP in the liver tissue of septic mice. So apoE23 exhibits an anti-inflammatory effect in the mouse model of S. typhimurium-induced sepsis. Further studies are required to understand the mechanisms underlying the anti-inflammatory effects of apoE23.

Spatiotemporal Distribution of Salmonella enterica in European Hedgehogs in Northern Italy

Growing attention is being given to the European hedgehog (Erinaceus europaeus) because of its synanthropic behaviour and its potential role in harbouring parasites, viruses, fungi and bacteria and disseminating them to several animals and humans. Salmonella are the most frequently detected zoonotic bacteria that hedgehogs could transmit through contaminating water and food sources with faeces. This study aimed to determine the prevalence and distribution of Salmonella spp. in wild hedgehogs in the Emilia-Romagna region (northern Italy). From 2019 to 2022, 212 European hedgehogs that died naturally were tested for Salmonella spp. through culture isolation. Positive samples were subjected to serological typing. A total of 82 samples tested positive for Salmonella spp., with the overall Bayesian posterior estimated prevalence ranging from 35% (95% CI: 23-47%) to a maximum of 45% (95% CI: 31-59%) during the years considered and with an overall prevalence calculated at 39% (95% CI: 33-45%). Salmonella enterica Enteritidis and Veneziana were the most prevalent detected serovars in 65% and 17% of the positive samples, respectively. Since 2021, S. Typhimurium, S. Typhimurium Monofasica, S. Zaiman, S. Hessarek, S. Muenster, S. Isangi serovars, S. enterica subsp. Diarizonae and S. enterica subsp. Houtenae have been detected. These findings show a high prevalence of Salmonella spp. in tested hedgehogs, suggesting an important role of this animal species in the epidemiology of potentially zoonotic serovars circulating in the Emilia-Romagna region.

SPI-1 virulence gene expression modulates motility of Salmonella Typhimurium in a proton motive force- and adhesins-dependent manner

Both the bacterial flagellum and the evolutionary related injectisome encoded on the Salmonella pathogenicity island 1 (SPI-1) play crucial roles during the infection cycle of Salmonella species. The interplay of both is highlighted by the complex cross-regulation that includes transcriptional control of the flagellar master regulatory operon flhDC by HilD, the master regulator of SPI-1 gene expression. Contrary to the HilD-dependent activation of flagellar gene expression, we report here that activation of HilD resulted in a dramatic loss of motility, which was dependent on the presence of SPI-1. Single cell analyses revealed that HilD-activation triggers a SPI-1-dependent induction of the stringent response and a substantial decrease in proton motive force (PMF), while flagellation remains unaffected. We further found that HilD activation enhances the adhesion of Salmonella to epithelial cells. A transcriptome analysis revealed a simultaneous upregulation of several adhesin systems, which, when overproduced, phenocopied the HilD-induced motility defect. We propose a model where the SPI-1-dependent depletion of the PMF and the upregulation of adhesins upon HilD-activation enable flagellated Salmonella to rapidly modulate their motility during infection, thereby enabling efficient adhesion to host cells and delivery of effector proteins.

Newly Isolated Virulent Salmophages for Biocontrol of Multidrug-Resistant Salmonella in Ready-to-Eat Plant-Based Food

Due to irrational antibiotic stewardship, an increase in the incidence of multidrug resistance of bacteria has been observed recently. Therefore, the search for new therapeutic methods for pathogen infection treatment seems to be necessary. One of the possibilities is the utilization of bacteriophages (phages)—the natural enemies of bacteria. Thus, this study is aimed at the genomic and functional characterization of two newly isolated phages targeting MDR Salmonella enterica strains and their efficacy in salmonellosis biocontrol in raw carrot–apple juice. The Salmonella phage vB_Sen-IAFB3829 (Salmonella phage strain KKP 3829) and Salmonella phage vB_Sen-IAFB3830 (Salmonella phage strain KKP 3830) were isolated against S. I (6,8:l,-:1,7) strain KKP 1762 and S. Typhimurium strain KKP 3080 host strains, respectively. Based on the transmission electron microscopy (TEM) and whole-genome sequencing (WGS) analyses, the viruses were identified as members of tailed bacteriophages from the Caudoviricetes class. Genome sequencing revealed that these phages have linear double-stranded DNA and sizes of 58,992 bp (vB_Sen-IAFB3829) and 50,514 bp (vB_Sen-IAFB3830). Phages retained their activity in a wide range of temperatures (from −20 °C to 60 °C) and active acidity values (pH from 3 to 11). The exposure of phages to UV radiation significantly decreased their activity in proportion to the exposure time. The application of phages to the food matrices significantly reduced the level of Salmonella contamination compared to the control. Genome analysis showed that both phages do not encode virulence or toxin genes and can be classified as virulent bacteriophages. Virulent characteristics and no possible pathogen factors make examined phages feasible to be potential candidates for food biocontrol.

Serotypes, Antimicrobial Resistance Profiles, and Virulence Factors of Salmonella Isolates in Chinese Edible Frogs (Hoplobatrachus rugulosus) Collected from Wet Markets in Hong Kong

Salmonella is an important agent of gastrointestinal disease in humans. While livestock, such as cattle, poultry, and pigs, are well-recognised animal reservoirs of Salmonella, there is a lack of data on Salmonella in edible frogs, even though frog meat is a popular food worldwide. In this study, 103 live edible Chinese frogs (Hoplobatrachus rugulosus) were collected from wet markets throughout Hong Kong. After euthanasia, faeces or cloacal swabs were examined for Salmonella. Overall, Salmonella spp. were isolated from 67 (65%, CI: 0.554-0.736) of the samples. The serotypes included S. Saintpaul (33%), S. Newport (24%), S. Bareilly (7%), S. Braenderup (4%), S. Hvittingfoss (4%), S. Stanley (10%), and S. Wandsworth (16%). Many isolates were phylogenetically related. A high number of genes encoding for resistance to clinically relevant antimicrobials, and a high number of virulence determinants, were identified. Antimicrobial susceptibility testing (AST) identified multidrug resistance (MDR) in 21% of the isolates. Resistance to ampicillin, ciprofloxacin, nalidixic acid, and tetracycline was common. These results demonstrate that a high percentage of live frogs sold for human consumption in wet markets are carriers of multidrug-resistant Salmonella. Public health recommendations for handling edible frogs should be considered, to mitigate the risk of Salmonella transmission to humans.

Human Salmonellosis: A Continuous Global Threat in the Farm-to-Fork Food Safety Continuum

Salmonella is one of the most common zoonotic foodborne pathogens and a worldwide public health threat. Salmonella enterica is the most pathogenic among Salmonella species, comprising over 2500 serovars. It causes typhoid fever and gastroenteritis, and the serovars responsible for the later disease are known as non-typhoidal Salmonella (NTS). Salmonella transmission to humans happens along the farm-to-fork continuum via contaminated animal- and plant-derived foods, including poultry, eggs, fish, pork, beef, vegetables, fruits, nuts, and flour. Several virulence factors have been recognized to play a vital role in attaching, invading, and evading the host defense system. These factors include capsule, adhesion proteins, flagella, plasmids, and type III secretion systems that are encoded on the Salmonella pathogenicity islands. The increased global prevalence of NTS serovars in recent years indicates that the control approaches centered on alleviating the food animals' contamination along the food chain have been unsuccessful. Moreover, the emergence of antibiotic-resistant Salmonella variants suggests a potential food safety crisis. This review summarizes the current state of the knowledge on the nomenclature, microbiological features, virulence factors, and the mechanism of antimicrobial resistance of Salmonella. Furthermore, it provides insights into the pathogenesis and epidemiology of Salmonella infections. The recent outbreaks of salmonellosis reported in different clinical settings and geographical regions, including Africa, the Middle East and North Africa, Latin America, Europe, and the USA in the farm-to-fork continuum, are also highlighted.

Biomedical applications of aptamer-modified chitosan nanomaterials: An updated review

Among polysaccharides of environmental and economic interest, chitosan (CS) is receiving much attention, particularly in the food and biotechnology industries to encapsulate active food ingredients and immobilize enzymes. CS nanoparticles (CS NPs) combine the intrinsic beneficial properties of both natural polymers and nanoscale particles such as quantum size effect, biocompatibility, biodegradability, and ease of modification, and have great potential for bioimaging, drug delivery, and biosensing applications. Aptamers are single-stranded oligonucleotides that can fold into predetermined structures and bind to the corresponding biomolecules. They are mainly used as targeting ligands in biosensors, disease diagnostic kits and treatment strategies. They can deliver contrast agents and drugs into cancer cells and tissues, control microorganism growth and precisely target pathogens. Aptamer-conjugated CS NPs can significantly improve the efficacy of conventional therapies, minimize their side effects on normal tissues, and overcome the enhanced permeability retention (EPR) effect. Further, aptamer-conjugated carbohydrate-based nanobiopolymers have shown excellent antibacterial and antiviral properties and can be used to develop novel biosensors for the efficient detection of antibiotics, toxins, and other biomolecules. This updated review aims to provide a comprehensive overview of the bioapplications of aptamer-conjugated CS NPs used as innovative diagnostic and therapeutic platforms, their limitations, and potential future directions.

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.

Molecular Detection of Virulence Factors in Salmonella serovars Isolated from Poultry and Human Samples

Salmonellosis is a common infectious disease in humans caused by Salmonella spp., which in recent years has shown an increase in its incidence, with products of avian origin being a common source of transmission. To present a successful infective cycle, there are molecular mechanisms such as virulence factors that provide characteristics that facilitate survival, colonization, and damage to the host. According to this, the study aims to characterize the virulence factors of Salmonella spp. strains isolated from broilers (n = 39) and humans (n = 10). The presence of 24 virulence genes was evaluated using end-point PCR. All the strains of Salmonella spp. isolated from broiler chickens revealed presence of 7/24 (29, 16%) virulence genes (lpfA, csgA, sitC, sipB, sopB, sopE, and sivH). Regarding the strains isolated from cases of gastroenteritis in humans, all strains contained (14/24, 58, 33%) virulence genes (lpfA, csgA, pagC, msgA, spiA, sitC, iroN, sipB, orgA, hilA, sopB, sifA, avrA, and sivH). In summary, the presence of virulence genes in different strains of Salmonella isolated from broilers and humans could be described as bacteria with potential pathogenicity due to the type and number of virulence genes detected. These findings are beneficial for the pathogenic monitoring of Salmonella in Colombia.

5'-Nucleotidase is dispensable for the growth of Salmonella Typhimurium but inhibits the bactericidal activity of macrophage extracellular traps

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a zoonotic pathogen that causes severe gastroenteritis. The 5'-nucleotidases of pathogens can dephosphorylate adenosine phosphates, boost adenosine levels and suppress the pro-inflammatory immune response. In our previous study, an extracellular nuclease, 5'-nucleotidase, was identified in the extracellular proteins of S. Typhimurium. However, the nuclease activity and the function of the 5'-nucleotidase of S. Typhimurium have not been explored. In the present study, deletion of the 5'-nucleotidase gene is dispensable for S. Typhimurium growth, even under environmental stress. Fluorescence microscopy revealed that the 5'-nucleotidase mutant induced more macrophage extracellular traps (METs) than the wild type did. Furthermore, recombinant 5'-nucleotidase protein (r5Nuc) could degrade λDNA, and the nuclease activity of r5Nuc was optimum at 37 °C and pH 6.0-7.0. The Mg²⁺ enhanced the nuclease activity of r5Nuc, whereas Zn²⁺ inhibited it. Meanwhile, deletion of the 5'-nucleotidase gene increased the bactericidal activity of METs, and r5Nuc could degrade METs and inhibit the bactericidal activity of METs. In conclusion, S. Typhimurium growth was independent of 5'-nucleotidase, but the nuclease activity of 5'-nucleotidase assisted S. Typhimurium to evade macrophage-mediated extracellular killing through degrading METs.

Bacterial Pathogens in the Food Industry: Antibiotic Resistance and Virulence Factors of Salmonella enterica Strains Isolated from Food Chain Links

Salmonella is one of the most important foodborne pathogens. Fifty-three strains of Salmonella deposited in the Culture Collection of Industrial Microorganisms-Microbiological Resources Center (IAFB) were identified using molecular and proteomic analyses. Moreover, the genetic similarity of the tested strains was determined using the PFGE method. Main virulence genes were identified, and phenotypical antibiotic susceptibility profiles and prevalence of resistance genes were analyzed. Subsequently, the occurrence of the main mechanisms of β-lactam resistance was determined. Virulence genes, invA, fimA, and stn were identified in all tested strains. Phenotypic tests, including 28 antibiotics, showed that 50.9% of the strains were MDR. The tet genes associated with tetracyclines resistance were the most frequently identified genes. Concerning the genes associated with ESBL-producing Salmonella, no resistance to the TEM and CTX-M type was identified, and only two strains (KKP 1597 and KKP 1610) showed resistance to SHV. No strains exhibited AmpC-type resistance but for six Salmonella strains, the efflux-related resistance of PSE-1 was presented. The high number of resistant strains in combination with multiple ARGs in Salmonella indicates the possible overuse of antibiotics. Our results showed that it is necessary to monitor antimicrobial resistance profiles in all food chain links constantly and to implement a policy of proper antibiotic stewardship to contain or at least significantly limit the further acquisition of antibiotic resistance among Salmonella strains.

Genomic virulence genes profile analysis of Salmonella enterica isolates from animal and human in China from 2004 to 2019

Salmonella is a momentously zoonotic and food-borne pathogen that seriously threats human and animal health around the world. Salmonella pathogenicity is closely related to its virulence genes profile. However, conventional virulence gene analysis methods cannot truly reveal whole virulence genes carried by Salmonella. In this study, whole genome sequencing in combination with Virulence Factor Database were applied to investigate whole virulence gene profiles of 243 Salmonella isolates from animals and humans in China from 2004 to 2019. The results showed that a total of 670 virulence genes were identified in Salmonella, among them, 319 virulence genes were found in all the Salmonella tested isolates, and 9 virulence genes were unique to Salmonella. The 670 virulence genes were classified into 14 categories according to their functions, and the genes related to adherence, effector delivery system, immune modulation, motility and nutritional/metabolic factors accounted for 84.63%. Relationships between virulence genes and serovars, sequence types indicated that strains belonged to the same serovar or sequence type had similar virulence genes profiles, however, isolates from different sources, years and locations of isolation had variable virulence gene profiles. In addition, copy number of virulence genes and homologous virulence genes shared with other pathogens were also analyzed in this study. In summary, we investigated pan-genomic virulence gene profiles and molecular epidemiology of Salmonella isolates from humans and animals in China from 2004 to 2019. These findings are beneficial for pathogenic monitoring, investigation of virulence evolution as well as prevention and control of Salmonella.

Curcumin Stimulates the Overexpression of Virulence Factors in Salmonella enterica Serovar Typhimurium: In Vitro and Animal Model Studies

Salmonella spp. is one of the most common food poisoning pathogens and the main cause of diarrheal diseases in humans in developing countries. The increased Salmonella resistance to antimicrobials has led to the search for new alternatives, including natural compounds such as curcumin, which has already demonstrated a bactericidal effect; however, in Gram-negatives, there is much controversy about this effect, as it is highly variable. In this study, we aimed to verify the antibacterial activity of curcumin against the Salmonella enterica serovar Typhimurium growth rate, virulence, and pathogenicity. The strain was exposed to 110, 220 or 330 µg/mL curcumin, and by complementary methods (spectrophotometric, pour plate and MTT assays), we determined its antibacterial activity. To elucidate whether curcumin regulates the expression of virulence genes, Salmonella invA, fliC and siiE genes were investigated by quantitative real-time reverse transcription (qRT-PCR). Furthermore, to explore the effect of curcumin on the pathogenesis process in vivo, a Caenorhabditis elegans infection model was employed. No antibacterial activity was observed, even at higher concentrations of curcumin. All concentrations of curcumin caused overgrowth (35−69%) and increased the pathogenicity of the bacterial strain through the overexpression of virulence factors. The latter coincided with a significant reduction in both the lifespan and survival time of C. elegans when fed with curcumin-treated bacteria. Our data provide relevant information that may support the selective antibacterial effects of curcumin to reconsider the indiscriminate use of this phytochemical, especially in outbreaks of pathogenic Gram-negative bacteria.

Regulation of Host-Pathogen Interactions via the Ubiquitin System

Ubiquitination is a posttranslational modification that regulates a multitude of cellular functions. Pathogens, such as bacteria and viruses, have evolved sophisticated mechanisms that evade or counteract ubiquitin-dependent host responses, or even exploit the ubiquitin system to their own advantage. This is largely done by numerous pathogen virulence factors that encode E3 ligases and deubiquitinases, which are often used as weapons in pathogen-host cell interactions. Moreover, upon pathogen attack, host cellular signaling networks undergo major ubiquitin-dependent changes to protect the host cell, including coordination of innate immunity, remodeling of cellular organelles, reorganization of the cytoskeleton, and reprogramming of metabolic pathways to restrict growth of the pathogen. Here we provide mechanistic insights into ubiquitin regulation of host-pathogen interactions and how it affects bacterial and viral pathogenesis and the organization and response of the host cell.

Functions of Small Non-Coding RNAs in Salmonella–Host Interactions

Simple Summary

In the process of infecting the host, Salmonella senses and adapts to the environment within the host, breaks through the host’s defense system, and survives and multiplies in the host cell. As a class of universal regulators encoded in intergenic space, an increasing number of small non-coding RNAs (sRNAs) have been found to be involved in a series of processes during Salmonella infection, and they play an important role in interactions with the host cell. In this review, we discuss how sRNAs help Salmonella resist acidic environmental stress by regulating acid resistance genes and modulate adhesion and invasion to non-phagocytic cells by regulating virulent genes such as fimbrial subunits and outer membrane proteins. In addition, sRNAs help Salmonella adapt to oxidative stress within host cells and promote survival within macrophages. Although the function of a variety of sRNAs has been studied during host–Salmonella interactions, many of sRNAs’ functions remain to be discovered.

Abstract

Salmonella species infect hosts by entering phagocytic and non-phagocytic cells, causing diverse disease symptoms, such as fever, gastroenteritis, and even death. Therefore, Salmonella has attracted much attention. Many factors are involved in pathogenesis, for example, the capsule, enterotoxins, Salmonella pathogenicity islands (SPIs), and corresponding regulators. These factors are all traditional proteins associated with virulence and regulation. Recently, small non-coding RNAs (sRNAs) have also been reported to function as critical regulators. Salmonella has become a model organism for studying sRNAs. sRNAs regulate gene expression by imperfect base-pairing with targets at the post-transcriptional level. sRNAs are involved in diverse biological processes, such as virulence, substance metabolism, and adaptation to stress environments. Although some studies have reported the crucial roles of sRNAs in regulating host–pathogen interactions, the function of sRNAs in host–Salmonella interactions has rarely been reviewed. Here, we review the functions of sRNAs during the infection of host cells by Salmonella, aiming to deepen our understanding of sRNA functions and the pathogenic mechanism of Salmonella.

Lipopolysaccharides and outer membrane proteins as main structures involved in complement evasion strategies of non-typhoidal Salmonella strains

Non-typhoidal Salmonella (NTS) infections pose a serious public health problem. In addition to the typical course of salmonellosis, an infection with Salmonella bacteria can often lead to parenteral infections and sepsis, which are particularly dangerous for children, the elderly and immunocompromised. Bacterial resistance to serum is a key virulence factor for the development of systemic infections. Salmonella, as an enterobacterial pathogen, has developed several mechanisms to escape and block the antibacterial effects of the complement system. In this review, we discuss the relevance of outer membrane polysaccharides to the complement evasion mechanisms of NTS strains. These include the influence of the overall length and density of the lipopolysaccharide molecules, modifications of the O-antigen lipopolysaccharide composition and the role of capsular polysaccharides in opsonization and protection of the outer membrane from the lytic action of complement. Additionally, we discuss specific outer membrane protein complement evasion mechanisms, such as recruitment of complement regulatory proteins, blocking assembly of late complement components to form the membrane attack complex and the proteolytic cleavage of complement proteins.

Molecular Epidemiology and Virulence of Non-Typhoidal Salmonella in Armenia

In this work, we analysed human isolates of nontyphoidal Salmonella enterica subsp. enterica (NTS), which were collected from salmonellosis cases in Armenia from 1996 to 2019. This disease became a leading food-borne bacterial infection in the region, with the younger age groups especially affected. The isolates were characterised by serotyping, Enterobacterial Repetitive Intergenic Consensus (ERIC-PCR) typing, and whole genome sequencing (WGS). The main serotypes were S. Typhimurium, S. Enteritidis, and S. Arizonae. ERIC-PCR indicated a high degree of clonality among S. Typhimurium strains, which were also multidrug-resistant and produced extended spectrum beta-lactamases. During the study period, the frequency of S. Typhimurium and S. Arizonae isolations decreased, but with the increase in S. Enteritidis and other NTS. A total of 42 NTS isolates were subjected to WGS and explored for virulence-related traits and the corresponding genetic elements. Some virulence and genetic factors were shared by all NTS serotypes, while the main differences were attributed to the serotype-specific diversity of virulence genes, SPIs, virulence plasmids, and phages. The results indicated the variability and dynamics in the epidemiology of salmonellosis and a high virulence potential of human NTS isolates circulating in the region.

A SARS-CoV-2 oral vaccine development strategy based on the attenuated Salmonella type III secretion system

Aims

This study aimed to provide a safe, stable and efficient SARS‐CoV‐2 oral vaccine development strategy based on the type III secretion system of attenuated Salmonella and a reference for the development of a SARS‐CoV‐2 vaccine.

Methods and Results

The attenuated Salmonella mutant ΔhtrA‐VNP was used as a vector to secrete the antigen SARS‐CoV‐2 based on the type III secretion system (T3SS). The Salmonella pathogenicity island 2 (SPI‐2)‐encoded T3SS promoter (sifB) was screened to express heterologous antigens (RBD, NTD, S2), and the SPI‐2‐encoded secretion system (sseJ) was employed to secrete this molecule (psifB‐sseJ‐antigen, abbreviated BJ‐antigen). Both immunoblotting and fluorescence microscopy revealed effective expression and secretion of the antigen into the cytosol of macrophages in vitro. The mixture of the three strains (BJ‐RBD/NTD/S2, named AisVax) elicited a marked increase in the induction of IgA or IgG S‐protein Abs after oral gavage, intraperitoneal and subcutaneous administration. Flow cytometric analysis proved that AisVax caused T‐cell activation, as shown by a significant increase in CD44 and CD69 expression. Significant production of IgA or IgG N‐protein Abs was also detected by using psifB‐sseJ‐N(FL), indicating the universality of this strategy.

Conclusions

Delivery of multiple SARS‐CoV‐2 antigens using the type III secretion system of attenuated Salmonella ΔhtrA‐VNP is a potential COVID‐19 vaccine strategy.

Significance and Impact of the Study

The attenuated Salmonella strain ΔhtrA‐VNP showed excellent performance as a vaccine vector. The Salmonella SPI‐2‐encoded T3SS showed highly efficient delivery of SARS‐COV‐2 antigens. Anti‐loss elements integrated into the plasmid stabilized the phenotype of the vaccine strain. Mixed administration of antigen‐expressing strains improved antibody induction.

First Report on Abortion Caused by Salmonella enterica subsp. enterica Serovar Enteritidis in Water Buffalo (Bubalus bubalis)

Salmonella enterica subsp. enterica Serovar Enteritidis is one of the major pathogens associated with enteric diseases in animals and humans. Thus, due to the importance of Salmonella spp. infections for animal production and public health, the aim of the present study was to describe the first detection of S. enteritidis in an aborted water buffalo fetus in southern Italy by characterizing the phylogroup profile and the antimicrobial susceptibility of the isolated pathogenic strains. The different clinical manifestations of salmonellosis in animals include diarrhea, abortion, pneumonia, septic arthritis, meningitis, and others, depending on the virulence of the serovars, infectious dose, and host immunity. This study reports the first case of abortion caused by Salmonella enterica subsp enterica serovar Enteritidis in water buffalo (Bubalus bubalis) in the Campania region, southern Italy. Complete necropsy was performed on the aborted water buffalo fetus under study, and samples and swabs from different organs were collected. Samples were processed by microbiological and molecular analyses to detect bacterial, viral, and protozoarian pathogens possibly responsible for abortion. Whole genome sequencing (WGS) was carried out to further characterize the isolated S. Enteritidis strain. Our findings highlight the crucial role of S. Enteritidis as a potential abortive agent in water buffalo and its presence should therefore be investigated in cases of bubaline abortion.

Flagellotropic Bacteriophages: Opportunities and Challenges for Antimicrobial Applications

Bacteriophages (phages) are the most abundant biological entities in the biosphere. As viruses that solely infect bacteria, phages have myriad healthcare and agricultural applications including phage therapy and antibacterial treatments in the foodservice industry. Phage therapy has been explored since the turn of the twentieth century but was no longer prioritized following the invention of antibiotics. As we approach a post-antibiotic society, phage therapy research has experienced a significant resurgence for the use of phages against antibiotic-resistant bacteria, a growing concern in modern medicine. Phages are extraordinarily diverse, as are their host receptor targets. Flagellotropic (flagellum-dependent) phages begin their infection cycle by attaching to the flagellum of their motile host, although the later stages of the infection process of most of these phages remain elusive. Flagella are helical appendages required for swimming and swarming motility and are also of great importance for virulence in many pathogenic bacteria of clinical relevance. Not only is bacterial motility itself frequently important for virulence, as it allows pathogenic bacteria to move toward their host and find nutrients more effectively, but flagella can also serve additional functions including mediating bacterial adhesion to surfaces. Flagella are also a potent antigen recognized by the human immune system. Phages utilizing the flagellum for infections are of particular interest due to the unique evolutionary tradeoff they force upon their hosts: by downregulating or abolishing motility to escape infection by a flagellotropic phage, a pathogenic bacterium would also likely attenuate its virulence. This factor may lead to flagellotropic phages becoming especially potent antibacterial agents. This review outlines past, present, and future research of flagellotropic phages, including their molecular mechanisms of infection and potential future applications.

Comparative Genomic Analysis of Salmonella enterica Serovar Typhimurium Isolates from Passerines Reveals Two Lineages Circulating in Europe, New Zealand, and the United States

Salmonella enterica serovar Typhimurium strains from passerines have caused wild bird deaths and human salmonellosis outbreaks in Europe, Oceania, and North America. Here, we performed comparative genomic analysis to explore the emergence, genetic relationship, and evolution of geographically dispersed passerine isolates. We found that passerine isolates from Europe and the United States clustered to form two lineages (EU and US passerine lineages), which were distinct from major S. Typhimurium lineages circulating in other diverse hosts (e.g., humans, cattle, pigs, chickens, and other avian hosts, such as pigeons and ducks). Further, passerine isolates from New Zealand clustered to form a sublineage (NZ passerine lineage) of the US passerine lineage. We inferred that the passerine isolates mutated at a rate of 3.2 × 10^-7 substitutions/site/year, and the US, EU, and NZ passerine lineages emerged in approximately 1952, 1970, and 1996, respectively. Isolates from the three lineages presented genetic similarity, such as lack of antimicrobial resistance genes and accumulation of the same virulence pseudogenes. In addition, genetic diversity due to microevolution existed in the three passerine lineages. Specifically, pseudogenization in the type 1 fimbrial gene fimC (deletion of G at position 87) was detected only in the US and NZ passerine isolates, while single-base deletions in type 3 secretion system effector genes (i.e., gogB, sseJ, and sseK2) cooccurred solely in the EU passerine isolates. These findings provide insights into the evolution, host adaptation, and epidemiology of S. Typhimurium in passerines. IMPORTANCE Passerine-associated S. Typhimurium strains have been linked to human salmonellosis outbreaks in recent years. Here, we investigated the phylogenetic relationship of globally distributed passerine isolates and profiled their genomic similarity and diversity. Our study reveals two passerine-associated S. Typhimurium lineages circulating in Europe, Oceania, and North America. Isolates from the two lineages presented phylogenetic and genetic signatures that were distinct from those of isolates from other hosts. The findings shed light on the host adaptation of S. Typhimurium in passerines and are important for source attribution of S. Typhimurium strains to avian hosts. Further, we found that S. Typhimurium definitive phage type 160 (DT160) from passerines, which caused decades-long human salmonellosis outbreaks in New Zealand and Australia, formed a sublineage of the US passerine lineage, suggesting that DT160 might have originated from passerines outside Oceania. Our study demonstrates the importance of whole-genome sequencing and genomic analysis of historical microbial collections to modern epidemiologic surveillance.

Nucleic Acid-Based Nanobiosensor (NAB) Used for Salmonella Detection in Foods: A Systematic Review

Salmonella bacteria is a foodborne pathogen found mainly in food products causing severe symptoms in the individual, such as diarrhea, fever, and abdominal cramps after consuming the infected food, which can be fatal in some severe cases. Rapid and selective methods to detect Salmonella bacteria can prevent outbreaks when ingesting contaminated food. Nanobiosensors are a highly sensitive, simple, faster, and lower cost method for the rapid detection of Salmonella, an alternative to conventional enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) techniques. This study systematically searched and analyzed literature data related to nucleic acid-based nanobiosensors (NABs) with nanomaterials to detect Salmonella in food, retrieved from three databases, published between 2010 and 2021. We extracted data and critically analyzed the effect of nanomaterial functionalized with aptamer or DNA at the limit of detection (LOD). Among the nanomaterials, gold nanoparticles (AuNPs) were the most used nanomaterial in studies due to their unique optical properties of the metal, followed by magnetic nanoparticles (MNPs) of Fe₃O₄, copper nanoparticles (CuNPs), and also hybrid nanomaterials multiwalled carbon nanotubes (c-MWCNT/AuNP), QD/UCNP-MB (quantum dotes upconverting nanoparticle of magnetic beads), and cadmium telluride quantum dots (CdTe QDs@MNPs) showed excellent LOD values. The transducers used for detection also varied from electrochemical, fluorescent, surface-enhanced Raman spectroscopy (SERS), RAMAN spectroscopy, and mainly colorimetric due to the possibility of visualizing the detection result with the naked eye. Furthermore, we show the magnetic separation system capable of detecting the target amplification of the genetic material. Finally, we present perspectives, future research, and opportunities to use point-of-care (POC) diagnostic devices as a faster and lower cost approach for detecting Salmonella in food as they prove to be viable for resource-constrained environments such as field-based or economically limited conditions.

Evidence for common ancestry and microevolution of passerine-adapted Salmonella enterica serovar Typhimurium in the UK and USA

The evolution of Salmonella enterica serovar Typhimurium (S. Typhimurium) within passerines has resulted in pathoadaptation of this serovar to the avian host in Europe. Recently, we identified an S. Typhimurium lineage from passerines in North America. The emergence of passerine-adapted S. Typhimurium in Europe and North America raises questions regarding its evolutionary origin. Here, we demonstrated that the UK and US passerine-adapted S. Typhimurium shared a common ancestor from ca. 1838, and larids played a key role in the clonal expansion by disseminating the common ancestor between North America and Europe. Further, we identified virulence gene signatures common in the passerine- and larid-adapted S. Typhimurium, including conserved pseudogenes in fimbrial gene lpfD and Type 3 Secretion System (T3SS) effector gene steC. However, the UK and US passerine-adapted S. Typhimurium also possessed unique virulence gene signatures (i.e. pseudogenes in fimbrial gene fimC and T3SS effector genes sspH2, gogB, sseJ and sseK2), and the majority of them (38/47) lost a virulence plasmid pSLT that was present in the larid-adapted S. Typhimurium. These results provide evidence that passerine-adapted S. Typhimurium share a common ancestor with those from larids, and the divergence of passerine- and larid-adapted S. Typhimurium might be due to pseudogenization or loss of specific virulence genes.

Salmonella enterica serovar Typhimurium from Wild Birds in the United States Represent Distinct Lineages Defined by Bird Type

Salmonella enterica serovar Typhimurium is typically considered a host generalist; however, certain isolates are associated with specific hosts and show genetic features of host adaptation. Here, we sequenced 131 S. Typhimurium isolates from wild birds collected in 30 U.S. states during 1978–2019. We found that isolates from broad taxonomic host groups including passerine birds, water birds (Aequornithes), and larids (gulls and terns) represented three distinct lineages and certain S. Typhimurium CRISPR types presented in individual lineages. We also showed that lineages formed by wild bird isolates differed from most isolates originating from domestic animal sources, and that genomes from these lineages substantially improved source attribution of Typhimurium genomes to wild birds by a machine learning classifier. Furthermore, virulence gene signatures that differentiated S. Typhimurium from passerines, water birds, and larids were detected. Passerine isolates tended to lack S. Typhimurium-specific virulence plasmids. Isolates from the passerine, water bird, and larid lineages had close genetic relatedness with human clinical isolates, including those from a 2021 U.S. outbreak linked to passerine birds. These observations indicate that S. Typhimurium from wild birds in the United States are likely host-adapted, and the representative genomic data set examined in this study can improve source prediction and facilitate outbreak investigation.

IMPORTANCE Within-host evolution of S. Typhimurium may lead to pathovars adapted to specific hosts. Here, we report the emergence of disparate avian S. Typhimurium lineages with distinct virulence gene signatures. The findings highlight the importance of wild birds as a reservoir for S. Typhimurium and contribute to our understanding of the genetic diversity of S. Typhimurium from wild birds. Our study indicates that S. Typhimurium may have undergone adaptive evolution within wild birds in the United States. The representative S. Typhimurium genomes from wild birds, together with the virulence gene signatures identified in these bird isolates, are valuable for S. Typhimurium source attribution and epidemiological surveillance.

Hibiscus Acid from Hibiscus sabdariffa L. Inhibits Flagellar Motility and Cell Invasion in Salmonella enterica

Extracts of Hibiscus sabdariffa L. (commonly called Rosselle or "Jamaica flower" in Mexico) have been shown to have antibiotic and antivirulence properties in several bacteria. Here, an organic extract of H. sabdariffa L. is shown to inhibit motility in Salmonella enterica serovars Typhi and Typhimurium. The compound responsible for this effect was purified and found to be the hibiscus acid. When tested, this compound also inhibited motility and reduced the secretion of both flagellin and type III secretion effectors. Purified hibiscus acid was not toxic in tissue-cultured eukaryotic cells, and it was able to reduce the invasion of Salmonella Typhimurium in epithelial cells. Initial steps to understand its mode of action showed it might affect membrane proton balance.

A Novel Dibenzoxazepine Attenuates Intracellular Salmonella Typhimurium Oxidative Stress Resistance

Salmonella enterica serovar Typhimurium is the leading cause of invasive nontyphoidal salmonellosis. Additionally, the emergence of multidrug-resistant S. Typhimurium has further increased the difficulty of controlling its infection. Previously, we showed that an antipsychotic drug, loxapine, suppressed intracellular Salmonella in macrophages. To exploit loxapine's antibacterial activity, we simultaneously evaluated the anti-intracellular Salmonella activity and cytotoxicity of newly synthesized loxapine derivatives using an image-based high-content assay. We identified that SW14 exhibits potent suppressive effects on intramacrophagic S. Typhimurium with an 50% effective concentration (EC₅₀) of 0.5 μM. SW14 also sensitized intracellular Salmonella to ciprofloxacin and cefixime and effectively controlled intracellular multidrug- and fluoroquinolone-resistant S. Typhimurium strains. However, SW14 did not affect bacterial growth in standard microbiological broth or minimal medium that mimics the phagosomal environment. Cellular autophagy blockade by 3-methyladenine (3-MA) or shATG7 elevated the susceptibility of intracellular Salmonella to SW14. Finally, reactive oxygen species (ROS) scavengers reduced the antibacterial efficacy of SW14, but the ROS levels in SW14-treated macrophages were not elevated. SW14 decreased the resistance of outer membrane-compromised S. Typhimurium to H₂O₂. Collectively, our data indicated that the structure of loxapine can be further optimized to develop new antibacterial agents by targeting bacterial resistance to host oxidative-stress defense. IMPORTANCE The incidence of diseases caused by pathogenic bacteria with resistance to common antibiotics is consistently increasing. In addition, Gram-negative bacteria are particularly difficult to treat with antibiotics, especially those that can invade and proliferate intracellularly. In order to find a new antibacterial compound against intracellular Salmonella, we established a cell-based high-content assay and identified SW14 from the derivatives of the antipsychotic drug loxapine. Our data indicate that SW14 has no effect on free bacteria in the medium but can suppress the intracellular proliferation of multidrug-resistant (MDR) S. Typhimurium in macrophages. We also found that SW14 can suppress the resistance of outer membrane compromised Salmonella to H₂O₂, and its anti-intracellular Salmonella activity can be reversed by reactive oxygen species (ROS) scavengers. Together, the findings suggest that SW14 might act via a virulence-targeted mechanism and that its structure has the potential to be further developed as a new therapeutic against MDR Salmonella.

Detection of invA virulence gene of multidrug-resistant Salmonella species isolated from the cloacal swab of broiler chickens in Blitar district, East Java, Indonesia

Background and Aim:

The increasing number of multidrug-resistant (MDR) Salmonella species on poultry farms in Indonesia has caused concern regarding human health. This study was conducted to determine the presence of the virulence gene invA in MDR Salmonella species isolated from the cloacal swab of broiler chickens in Blitar district, East Java Province, Indonesia.

Materials and Methods:

Cloacal swab samples were collected by purposive sampling from 15 farms in four districts. Isolation and identification of bacteria were performed using standard microbiological techniques. Confirmation of MDR isolates was done using five different classes of antibiotics, including the beta-lactam, aminoglycoside, fluoroquinolone, phenicol, and monobactam groups. An antibiotic susceptibility test was conducted using the Kirby–Bauer disk diffusion method, and a polymerase chain reaction method was used to screen for the presence of invA.

Results:

It was observed that 32.26% (50/155) of the samples were positive for Salmonella species. Of these 50 Salmonella isolates, 7 (14%) were identified as MDR strains. An important finding was the detection of invA in all the seven MDR Salmonella strains (100%) isolated from the cloacal swab of broiler chickens in Blitar district, East Java Province.

Conclusion:

Veterinarians have an extremely important role in monitoring the use of antibiotics in farm animals to mitigate the rapid spread of MDR organisms in our environment, which can otherwise cause serious economic losses and also public health issues.

Eugenol exposure in vitro inhibits the expressions of T3SS and TIF virulence genes in Salmonella Typhimurium and reduces its pathogenicity to chickens

BACKGROUND

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a common food-borne pathogen, which has the ability to infect a wide range of hosts. The increasing emergence of drug-resistant strains urgently requires new alternative therapies. Eugenol has been shown to be very effective against drug-resistant strains of Gram-negative and Gram-positive bacteria. The purpose of this study is to explore the effects of eugenol on the virulence factors and pathogenicity of S. Typhimurium.

METHODS

The antibacterial activity of eugenol was investigated via the changes of cell morphology, fimbriae related-genes and virulence factors of S. Typhimurium, then the pathogenicity of S. Typhimurium pretreated by eugenol to chickens was evaluated.

RESULTS

Susceptibility testing showed that eugenol possessed significant antimicrobial activity. Scanning electron microscope analysis showed eugenol treatment deformed the morphology with damaged fimbriae structure of S. Typhimurium. Real time PCR assay confirmed eugenol significantly down-regulated the expressions of virulence factors (hilA, hilD, sipA, sipC, spiC, misL) of Type III secretion system (T3SS) and adherence genes (fimA, fimH, fimD, fimY, fimZ, stm0551) of Type I fimbriae (TIF). Animal experiment proved that the pathogenicity of S. Typhimurium exposed by eugenol was reduced, which was evidenced by the higher survival rate, weight gains and organs indexes, the lower bacterial loads in organs. Meanwhile, the duodenal histopathological changes were mitigated, with a significantly decline in the expressions of TNF-α, IL-6 and IL-18.

CONCLUSION

In summary, eugenol pretreatment may alleviate the pathogenicity of the S. Typhimurium to chickens via wrecking the fimbriae and inhibiting the mRNA expressions of virulence factors and adhesion molecules. These data dedicated the potential mechanisms of eugenol against S. Typhimurium in vitro.

The Distribution of Campylobacter jejuni Virulence Genes in Genomes Worldwide Derived from the NCBI Pathogen Detection Database

Campylobacter jejuni (C. jejuni) is responsible for 80% of human campylobacteriosis and is the leading cause of gastroenteritis globally. The relevant public health risks of C. jejuni are caused by particular virulence genes encompassing its virulome. We analyzed 40,371 publicly available genomes of C. jejuni deposited in the NCBI Pathogen Detection Database, combining their epidemiologic metadata with an in silico bioinformatics analysis to increase our current comprehension of their virulome from a global perspective. The collection presented a virulome composed of 126 identified virulence factors that were grouped in three clusters representing the accessory, the softcore, and the essential core genes according to their prevalence within the genomes. The multilocus sequence type distribution in the genomes was also investigated. An unexpected low prevalence of the full-length flagellin flaA and flaB locus of C. jejuni genomes was revealed, and an essential core virulence gene repertoire prevalent in more than 99.99% of genomes was identified. Altogether, this is a pioneer study regarding Campylobacter jejuni that has compiled a significant amount of data about the Multilocus Sequence Type and virulence factors concerning their global prevalence and distribution over this database.

Salmonella Typhimurium ST313 isolated in Brazil revealed to be more invasive and inflammatory in murine colon compared to ST19 strains

Salmonella Typhimurium (ST313) has caused an epidemic of invasive disease in sub-Saharan Africa and has been recently identified in Brazil. As the virulence of this ST is poorly understood, the present study aimed to (i) perform the RNA-seq in vitro of S. Typhimurium STm30 (ST313) grown in Luria-Bertani medium at 37°C; (ii) compare it with the RNA-seq of the S. Typhimurium SL1344 (ST19) and S. Typhimurium STm11 (ST19) strains under the same growing conditions; and (iii) examine the colonization capacity and expression of virulence genes and cytokines in murine colon. The STm30 (ST313) strain exhibited stronger virulence and was associated with a more inflammatory profile than the strains SL1344 (ST19) and STm11 (ST19), as demonstrated by transcriptome and in vivo assay. The expression levels of the hilA, sopD2, pipB, and ssaS virulence genes, other Salmonella pathogenicity islands SPI-1 and SPI-2 genes or effectors, and genes of the cytokines IL-1β, IFN-γ, TNF-α, IL-6, IL-17, IL-22, and IL-12 were increased during ST313 infection in C57BL/6J mice. In conclusion, S. Typhimurium STm30 (ST313) isolated from human feces in Brazil express higher levels of pathogenesis-related genes at 37°C and has stronger colonization and invasion capacity in murine colon due to its high expression levels of virulence genes, when compared with the S. Typhimurium SL1344 (ST19) and STm11 (ST19) strains. STm30 (ST313) also induces stronger expression of pro-inflammatory cytokines in this organ, suggesting that it causes more extensive tissue damage.

Salmonella enterica BcfH Is a Trimeric Thioredoxin-Like Bifunctional Enzyme with Both Thiol Oxidase and Disulfide Isomerase Activities

Aims: Thioredoxin (TRX)-fold proteins are ubiquitous in nature. This redox scaffold has evolved to enable a variety of functions, including redox regulation, protein folding, and oxidative stress defense. In bacteria, the TRX-like disulfide bond (Dsb) family mediates the oxidative folding of multiple proteins required for fitness and pathogenic potential. Conventionally, Dsb proteins have specific redox functions with monomeric and dimeric Dsbs exclusively catalyzing thiol oxidation and disulfide isomerization, respectively. This contrasts with the eukaryotic disulfide forming machinery where the modular TRX protein disulfide isomerase (PDI) mediates thiol oxidation and disulfide reshuffling. In this study, we identified and structurally and biochemically characterized a novel Dsb-like protein from Salmonella enterica termed bovine colonization factor protein H (BcfH) and defined its role in virulence. Results: In the conserved bovine colonization factor (bcf) fimbrial operon, the Dsb-like enzyme BcfH forms a trimeric structure, exceptionally uncommon among the large and evolutionary conserved TRX superfamily. This protein also displays very unusual catalytic redox centers, including an unwound α-helix holding the redox active site and a trans-proline instead of the conserved cis-proline active site loop. Remarkably, BcfH displays both thiol oxidase and disulfide isomerase activities contributing to Salmonella fimbrial biogenesis. Innovation and Conclusion: Typically, oligomerization of bacterial Dsb proteins modulates their redox function, with monomeric and dimeric Dsbs mediating thiol oxidation and disulfide isomerization, respectively. This study demonstrates a further structural and functional malleability in the TRX-fold protein family. BcfH trimeric architecture and unconventional catalytic sites permit multiple redox functions emulating in bacteria the eukaryotic PDI dual oxidoreductase activity. Antioxid. Redox Signal. 35, 21-39.

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.

Phylogenetic relationship and genomic characterization of Salmonella Typhimurium strains isolated from swine in Brazil

Salmonella Typhimurium has been transmitted between humans and animals. Although, Brazil has been one of the largest pork meat exporters worldwide, there are few studies that characterized epidemiologically S. Typhimurium strains from swine. The aims of this work were to study the phylogenetic relationship of S. Typhimurium genomes isolated from swine in Brazil among themselves and with other genomes isolated from several sources and countries using wgMLST and cgMLST and to perform the search of Salmonella pathogenicity islands (SPIs). In addition, for S. Typhimurium strains from swine to compare the virulence and antimicrobial resistance genes by VFDB and ResFinder, genetic content by BLAST Atlas and orthologous proteins clusters by OrthoVenn. The constructed phylogenetic trees by wgMLST and cgMLST grouped the majority (92.3% and 80.7%, respectively) of the strains isolated from swine in Brazil into the same group. All the isolates contained important SPIs (SPI-1, SPI-2, SPI-3, SPI-5 and SPI-9). A total of 100 and 31 virulence and resistance genes were detected in the S. Typhimurium strains isolated from swine, respectively. The BLAST Atlas and orthologous proteins analysis found regions of phages and differences in metabolic, regulatory and cellular processes among S. Typhimurium LT2 and S. Typhimurium isolates from swine. In conclusion, molecular typing based in the wgMLST and cgMLST suggested that the S. Typhimurium isolates from swine studied were genetically related. The pathogenic potential of the strains studied was corroborated by the presence of important SPIs and virulence genes. The high number of antimicrobial resistance genes detected is worrying and reinforced their potential risk in swine in Brazil. The comparison by BLAST Atlas suggested differences in mobile genetic elements among S. Typhimurium LT2 and S. Typhimurium isolates from swine in Brazil. The orthologous proteins analysis revealed unique genes related to important cellular processes in the strains from swine.

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.