Differential expression of Salmonella type III secretion system factors InvJ, PrgJ, SipC, SipD, SopA and SopB in cultures and in mice

Phenotypic and genotypic characterization of antimicrobial resistance and virulence profiles of Salmonella enterica serotypes isolated from necropsied horses in Kentucky

Salmonella is a foodborne pathogen that poses a significant threat to global public health. It affects several animal species, including horses. Salmonella infections in horses can be either asymptomatic or cause severe clinical illness. Infections caused by Salmonella are presently controlled with antibiotics. Due to the formation of biofilms and the emergence of antimicrobial resistance, the treatment has become more complicated. Our study focused on investigating the prevalence of Salmonella enterica in necropsied horses, assessing the capability for biofilm formation, and motility, determining the phenotypic and genotypic profiles of antibiotic resistance, and detecting virulence genes. A total of 2,182 necropsied horses were tested for the presence of Salmonella. Intestinal samples were enriched in selenite broth and cultured on hektoen and eosin methylene blue agar plates, whereas other samples were directly cultured on aforementioned plates. Confirmation of the serotypes was performed according to the Kauffmann-White-Le Minor Scheme followed by biofilm formation screening using crystal violet assay. The resistance profile of the isolates was determined by broth microdilution assay using the Sensititre️ Vet (Equine EQUIN2F). The genotypic antimicrobial resistance (AMR) and virulence profiles were detected using polymerase chain reaction (PCR). The overall prevalence of Salmonella was 1.19% (26/2182), with 11 different serotypes identified. Salmonella Typhimurium was the most prevalent serotype with 19.2% prevalence. All of the isolates were identified as biofilm producers and motile. Virulence genes related to invasion (invA, hilA, mgtC, and spiA), biofilm formation (csgA and csgB), and motility (filA, motA, flgG, figG, flgH, fimC, fimD, and fimH) of Salmonella were detected among 100% of the isolates. An overall 11.4% of the isolates were identified as multidrug-resistant (MDR), with resistance to gentamicin, amikacin, ampicillin, ceftazidime, ceftiofur, chloramphenicol, and trimethoprim/sulfamethoxazole. We found that beta-lactamase-producing genes blaTEM, blaCTXM, and blaSHV2 were identified in 11.5% of the isolates, while only 3.8% carried the blaOXA-9 gene. The presence of MDR pathogenic Salmonella in horses is alarming for human and animal health, especially when they have a high affinity for forming biofilm. Our study found horses as potential sources of pathogenic Salmonella transmission to humans. Thus, it is important to perform continuous monitoring and surveillance studies to track the source of infection and develop preventive measures.

IMPORTANCE

This study focuses on understanding how Salmonella, specifically isolated from horses, can resist antibiotics and cause disease. Salmonella is a well-known foodborne pathogen that can pose risks not only to animals but also to humans. By studying the bacteria from necropsied horses, the research aims to uncover how certain Salmonella strains develop resistance to antibiotics and which genetic factors make them more dangerous. In addition to antibiotic resistance, the research explores the biofilm-forming ability of these strains, which enhances their survival in harsh environments. The study also investigates their motility, a factor that contributes to the spread of infection. The findings can improve treatment strategies for horses and help prevent the transmission of resistant bacteria to other animals as well as humans. Ultimately, the research could contribute to better management of antibiotic resistance in both veterinary and public health contexts, helping to safeguard animal welfare and public health.

From intestine to beyond: Salmonella entry factors display distinct transcription pattern upon infection in murine models

The infectious process of bacteria of the genus Salmonella requires the finely regulated use of various virulence factors. Among them, the type 3 secretion system-1 (T3SS-1) and the Rck and PagN invasins are involved in the internalization of the pathogen within eukaryotic cells, but their precise role in the host and in the pathogenic process is still poorly understood. In this study, we aimed to determine the kinetics of expression of these entry factors in a typhoid fever-like and a gastroenteritis model in mice by in vivo imaging using bioluminescent Salmonella Typhimurium reporter strains carrying chromosomal transcriptional fusions. Only pagN and T3SS-1 transcription has been clearly identified. Independently of the pathological model, the caecum was identified as the main transcription site of both pagN and the T3SS-1-encoding gene both at early and late stages of the infection. An intense transcription of pagN was also observed in deep organs in the typhoid fever-like model, while that of T3SS-1 remained quite sporadic in these organs, and mainly focused on the intestine all along the infection. This work will help to understand the respective role of these entry factors at the cellular level in the pathogenesis of Salmonella in vivo.

SopA inactivation or reduced expression is selected in intracellular Salmonella and contributes to systemic Salmonella infection

Pseudogenes are accumulated in host-restricted Salmonella enterica serovars, while pseudogenization is primarily regarded as a process that purges unnecessary genes from the genome. Here we showed that the inactivation of sopA, which encodes an effector of Salmonella Pathogenicity Island 1, in human-restricted S. enterica serovar Typhi (S. Ty) and Paratyphi A (S. PA) is under positive selection and aimed to reduce bacterial cytotoxicity toward host macrophages. Moreover, we found that the expression of sopA in Salmonella Typhimurium (S. Tm), a broad-host-range serovar which causes systemic disease in mice, was negatively regulated during mice infection and survival in murine macrophages. The sopA repression in S. Tm is mediated by IsrM, a small RNA absent from the genome of S. Ty and S. PA. Due to the lack of IsrM, sopA expression was unregulated in S. Ty and S. PA, which might have facilitated the convergent inactivation of sopA in these two serovars. In conclusion, our findings demonstrate that sopA inactivation or intracellular repression is the target of positive selection during the systemic infection caused by S. enterica serovars.

Screening and identification of SipC-interacting proteins in Salmonella enteritidis using Gal4 yeast two-hybrid system in duck

The zoonotic pathogen Salmonella not only reduces the production performance in ducks, but also poses a serious threat to human health through eggs and pollutes water bodies through feces. SipC, an effector protein of type III secretion systems (T3SS) in Salmonella, mediates translocation of effectors into the eukaryotic host. However, the precise role of SipC effectors remains unknown in ducks. In this study, the SipC from duck granulosa cells (dGCs) was selected as bait, and the SipC-interacting proteins in Salmonella enteritidis (SE) were screened using Gal4 yeast two-hybrid system in duck. Twelve SipC-interacting proteins were identified. Among those, the p53-effector related to PMP-22 (PERP) and TGF-β activated kinase 1-binding protein 2 (TAB2) were selected to further confirm the function by GST pull-down in vitro. Over-expression of PERP resulted in not only increasing SE adhesion and invasion but also triggering the production of IL-1β and IFN-α in SE infected dGCs, while knock-down PERP showed the opposite tendency (P < 0.01). In addition, TAB2 significantly induced the production of IL-6, IL-1β, IFN-α, and INF-γ in SE infected dGCs (P < 0.05), but did not cause obvious changes in SE adhesion and invasion. When the sipC in SE was deleted, the activities of duck PERP and TAB2 were abolished because they could not bind to SipC. Taken together, although the protein of PERP and TAB2 can interact with SipC, their mechanisms were different in duck challenged by SE. Therefore, PERP was involved in SE invasion and inflammatory response of dGC ovaries, and TAB2 only contributed to dGCs inflammatory response, which provided critical insights about the mechanism in host- bacterium protein interactions during Salmonella invasion in duck.

Salmonella Pathogenicity Island 1 (SPI-1) and Its Complex Regulatory Network

Salmonella species can infect a diverse range of birds, reptiles, and mammals, including humans. The type III protein secretion system (T3SS) encoded by Salmonella pathogenicity island 1 (SPI-1) delivers effector proteins required for intestinal invasion and the production of enteritis. The T3SS is regarded as the most important virulence factor of Salmonella. SPI-1 encodes transcription factors that regulate the expression of some virulence factors of Salmonella, while other transcription factors encoded outside SPI-1 participate in the expression of SPI-1-encoded genes. SPI-1 genes are responsible for the invasion of host cells, regulation of the host immune response, e.g., the host inflammatory response, immune cell recruitment and apoptosis, and biofilm formation. The regulatory network of SPI-1 is very complex and crucial. Here, we review the function, effectors, and regulation of SPI-1 genes and their contribution to the pathogenicity of Salmonella.

Structures of Type III Secretion System Needle Filaments

Among the Gram-negative bacterial secretion systems, type III secretion systems (T3SS) possess a unique extracellular molecular apparatus called the needle. This macromolecular protein assembly is a nanometre-size filament formed by the helical arrangement of hundreds of copies of a single, small protein, which is highly conserved between T3SSs from animal to plant bacterial pathogens. The needle filament forms a hollow tube with a channel ~20 Å in diameter that serves as a conduit for proteins secreted into the targeted host cell. In the past ten years, technical breakthroughs in biophysical techniques such as cryo-electron microscopy (cryo-EM) and solid-state NMR (SSNMR) spectroscopy have uncovered atomic resolution details about the T3SS needle assembly. Several high-resolution structures of Salmonella typhimurium and Shigella flexneri T3SS needles have been reported demonstrating a common structural fold. These structural models have been used to explain the active role of the needle in transmitting the host-cell contact signal from the tip to the base of the T3SS through conformational changes as well as during the injection of effector proteins. In this chapter, we summarize the current knowledge about the structure and the role of the T3SS needle during T3SS assembly and effector secretion.

Secretion of Salmonella Pathogenicity Island 1-Encoded Type III Secretion System Effectors by Outer Membrane Vesicles in Salmonella enterica Serovar Typhimurium

Outer membrane vesicles (OMVs) are spherical membranous structures released by Gram-negative bacteria. Several bacterial pathogens utilize OMVs as vehicles for the delivery of virulence factors into host cells. Results of our previous study on proteomic analysis revealed that OMVs isolated from Salmonellaenterica serovar Typhimurium had virulence effectors that are known to be translocated by Salmonella pathogenicity island 1 (SPI-1)-encoded type III secretion system (T3SS1) into the host cell. In the present study, immunoblot analysis confirmed the secretion of the six T3SS1 effector proteins, namely SipB and SipC (translocators of T3SS1), and SipA, SopA, SopB, and SopE2 (effectors translocated by T3SS1), by OMVs. Results of proteinase K treatment revealed the localization of these T3SS1 effector proteins on the outer surface of OMVs. SipC and SopE2 were secreted by OMVs independent of the three secretion systems T3SS1, T3SS2, and flagella, signifying OMVs to be an alternative delivery system to T3SSs. T3SS1 effectors SipA, SipC, and SopE2 were internalized into the cytoplasm of the host cell by OMVs independent of cellular Salmonella–host cell contact. In epithelial cells, addition of OMVs harboring T3SS1 effectors stimulated the production of F-actin, thereby complementing the attenuated invasion of ΔsopE2 into host cells. These results suggest that S. Typhimurium might exploit OMVs as a long-distance vehicle to deliver T3SS1 effectors into the cytoplasm of the host cell independent of bacteria–host cell interaction.

Comparative study of Salmonella enterica serovar Enteritidis genes expressed within avian and murine macrophages via selective capture of transcribed sequences (SCOTS)

Salmonella enterica serovar Enteritidis (SE) is a communicable zoonotic bacterium. Macrophages are essential for Salmonella survival, transmission, and infection. In this study, selective capture of transcribed sequences (SCOTS) was used to screen genes preferentially expressed by SE during contact with macrophages from different hosts. We found 57 predicted genes and 52 genes expressed by SE during interaction with avian HD-11 and murine RAW264.7 cells, respectively. These expressed genes were involved in virulence, metabolism, stress response, transport, regulation, and other functions. Although genes related to survival or metabolic pathways were needed during SE infection, different gene expression profiles of SE occurred in the two macrophage cell lines. qRT-PCR results confirmed that most screened genes were upregulated during infection in contrast to the observation during in vitro cultivation, with different expression levels in infected avian macrophages at 2-h and 7-h post-infection. In addition, in vitro and in vivo competition assays confirmed that SEN3610 (a putative deoR family regulator) and rfaQ (related to LPS synthesis) were closely related to SE virulence in both mice and chickens. Three putative transcriptional regulators, SEN2967, SEN4299, and rtcR, were related to SE colonization in mice, while the ycaM mutation caused decreased infection and survival of SE in HD-11 cells without influencing virulence in mice or chicken. Genes showing differential expression between SE-infected avian and murine macrophages indicate specific pathogen adaptation to enable infection of various hosts.

Bacterial type III secretion system as a protein delivery tool for a broad range of biomedical applications

A protein delivery tool based on bacterial type III secretion system (T3SS) has been broadly applied in biomedical researches. In this review, we summarize various applications of the T3SS-mediate protein delivery which enables translocation of proteins directly into mammalian cells without protein purification. Some of the remarkable advancements include delivery of antigens for therapeutic vaccines, nucleases for genome editing, transcription factors for cellular reprogramming and stem cells differentiation, and signaling molecules for post-translational proteomics studies. With continued improvement of the T3SS-mediated protein delivery tools, even wider application of the technology is anticipated.

Structure and Function of the Fecal Microbiota in Diarrheic Neonatal Piglets

Diarrhea is a leading cause of increased mortality in neonatal and young piglets. Aberration of the gut microbiota is one important factor in the etiology of piglet diarrhea. However, information regarding the structure and function of the gut microbiome in diarrheic neonatal piglets is limited. To investigate the composition and functional potential of the fecal microbiota in neonatal piglets, we performed 16S rRNA gene sequencing on 20 fecal samples from diarrheic piglets and healthy controls, and metagenomics sequencing on a subset of six samples. We found striking compositional and functional differences in fecal microbiota between diarrheic and healthy piglets. Neonatal piglet diarrhea was associated with increases in the relative abundance of Prevotella, Sutterella, and Campylobacter, as well as Fusobacteriaceae. The increased relative abundance of Prevotella was correlated with the reduction in Escherichia coli and the majority of beneficial bacteria that belonging to the Firmicutes phylum (e.g., Enterococcus, Streptococcus, Lactobacillus, Clostridium, and Blautia) in diarrheic piglets. The differentially functional gene abundances in diarrheic piglets were an increase in bacterial ribosome, and contributed primarily by the genera Prevotella, this indicates a growth advantage of the Prevotella in diarrheic conditions. Additional functional gene sets were associated with the reduction of polyamine transport, monosaccharide and sugar-specific PTS transport, amino acid transport, and two-component regulatory system. These profiles likely impact the ability to transport and uptake nutrients, as well as the ability to fight microbial infections in the piglet gut ecosystem. This work identifies a potential role for Prevotella in the community-wide microbial aberration and dysfunction that underpins the pathogenesis of piglet diarrhea. Identification of these microbial and functional signatures may provide biomarkers of neonatal piglet diarrhea.

Salmonella Virulence Factor SsrAB Regulated Factor Modulates Inflammatory Responses by Enhancing the Activation of NF-κB Signaling Pathway

Effector proteins encoded by Salmonella pathogenicity islands play a key role in promoting bacterial intracellular survival, colonization, and pathogenesis. In this study, we investigated the function of the virulence-associated effector SrfA (SsrAB regulated factor) both in macrophages in vitro and in infected mice in vivo. SrfA was secreted into the cytoplasm during S. Typhimurium infection and disassociated IL-1R-associated kinase-1 (IRAK-1) from the IRAK-1-Toll interacting protein (Tollip) complex by interacting with Tollip. The released IRAK-1 was phosphorylated and subsequently activated the NF-κB signaling pathway, which enhanced the LPS-induced expression of inflammatory cytokines, such as IL-8, IL-1β, and TNF-α. The coupling of ubiquitin to endoplasmic reticulum degradation aa 183-219 domain of Tollip is the binding region for SrfA, and both the MDaa207-226 and CTaa357-377 regions of SrfA mediate binding to Tollip and NF-κB signaling activation. Deletion of SrfA in S. Typhimurium had no notable effects on its replication but impaired the induction of NF-κB activation in infected macrophages. The mice infected with srfA-deficient bacteria exhibited a decreased inflammatory response and an increased survival rate compared with those infected with wild-type S. Typhimurium. We conclude that SrfA is a novel Salmonella virulence effector that helps modulate host inflammatory responses by promoting NF-κB signaling activation.

AvrA effector protein of Salmonella enterica serovar Enteritidis is expressed and translocated in mesenteric lymph nodes at late stages of infection in mice

Salmonellosis is a major health problem worldwide. Salmonella enterica serovar Enteritidis (S. Enteritidis) has been a primary cause of Salmonella outbreaks in many countries. AvrA is an SPI-1 effector protein involved in the enteritis pathway, with critical roles in inhibiting inflammation and apoptosis. In this work, we constructed an AvrA-FLAG-tagged strain of S. Enteritidis to analyse the expression profile of AvrA in vitro, in cell culture and in vivo. AvrA expression and secretion were observed in vitro under culture conditions that mimicked intestinal and intracellular environments. In agreement, bacteria isolated from infected cell monolayers expressed and translocated AvrA for at least 24 h post-inoculation. For in vivo experiments, BALB/c mice were inoculated by the natural route of infection with the AvrA-FLAG strain. Infecting bacteria and infected cells were recovered from mesenteric lymph nodes (MLN). Our results showed that AvrA continues to be synthesized in vivo up to day 8 post-inoculation. Moreover, AvrA translocation was detected in the cytosol of cells isolated from MLN 8 days after infection. Interestingly, we observed that AvrA is secreted by both type three secretion system (T3SS)-1 and T3SS-2. In summary, these findings indicate that AvrA expression is not constrained to the initial host-bacteria encounter in the intestinal environment as defined previously. The AvrA effector may participate also in systemic S. Enteritidis infection.

Type III secretion of ExoU is critical during early Pseudomonas aeruginosa pneumonia

The Pseudomonas aeruginosa type III secretion system has been associated with poor outcomes in both animal models and human patients. Despite a large number of studies exploring the regulation of type III secretion in vitro, little is known about the timing of secretion during mammalian infection. Here we demonstrate that the exoU gene, which encodes the highly cytotoxic type III effector ExoU, is induced early during acute P. aeruginosa pneumonia. Immunofluorescence microscopy indicated that the amount of ExoU protein in the lung also increased over time. The importance of early expression was examined using a strain of P. aeruginosa with inducible production of ExoU. Delays in expression as short as 3 h led to reduced bacterial burdens in the lungs of mice and improved survival. Our results demonstrate that early expression of exoU is critical to bacterial survival during pneumonia and suggest that therapeutic interventions that delay ExoU secretion for even short periods of time may be efficacious.

Pseudomonas aeruginosa is a major contributor to the large numbers of health care-associated infections occurring annually, particularly for immunocompromised patients. Although this organism possesses many virulence factors, the type III secretion system plays an especially important role in both animal models and humans. This system forms a needle-like apparatus that injects toxins directly into eukaryotic cells. The most toxic protein secreted by this molecular machine is ExoU, which causes rapid cell death. In this study, we demonstrated that exoU was expressed and ExoU was produced early during acute pneumonia in a mouse model. Delaying expression of exoU by as little as 3 h enhanced clearance of bacteria and survival of infected mice. Our findings highlight the importance of understanding the regulation of virulence factor expression during infection when designing therapeutic strategies to inhibit the toxic effects of these proteins.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

The Salmonella type III secretion system inner rod protein PrgJ is partially folded

The type III secretion system (T3SS) is essential in the pathogenesis of many bacteria. The inner rod is important in the assembly of the T3SS needle complex. However, the atomic structure of the inner rod protein is currently unknown. Based on computational methods, others have suggested that the Salmonella inner rod protein PrgJ is highly helical, forming a folded 3 helix structure. Here we show by CD and NMR spectroscopy that the monomeric form of PrgJ lacks a tertiary structure, and the only well-structured part of PrgJ is a short α-helix at the C-terminal region from residues 65-82. Disruption of this helix by glycine or proline mutation resulted in defective assembly of the needle complex, rendering bacteria incapable of secreting effector proteins. Likewise, CD and NMR data for the Shigella inner rod protein MxiI indicate this protein lacks a tertiary structure as well. Our results reveal that the monomeric forms of the T3SS inner rod proteins are partially folded.

A Salmonella small non-coding RNA facilitates bacterial invasion and intracellular replication by modulating the expression of virulence factors

Small non-coding RNAs (sRNAs) that act as regulators of gene expression have been identified in all kingdoms of life, including microRNA (miRNA) and small interfering RNA (siRNA) in eukaryotic cells. Numerous sRNAs identified in Salmonella are encoded by genes located at Salmonella pathogenicity islands (SPIs) that are commonly found in pathogenic strains. Whether these sRNAs are important for Salmonella pathogenesis and virulence in animals has not been reported. In this study, we provide the first direct evidence that a pathogenicity island-encoded sRNA, IsrM, is important for Salmonella invasion of epithelial cells, intracellular replication inside macrophages, and virulence and colonization in mice. IsrM RNA is expressed in vitro under conditions resembling those during infection in the gastrointestinal tract. Furthermore, IsrM is found to be differentially expressed in vivo, with higher expression in the ileum than in the spleen. IsrM targets the mRNAs coding for SopA, a SPI-1 effector, and HilE, a global regulator of the expression of SPI-1 proteins, which are major virulence factors essential for bacterial invasion. Mutations in IsrM result in disregulation of expression of HilE and SopA, as well as other SPI-1 genes whose expression is regulated by HilE. Salmonella with deletion of isrM is defective in bacteria invasion of epithelial cells and intracellular replication/survival in macrophages. Moreover, Salmonella with mutations in isrM is attenuated in killing animals and defective in growth in the ileum and spleen in mice. Our study has shown that IsrM sRNA functions as a pathogenicity island-encoded sRNA directly involved in Salmonella pathogenesis in animals. Our results also suggest that sRNAs may represent a distinct class of virulence factors that are important for bacterial infection in vivo.

Regulated expression of virulence factors is essential for infection by human pathogens such as Salmonella. Small non-coding RNAs (sRNAs) that act as regulators of gene expression have been identified in all kingdoms of life, and many sRNAs in Salmonella are encoded by genes located at Salmonella pathogenicity islands commonly found in pathogenic strains. In this study, we demonstrated that a pathogenicity island-encoded sRNA directly targets the expression of both a global regulator of virulence genes as well as a specific virulence factor critical for Salmonella pathogenesis. The sRNA is important for Salmonella invasion of epithelial cells, replication inside macrophages, and virulence/colonization in mice, representing the first example of a pathogenicity island-encoded sRNA that is directly involved in Salmonella pathogenesis in vivo. Our study suggests that sRNA may function as a distinct class of virulence factors that significantly contribute to bacterial infection in vivo. Furthermore, our results raise the possibility of developing new strategies against bacterial infection by preventing the expression of regulatory sRNAs.

SopB effector protein of Salmonella Typhimurium is translocated in mesenteric lymph nodes during murine salmonellosis

Salmonella Typhimurium harbors two Salmonella pathogenicity islands (SPIs), each encoding a type three secretion system for virulence proteins. Although there is increasing evidence of postinvasion roles for SPI-1, it has been generally accepted that SPI-1 genes are downregulated following the invasion process. Here, we analyzed the expression and translocation of SopB in vitro, in cell culture and in vivo. To this end, a sopB-FLAG-tagged strain of Salmonella Typhimurium was obtained by epitope tagging. Tagged proteins were detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis and immunoblotting with anti-FLAG antibodies. SopB expression was observed in vitro under cultured conditions that mimic the intestinal niche and different intracellular environments. In agreement, bacteria isolated from infected monolayers expressed and translocated SopB for at least 24 h postinoculation. For in vivo experiments, BALB/c mice were inoculated intraperitoneally with the tagged strain of Salmonella Typhimurium. Infecting bacteria and infected cells were recovered from mesenteric lymph nodes. Our results showed that SopB continues to be synthesized in vivo during 5 days after inoculation. Interestingly, translocation of SopB was detected in the cytosol of cells isolated from lymph nodes 1 day after infection. Altogether, these findings indicate that the expression and translocation of SopB during Salmonella infection is not constrained to the initial host-bacteria encounter in the intestinal environment as defined previously.

Discovery of novel secreted virulence factors from Salmonella enterica serovar Typhimurium by proteomic analysis of culture supernatants

Salmonella enterica serovar Typhimurium is a leading cause of acute gastroenteritis throughout the world. This pathogen has two type III secretion systems (TTSS) encoded in Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2) that deliver virulence factors (effectors) to the host cell cytoplasm and are required for virulence. While many effectors have been identified and at least partially characterized, the full repertoire of effectors has not been catalogued. In this proteomic study, we identified effector proteins secreted into defined minimal medium designed to induce expression of the SPI-2 TTSS and its effectors. We compared the secretomes of the parent strain to those of strains missing essential (ssaK::cat) or regulatory (ΔssaL) components of the SPI-2 TTSS. We identified 20 known SPI-2 effectors. Excluding the translocon components SseBCD, all SPI-2 effectors were biased for identification in the ΔssaL mutant, substantiating the regulatory role of SsaL in TTS. To identify novel effector proteins, we coupled our secretome data with a machine learning algorithm (SIEVE, SVM-based identification and evaluation of virulence effectors) and selected 12 candidate proteins for further characterization. Using CyaA' reporter fusions, we identified six novel type III effectors and two additional proteins that were secreted into J774 macrophages independently of a TTSS. To assess their roles in virulence, we constructed nonpolar deletions and performed a competitive index analysis from intraperitoneally infected 129/SvJ mice. Six mutants were significantly attenuated for spleen colonization. Our results also suggest that non-type III secretion mechanisms are required for full Salmonella virulence.