Characterization of pertussis-like toxin from Salmonella spp. that catalyzes ADP-ribosylation of G proteins

A multidrug-resistant Salmonella enterica Typhimurium DT104 complex lineage circulating among humans and cattle in the USA lost the ability to produce pertussis-like toxin ArtAB

Salmonella enterica subsp. enterica serotype Typhimurium definitive type 104 (DT104) can infect both humans and animals and is often multidrug-resistant (MDR). Previous studies have indicated that, unlike most S . Typhimurium, the overwhelming majority of DT104 strains produce pertussis-like toxin ArtAB via prophage-encoded genes artAB . However, DT104 that lack artAB have been described on occasion. Here, we identify an MDR DT104 complex lineage circulating among humans and cattle in the USA, which lacks artAB (i.e. the ‘U.S. artAB -negative major clade’; n =42 genomes). Unlike most other bovine- and human-associated DT104 complex strains from the USA (n =230 total genomes), which harbour artAB on prophage Gifsy-1 (n =177), members of the U.S. artAB -negative major clade lack Gifsy-1, as well as anti-inflammatory effector gogB . The U.S. artAB -negative major clade encompasses human- and cattle-associated strains isolated from ≥11 USA states over a 20-year period. The clade was predicted to have lost artAB , Gifsy-1 and gogB circa 1985–1987 (95 % highest posterior density interval 1979.0–1992.1). When compared to DT104 genomes from other regions of the world (n =752 total genomes), several additional, sporadic artAB , Gifsy-1 and/or gogB loss events among clades encompassing five or fewer genomes were observed. Using phenotypic assays that simulate conditions encountered during human and/or bovine digestion, members of the U.S. artAB -negative major clade did not differ from closely related Gifsy-1/artAB /gogB -harbouring U.S. DT104 complex strains (ANOVA raw P >0.05); thus, future research is needed to elucidate the roles that artAB , gogB and Gifsy-1 play in DT104 virulence in humans and animals.

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.

Antimicrobial resistance and genomic characterization of Salmonella enterica isolates from chicken meat

This study investigated genotypic and phenotypic antimicrobial resistance profiles, phylogenic relatedness, plasmid and virulence composition of 39 Salmonella enterica strains isolated from chicken meat samples using whole genome sequencing (WGS) technology. Four distinct serotypes were identified; Salmonella Minnesota (16/39, 41%), Salmonella Infantis (13/39, 33.3%), Salmonella Enteritidis (9/39, 23.1%), and one isolate was detected for Salmonella Kentucky (1/39, 2.6%), with sequence types (STs) as followed: ST548, ST32, ST11, and ST198, respectively. Phenotypic resistance to tetracycline (91.2%), ampicillin (82.4%), sulfisoxazole (64.7%), and nalidixic acid (61.6%) was the most observed. Resistome analysis revealed the presence of resistance genes to aminoglycosides, β-lactamase, sulfonamides, trimethoprim, phenicol, lincosamide, macrolides, and tetracyclines. Plasmidome showed the presence of eight incompatibility groups, including IncA/C2, IncFIB(K)_1_Kpn3, Col440I_1, IncR, IncX1, IncI1_1_Alpha, IncFIB(S)/IncFII(S), IncHI2/IncHI2A, IncX2 and ColpVC plasmids across the 39 genomes. Three resistance genes, sul2, tetA and bla_CMY-2, were predicted to be located on IncA/C2 plasmid in S. Minnesota isolates, whereas all S. Infantis isolates were positive to IncFIB(K)_1_Kpn3 plasmid that carries bla _CTX-M-65 gene. Eleven Salmonella pathogenicity islands and up to 131 stress and/or virulence genes were identified in the evaluated genomes. Phylogenetic analysis showed four phylogroups that were consistent with the identified ST profiles with a high level of inter-diversity between isolates. This is the first genomic characterization of Salmonella isolates from retail chicken meat in Saudi Arabia using WGS technology. The availability of Salmonella genomes from multiple geographic locations, including Saudi Arabia, would be highly beneficial in future source-tracking, especially during epidemiological surveillance and outbreak investigations.

The evolutionary diversification of the Salmonella artAB toxin locus

Salmonella enterica is a diverse species of bacterial pathogens comprised of >2,500 serovars with variable host ranges and virulence properties. Accumulating evidence indicates that two AB₅-type toxins, typhoid toxin and ArtAB toxin, contribute to the more severe virulence properties of the Salmonella strains that encode them. It was recently discovered that there are two distinct types of artAB-like genetic elements in Salmonella: those that encode ArtAB toxins (artAB elements) and those in which the artA gene is degraded and the ArtB homolog, dubbed PltC, serves as an alternative delivery subunit for typhoid toxin (pltC elements). Here, we take a multifaceted approach to explore the evolutionary diversification of artAB-like genetic elements in Salmonella. We identify 7 subtypes of ArtAB toxins and 4 different PltC sequence groups that are distributed throughout the Salmonella genus. Both artAB and pltC are encoded within numerous diverse prophages, indicating a central role for phages in their evolutionary diversification. Genetic and structural analyses revealed features that distinguish pltC elements from artAB and identified evolutionary adaptations that enable PltC to efficiently engage typhoid toxin A subunits. For both pltC and artAB, we find that the sequences of the B subunits are especially variable, particularly amongst amino acid residues that fine tune the chemical environment of their glycan binding pockets. This study provides a framework to delineate the remarkably complex collection of Salmonella artAB/pltC-like genetic elements and provides a window into the mechanisms of evolution for AB₅-type toxins.

Intra-macrophage expression of ArtAB toxin gene in Salmonella

Salmonella enterica subspecies enterica serovar Typhimurium (S. Typhimurium) definitive phage type 104 (DT104), S. Worthington, and S. bongori produce ArtAB toxin, which catalyses ADP-ribosylation of pertussis toxin-sensitive G protein. ArtAB gene (artAB) is encoded on a prophage in Salmonella, and prophage induction by SOS-inducing agents is associated with increases in ArtAB production in vitro. However, little is known about the expression of artAB in vivo. Here, we showed a significant increase in artAB transcription of DT104 within macrophage-like RAW264.7 cells. Intracellular expression of ArtAB was also observed by immunofluorescence staining. The induced expression of artAB in DT104 and S. bongori was enhanced by treatment of RAW264.7 cells with phorbol 12-myristate 13-acetate (PMA), which stimulates the production of reactive oxygen species (ROS); however, such induction was not observed in S. Worthington. Upregulation of oxyR, a major regulator of oxidative stress, and cI, a repressor of prophage induction, was observed in S. Worthington within RAW264.7 cells treated with PMA but not in the DT104 strain. Although the expression of oxyR was increased, artAB was upregulated in S. bongori, which lacks the cI gene in the incomplete artAB-encoded prophage. Taken together, oxidative stress plays a role in the production of artAB toxins in macrophages, and high expression levels of oxyR and cI are responsible for the low expression of artAB. Therefore, strain variation in the level of artAB expression within macrophages could be explained by differences in the oxidative stress response of bacteria and might be reflected in its virulence.

Contribution of a Novel Pertussis Toxin-Like Factor in Mediating Persistent Otitis Media

Chronic otitis media (COM) is the long-term infection and inflammation of the middle ears typically caused by upper respiratory tract pathogens that are able to ascend the Eustachian tube. Our understanding of contributing factors is limited because human otopathogens cannot naturally colonize or persist in the middle ears of mice. We recently described a natural COM in mice caused by Bordetella pseudohinzii and proposed this as an experimental system to study bacterial mechanisms of immune evasion that allow persistent infection of the middle ear. Here we describe a novel pertussis toxin (PTx)-like factor unique to B. pseudohinzii, apparently acquired horizontally, that is associated with its particularly efficient persistence and pathogenesis. The catalytic subunit of this toxin, PsxA, has conserved catalytic sites and substantial predicted structural homology to pertussis toxin catalytic subunit PtxA. Deletion of the gene predicted to encode the catalytic subunit, psxA, resulted in a significant decrease in persistence in the middle ears. The defect was not observed in mice lacking T cells, indicating that PsxA is necessary for persistence only when T cells are present. These results demonstrate the role of a novel putative toxin in the persistence of B. pseudohinzii and its generation of COM. This PsxA-mediated immune evasion strategy may similarly be utilized by human otopathogens, via other PTx-like toxins or alternative mechanisms to disrupt critical T cell functions necessary to clear bacteria from the middle ear. This work demonstrates that this experimental system can allow for the detailed study of general strategies and specific mechanisms that otopathogens use to evade host immune responses to persist in the middle ear to cause COM.

AB5 Enterotoxin-Mediated Pathogenesis: Perspectives Gleaned from Shiga Toxins

Foodborne diseases affect an estimated 600 million people worldwide annually, with the majority of these illnesses caused by Norovirus, Vibrio, Listeria, Campylobacter, Salmonella, and Escherichia coli. To elicit infections in humans, bacterial pathogens express a combination of virulence factors and toxins. AB₅ toxins are an example of such toxins that can cause various clinical manifestations, including dehydration, diarrhea, kidney damage, hemorrhagic colitis, and hemolytic uremic syndrome (HUS). Treatment of most bacterial foodborne illnesses consists of fluid replacement and antibiotics. However, antibiotics are not recommended for infections caused by Shiga toxin-producing E. coli (STEC) because of the increased risk of HUS development, although there are conflicting views and results in this regard. Lack of effective treatment strategies for STEC infections pose a public health threat during outbreaks; therefore, the debate on antibiotic use for STEC infections could be further explored, along with investigations into antibiotic alternatives. The overall goal of this review is to provide a succinct summary on the mechanisms of action and the pathogenesis of AB₅ and related toxins, as expressed by bacterial foodborne pathogens, with a primary focus on Shiga toxins (Stx). The role of Stx in human STEC disease, detection methodologies, and available treatment options are also briefly discussed.

Community-Acquired Respiratory Distress Syndrome Toxin: Unique Exotoxin for M. pneumoniae

Mycoplasma pneumoniae infection often causes respiratory diseases in humans, particularly in children and adults with atypical pneumonia and community-acquired pneumonia (CAP), and is often exacerbated by co-infection with other lung diseases, such as asthma, bronchitis, and chronic obstructive pulmonary disorder. Community-acquired respiratory distress syndrome toxin (CARDS TX) is the only exotoxin produced by M. pneumoniae and has been extensively studied for its ADP-ribosyltransferase (ADPRT) activity and cellular vacuolization properties. Additionally, CARDS TX induces inflammatory responses, resulting in cell swelling, nuclear lysis, mucus proliferation, and cell vacuolization. CARDS TX enters host cells by binding to the host receptor and is then reverse transported to the endoplasmic reticulum to exert its pathogenic effects. In this review, we focus on the structural characteristics, functional activity, distribution and receptors, mechanism of cell entry, and inflammatory response of CARDS TX was examined. Overall, the findings of this review provide a theoretical basis for further investigation of the mechanism of M. pneumoniae infection and the development of clinical diagnosis and vaccines.

Characterization of Basal Transcriptomes Identifies Potential Metabolic and Virulence-Associated Adaptations Among Diverse Nontyphoidal Salmonella enterica Serovars

The zoonotic pathogen Salmonella enterica includes >2,600 serovars, which differ in the range of hosts they infect and the severity of disease they cause. To further elucidate the mechanisms behind these differences, we performed transcriptomic comparisons of nontyphoidal Salmonella (NTS) serovars with the model for NTS pathogenesis, S. Typhimurium. Specifically, we used RNA-seq to characterize the understudied NTS serovars S. Javiana and S. Cerro, representing a serovar frequently attributed to human infection via contact with amphibians and reptiles, and a serovar primarily associated with cattle, respectively. Whole-genome sequence (WGS) data were utilized to ensure that strains characterized with RNA-seq were representative of their respective serovars. RNA extracted from representative strains of each serovar grown to late exponential phase in Luria-Bertani (LB) broth showed that transcript abundances of core genes were significantly higher (p<0.001) than those of accessory genes for all three serovars. Inter-serovar comparisons identified that transcript abundances of genes in Salmonella Pathogenicity Island (SPI) 1 were significantly higher in both S. Javiana and S. Typhimurium compared to S. Cerro. Together, our data highlight potential transcriptional mechanisms that may facilitate S. Cerro and S. Javiana survival in and adaptation to their respective hosts and impact their ability to cause disease in others. Furthermore, our analyses demonstrate the utility of omics approaches in advancing our understanding of the diversity of metabolic and virulence mechanisms of different NTS serovars.

Cellular Activity of Salmonella Typhimurium ArtAB Toxin and Its Receptor-Binding Subunit

Salmonellosis is among the most reported foodborne illnesses in the United States. The Salmonellaenterica Typhimurium DT104 phage type, which is associated with multidrug-resistant disease in humans and animals, possesses an ADP-ribosylating toxin called ArtAB. Full-length artAB has been found on a number of broad-host-range non-typhoidal Salmonella species and serovars. ArtAB is also homologous to many AB₅ toxins from diverse Gram-negative pathogens, including cholera toxin (CT) and pertussis toxin (PT), and may be involved in Salmonella pathogenesis, however, in vitro cellular toxicity of ArtAB has not been characterized. artAB was cloned into E. coli and initially isolated using a histidine tag (ArtABHIS) and nickel chromatography. ArtABHIS was found to bind to African green monkey kidney epithelial (Vero) cells using confocal microscopy and to interact with glycans present on fetuin and monosialotetrahexosylganglioside (GM1) using ELISA. Untagged, or native, holotoxin (ArtAB), and the pentameric receptor-binding subunit (ArtB) were purified from E. coli using fetuin and d-galactose affinity chromatography. ArtAB and ArtB metabolic and cytotoxic activities were determined using Vero and Chinese hamster ovary (CHO) epithelial cells. Vero cells were more sensitive to ArtAB, however, incubation with both cell types revealed only partial cytotoxicity over 72 h, similar to that induced by CT. ArtAB induced a distinctive clustering phenotype on CHO cells over 72 h, similar to PT, and an elongated phenotype on Vero cells, similar to CT. The ArtB binding subunit alone also had a cytotoxic effect on CHO cells and induced morphological rounding. Results indicate that this toxin induces distinctive cellular outcomes. Continued biological characterization of ArtAB will advance efforts to prevent disease caused by non-typhoidal Salmonella.

ADP-ribosylation systems in bacteria and viruses

ADP-ribosylation is an ancient posttranslational modification present in all kingdoms of life. The system likely originated in bacteria where it functions in inter- and intra-species conflict, stress response and pathogenicity. It was repeatedly adopted via lateral transfer by eukaryotes, including humans, where it has a pivotal role in epigenetics, DNA-damage repair, apoptosis, and other crucial pathways including the immune response to pathogenic bacteria and viruses. In other words, the same ammunition used by pathogens is adapted by eukaryotes to fight back. While we know quite a lot about the eukaryotic system, expanding rather patchy knowledge on bacterial and viral ADP-ribosylation would give us not only a better understanding of the system as a whole but a fighting advantage in this constant arms race. By writing this review we hope to put into focus the available information and give a perspective on how this system works and can be exploited in the search for therapeutic targets in the future. The relevance of the subject is especially highlighted by the current situation of being amid the world pandemic caused by a virus harbouring and dependent on a representative of such a system.

Recent Advances in the Detection of Antibiotic and Multi-Drug Resistant Salmonella: An Update

Antibiotic and multi-drug resistant (MDR) Salmonella poses a significant threat to public health due to its ability to colonize animals (cold and warm-blooded) and contaminate freshwater supplies. Monitoring antibiotic resistant Salmonella is traditionally costly, involving the application of phenotypic and genotypic tests over several days. However, with the introduction of cheaper semi-automated devices in the last decade, strain detection and identification times have significantly fallen. This, in turn, has led to efficiently regulated food production systems and further reductions in food safety hazards. This review highlights current and emerging technologies used in the detection of antibiotic resistant and MDR Salmonella.

Genomic investigation of antimicrobial resistance determinants and virulence factors in Salmonella enterica serovars isolated from contaminated food and human stool samples in Brazil

This study investigated the antimicrobial resistance determinants, virulence factors and identified serovars in 37 Salmonella enterica strains isolated from human stool and contaminated foods linked to outbreaks that occurred in Brazil over 7 years using whole genome sequencing (WGS). Phylogenetic analysis of selected serovars (S. Typhimurium, S. Infantis, S. London, and S. Johannesburg) was performed. Ten distinct serovars were identified and, 51% of the tested strains (n = 19) showed disagreement with the previous conventional serotyping. The antimicrobial resistance (AMR) determinants or plasmids varied among the strains. Resistome analysis revealed the presence of resistance genes to aminoglycosides [aac (6')-laa, aph (3″)-lb, aph (6)-ld, aadA1 and aadA2], sulfonamides (sul1), trimethoprin (dfrA8), fosfomycin (fosA7) and tetracyclines (tetA, tetB, tetC), as well as point mutations in parC (T57S) and gyrA (S83F). Plasmidome showed the presence of IncHI2, IncHI2A, IncFIB (S), IncFII (S), IncI1 and p0111 plasmids. Eight Salmonella pathogenicity islands and up to 102 stress and/or virulence genes were identified in the evaluated genomes. Virulence genes of K88 fimbrial adhesin were first reported in S. enterica (S. Pomona, S. Bredeney and S. Mbandaka strains). pilW gene was first identified in S. Pomona. Phylogenetic analysis showed that some serovars circulated in Brazil for decades, primarily within the poultry production chain. Findings highlighted the virulence and AMR determinants in strains that may lead to recurring food outbreaks.

The Majority of Typhoid Toxin-Positive Salmonella Serovars Encode ArtB, an Alternate Binding Subunit

While previous reports had suggested that the typhoid toxin (TT) could potentially use ArtB as an alternate binding subunit, this was thought to play a minor role in the evolution and biology of the toxin. In this study, we establish that both TT genes and artB are widespread among Salmonella enterica subsp. enterica, suggesting that TT likely plays a broader role in Salmonella virulence that extends beyond its proposed role in typhoid fever.

Salmonella enterica encodes a wide array of virulence factors. One novel virulence factor, an A₂B₅ toxin known as the typhoid toxin (TT), was recently identified among a variety of S. enterica serovars. While past studies have shown that some serovars encode both the TT (active subunits CdtB and PltA and binding subunit PltB) and a second binding subunit (ArtB), these serovars were thought to be the exception. Here, we show that genes encoding the TT are detected in more than 100 serovars representing distinct phylogenetic lineages of S. enterica subsp. enterica, although clade B and section Typhi are significantly more likely to encode TT genes than serovars from other clades. Furthermore, we show that 81% of these TT-positive serovars also encode artB, suggesting that the cooccurrence of both toxin binding subunits is considerably more common than previously thought. A combination of in silico modeling, bacterial two-hybrid system screening, and tandem affinity purification (TAP) of toxin subunits suggests that ArtB and PltB interact in vitro, at least under some growth conditions. While different growth conditions yielded slightly higher transcript abundances of artB and pltB, both genes had their highest relative transcript abundances when Salmonella was grown under low-Mg²⁺ conditions, suggesting that ArtB and PltB may compete for inclusion in the TT. Together, our results suggest that ArtB likely plays an important and previously underappreciated role in the biology of the TT produced by typhoidal and nontyphoidal Salmonella.

IMPORTANCE While previous reports had suggested that the typhoid toxin (TT) could potentially use ArtB as an alternate binding subunit, this was thought to play a minor role in the evolution and biology of the toxin. In this study, we establish that both TT genes and artB are widespread among Salmonella enterica subsp. enterica, suggesting that TT likely plays a broader role in Salmonella virulence that extends beyond its proposed role in typhoid fever. Furthermore, our data suggest the selective maintenance of both toxin binding subunits, which may compete for inclusion in the holotoxin. Last, our data support the importance of characterizing diverse nontyphoidal Salmonella (NTS) serovars, as the presence of classically defined typhoidal virulence factors among NTS serovars continues to challenge the typhoid-nontyphoid Salmonella paradigm.

Twentieth-century emergence of antimicrobial resistant human- and bovine-associated Salmonella enterica serotype Typhimurium lineages in New York State

Salmonella enterica serotype Typhimurium (S. Typhimurium) boasts a broad host range and can be transmitted between livestock and humans. While members of this serotype can acquire resistance to antimicrobials, the temporal dynamics of this acquisition is not well understood. Using New York State (NYS) and its dairy cattle farms as a model system, 87 S. Typhimurium strains isolated from 1999 to 2016 from either human clinical or bovine-associated sources in NYS were characterized using whole-genome sequencing. More than 91% of isolates were classified into one of four major lineages, two of which were largely susceptible to antimicrobials but showed sporadic antimicrobial resistance (AMR) gene acquisition, and two that were largely multidrug-resistant (MDR). All four lineages clustered by presence and absence of elements in the pan-genome. The two MDR lineages, one of which resembled S. Typhimurium DT104, were predicted to have emerged circa 1960 and 1972. The two largely susceptible lineages emerged earlier, but showcased sporadic AMR determinant acquisition largely after 1960, including acquisition of cephalosporin resistance-conferring genes after 1985. These results confine the majority of AMR acquisition events in NYS S. Typhimurium to the twentieth century, largely within the era of antibiotic usage.

Influence of SOS-inducing agents on the expression of ArtAB toxin gene in Salmonella enterica and Salmonella bongori

Salmonella enterica subspecies enterica serovar Typhimurium (S. Typhimurium) definitive phage type 104 (DT104), S. enterica subspecies enterica serovar Worthington (S. Worthington) and S. bongori produce ArtA and ArtB (ArtAB) toxin homologues, which catalyse ADP-ribosylation of pertussis toxin-sensitive G protein. ArtAB gene (artAB) is encoded on prophage in DT104 and its expression is induced by mitomycin C (MTC) and hydrogen peroxide (H₂O₂) that trigger the bacterial SOS response. Although the genetic regulatory mechanism associated with artAB expression is not characterized, it is thought to be associated with prophage induction, which occurs when the RecA-mediated SOS response is triggered. Here we show that subinhibitory concentration of quinolone antibiotics that are SOS-inducing agents, also induce ArtAB production in these Salmonella strains. Both MTC and fluoroquinolone antibiotics such as enrofloxacin-induced artA and recA transcription and artAB-encoding prophage (ArtAB-prophage) in DT104 and S. Worthington. However, in S. bongori, which harbours artAB genes on incomplete prophage, artA transcription was induced by MTC and enrofloxacin, but prophage induction was not observed. Taken together, these results suggest that SOS response followed by induction of artAB transcription is essential for ArtAB production. H₂O₂-mediated induction of ArtAB prophage and efficient production of ArtAB was observed in DT104 but not in S. Worthington and S. bongori. Therefore, induction of artAB expression with H₂O₂ is strain-specific, and the mode of action of H₂O₂ as an SOS-inducing agent might be different from those of MTC and quinolone antibiotics.

The ADP-Ribosylating Toxins of Salmonella

A number of pathogenic bacteria utilize toxins to mediate disease in a susceptible host. The foodborne pathogen Salmonella is one of the most important and well-studied bacterial pathogens. Recently, whole genome sequence characterizations revealed the presence of multiple novel ADP-ribosylating toxins encoded by a variety of Salmonella serovars. In this review, we discuss both the classical (SpvB) and novel (typhoid toxin, ArtAB, and SboC/SeoC) ADP-ribosylating toxins of Salmonella, including the structure and function of these toxins and our current understanding of their contributions to virulence.

Embracing Diversity: Differences in Virulence Mechanisms, Disease Severity, and Host Adaptations Contribute to the Success of Nontyphoidal Salmonella as a Foodborne Pathogen

Not all Salmonella enterica serovars cause the same disease. S. enterica represents an incredibly diverse species comprising >2,600 unique serovars. While some S. enterica serovars are host-restricted, others infect a wide range of hosts. The diseases that nontyphoidal Salmonella (NTS) serovars cause vary considerably, with some serovars being significantly more likely to cause invasive disease in humans than others. Furthermore, while genomic analyses have advanced our understanding of the genetic diversity of these serovars, they have not been able to fully account for the observed clinical differences. One overarching challenge is that much of what is known about Salmonella's general biology and virulence strategies is concluded from studies examining a select few serovars, especially serovar Typhimurium. As targeted control strategies have been implemented to control select serovars, an increasing number of foodborne outbreaks involving serovars that are less frequently associated with human clinical illness are being detected. Harnessing what is known about the diversity of NTS serovars represents an important factor in achieving the ultimate goal of reducing salmonellosis-associated morbidity and mortality worldwide. In this review we summarize the current understanding of the differences and similarities among NTS serovars, highlighting the virulence mechanisms, genetic differences, and sources that characterize S. enterica diversity and contribute to its success as a foodborne pathogen.

Targeting ADP-ribosylation as an antimicrobial strategy

ADP-ribosylation (ADPr) is an ancient reversible modification of cellular macromolecules controlling major biological processes as diverse as DNA damage repair, transcriptional regulation, intracellular transport, immune and stress responses, cell survival and proliferation. Furthermore, enzymatic reactions of ADPr are central in the pathogenesis of many human diseases, including infectious conditions. By providing a review of ADPr signalling in bacterial systems, we highlight the relevance of this chemical modification in the pathogenesis of human diseases depending on host-pathogen interactions. The post-antibiotic era has raised the need to find alternative approaches to antibiotic administration, as major pathogens becoming resistant to antibiotics. An in-depth understanding of ADPr reactions provides the rationale for designing novel antimicrobial strategies for treatment of infectious diseases. In addition, the understanding of mechanisms of ADPr by bacterial virulence factors offers important hints to improve our knowledge on cellular processes regulated by eukaryotic homologous enzymes, which are often involved in the pathogenesis of human diseases.

The Typhoid Toxin Produced by the Nontyphoidal Salmonella enterica Serotype Javiana Is Required for Induction of a DNA Damage Response In Vitro and Systemic Spread In Vivo

The Salmonella cytolethal distending toxin (S-CDT), first described as the “typhoid toxin” in Salmonella enterica subsp. enterica serotype Typhi, induces DNA damage in eukaryotic cells. Recent studies have shown that more than 40 nontyphoidal Salmonella (NTS) serotypes carry genes that encode S-CDT, yet very little is known about the activity, function, and role of S-CDT in NTS. Here we show that deletion of genes encoding the binding subunit (pltB) and a bacteriophage muramidase predicted to play a role in toxin export (ttsA) does not abolish toxin activity in the S-CDT-positive NTS Salmonella enterica subsp. enterica serotype Javiana. However, S. Javiana strains harboring deletions of both pltB and its homolog artB, had a complete loss of S-CDT activity, suggesting that S. Javiana carries genes encoding two variants of the binding subunit. S-CDT-mediated DNA damage, as determined by phosphorylation of histone 2AX (H2AX), producing phosphorylated H2AX (γH2AX), was restricted to epithelial cells in S and G₂/M phases of the cell cycle and did not result in apoptosis or cell death. Compared to mice infected with a ΔcdtB strain, mice infected with wild-type S. Javiana had significantly higher levels of S. Javiana in the liver, but not in the spleen, ileum, or cecum. Overall, we show that production of active S-CDT by NTS serotype S. Javiana requires different genes (cdtB, pltA, and either pltB or artB) for expression of biologically active toxin than those reported for S-CDT production by S. Typhi (cdtB, pltA, pltB, and ttsA). However, as in S. Typhi, NTS S-CDT influences the outcome of infection both in vitro and in vivo.

Nontyphoidal Salmonella (NTS) are a major cause of bacterial food-borne illness worldwide; however, our understanding of virulence mechanisms that determine the outcome and severity of nontyphoidal salmonellosis is incompletely understood. Here we show that S-CDT produced by NTS plays a significant role in the outcome of infection both in vitro and in vivo, highlighting S-CDT as an important virulence factor for nontyphoidal Salmonella serotypes. Our data also contribute novel information about the function of S-CDT, as S-CDT-mediated DNA damage occurs only during certain phases of the cell cycle, and the resulting damage does not induce cell death as assessed using a propidium iodide exclusion assay. Importantly, our data support that, despite having genetically similar S-CDT operons, NTS serotype S. Javiana has different genetic requirements than S. Typhi, for the production and export of active S-CDT.