The related effector proteins SopD and SopD2 from Salmonella enterica serovar Typhimurium contribute to virulence during systemic infection of mice

Functional Divergence of the Paralog Salmonella Effector Proteins SopD and SopD2 and Their Contributions to Infection

Salmonella enterica is a leading cause of bacterial food-borne illness in humans and is responsible for millions of cases annually. A critical strategy for the survival of this pathogen is the translocation of bacterial virulence factors termed effectors into host cells, which primarily function via protein-protein interactions with host proteins. The Salmonella genome encodes several paralogous effectors believed to have arisen from duplication events throughout the course of evolution. These paralogs can share structural similarities and enzymatic activities but have also demonstrated divergence in host cell targets or interaction partners and contributions to the intracellular lifecycle of Salmonella. The paralog effectors SopD and SopD2 share 63% amino acid sequence similarity and extensive structural homology yet have demonstrated divergence in secretion kinetics, intracellular localization, host targets, and roles in infection. SopD and SopD2 target host Rab GTPases, which represent critical regulators of intracellular trafficking that mediate diverse cellular functions. While SopD and SopD2 both manipulate Rab function, these paralogs display differences in Rab specificity, and the effectors have also evolved multiple mechanisms of action for GTPase manipulation. Here, we highlight this intriguing pair of paralog effectors in the context of host-pathogen interactions and discuss how this research has presented valuable insights into effector evolution.

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.

Genomic and phenotypic characterization of Salmonella enterica serovar Kentucky

Non-typhoidal Salmonella are extremely diverse and different serovars can exhibit varied phenotypes, including host adaptation and the ability to cause clinical illness in animals and humans. In the USA, Salmonella enterica serovar Kentucky is infrequently found to cause human illness, despite being the top serovar isolated from broiler chickens. Conversely, in Europe, this serovar falls in the top 10 serovars linked to human salmonellosis. Serovar Kentucky is polyphyletic and has two lineages, Kentucky-I and Kentucky-II; isolates belonging to Kentucky-I are frequently isolated from poultry in the USA, while Kentucky-II isolates tend to be associated with human illness. In this study, we analysed whole-genome sequences and associated metadata deposited in public databases between 2017 and 2021 by federal agencies to determine serovar Kentucky incidence across different animal and human sources. Of 5151 genomes, 90.3 % were from isolates that came from broilers, while 5.9 % were from humans and 3.0 % were from cattle. Kentucky-I isolates were associated with broilers, while isolates belonging to Kentucky-II and a new lineage, Kentucky-III, were more commonly associated with cattle and humans. Very few serovar Kentucky isolates were associated with turkey and swine sources. Phylogenetic analyses showed that Kentucky-III genomes were more closely related to Kentucky-I, and this was confirmed by CRISPR-typing and multilocus sequence typing (MLST). In a macrophage assay, serovar Kentucky-II isolates were able to replicate over eight times better than Kentucky-I isolates. Analysis of virulence factors showed unique patterns across these three groups, and these differences may be linked to their association with different hosts.

Speaking the host language: how Salmonella effector proteins manipulate the host

Salmonella injects over 40 virulence factors, termed effectors, into host cells to subvert diverse host cellular processes. Of these 40 Salmonella effectors, at least 25 have been described as mediating eukaryotic-like, biochemical post-translational modifications (PTMs) of host proteins, altering the outcome of infection. The downstream changes mediated by an effector's enzymatic activity range from highly specific to multifunctional, and altogether their combined action impacts the function of an impressive array of host cellular processes, including signal transduction, membrane trafficking, and both innate and adaptive immune responses. Salmonella and related Gram-negative pathogens have been a rich resource for the discovery of unique enzymatic activities, expanding our understanding of host signalling networks, bacterial pathogenesis as well as basic biochemistry. In this review, we provide an up-to-date assessment of host manipulation mediated by the Salmonella type III secretion system injectosome, exploring the cellular effects of diverse effector activities with a particular focus on PTMs and the implications for infection outcomes. We also highlight activities and functions of numerous effectors that remain poorly characterized.

Pathogen genomics and phage-based solutions for accurately identifying and controlling Salmonella pathogens

Salmonella is a food-borne pathogen often linked to poultry sources, causing gastrointestinal infections in humans, with the numbers of multidrug resistant (MDR) isolates increasing globally. To gain insight into the genomic diversity of common serovars and their potential contribution to disease, we characterized antimicrobial resistance genes, and virulence factors encoded in 88 UK and 55 Thai isolates from poultry; the presence of virulence genes was detected through an extensive virulence determinants database compiled in this study. Long-read sequencing of three MDR isolates, each from a different serovar, was used to explore the links between virulence and resistance. To augment current control methods, we determined the sensitivity of isolates to 22 previously characterized Salmonella bacteriophages. Of the 17 serovars included, Salmonella Typhimurium and its monophasic variants were the most common, followed by S. Enteritidis, S. Mbandaka, and S. Virchow. Phylogenetic analysis of Typhumurium and monophasic variants showed poultry isolates were generally distinct from pigs. Resistance to sulfamethoxazole and ciprofloxacin was highest in isolates from the UK and Thailand, respectively, with 14-15% of all isolates being MDR. We noted that >90% of MDR isolates were likely to carry virulence genes as diverse as the srjF, lpfD, fhuA, and stc operons. Long-read sequencing revealed the presence of global epidemic MDR clones in our dataset, indicating they are possibly widespread in poultry. The clones included MDR ST198 S. Kentucky, harboring a Salmonella Genomic Island-1 (SGI)-K, European ST34 S. 1,4,[5],12:i:-, harboring SGI-4 and mercury-resistance genes, and a S. 1,4,12:i:- isolate from the Spanish clone harboring an MDR-plasmid. Testing of all isolates against a panel of bacteriophages showed variable sensitivity to phages, with STW-77 found to be the most effective. STW-77 lysed 37.76% of the isolates, including serovars important for human clinical infections: S. Enteritidis (80.95%), S. Typhimurium (66.67%), S. 1,4,[5],12:i:- (83.3%), and S. 1,4,12: i:- (71.43%). Therefore, our study revealed that combining genomics and phage sensitivity assays is promising for accurately identifying and providing biocontrols for Salmonella to prevent its dissemination in poultry flocks and through the food chain to cause infections in humans.

A nontyphoidal Salmonella serovar domestication accompanying enhanced niche adaptation

Invasive nontyphoidal Salmonella (iNTS) causes extraintestinal infections with ~15% case fatality in many countries. However, the mechanism by which iNTS emerged in China remains unaddressed. We conducted clinical investigations of iNTS infection with recurrent treatment failure, caused by underreported Salmonella enterica serovar Livingstone (SL). Genomic epidemiology demonstrated five clades in the SL population and suggested that the international animal feed trade was a likely vehicle for their introduction into China, as evidenced by multiple independent transmission incidents. Importantly, isolates from Clade-5-I-a/b, predominant in China, showed an invasive nature in mice, chicken and zebrafish infection models. The antimicrobial susceptibility testing revealed most isolates (> 96%) in China are multidrug-resistant (MDR). Overall, we offer exploiting genomics in uncovering international transmission led by the animal feed trade and highlight an emerging hypervirulent clade with increased resistance to frontline antibiotics.

Endomembrane remodeling and dynamics in Salmonella infection

Salmonellae are bacteria that cause moderate to severe infections in humans, depending on the strain and the immune status of the infected host. These pathogens have the particularity of residing in the cells of the infected host. They are usually found in a vacuolar compartment that the bacteria shape with the help of effector proteins. Following invasion of a eukaryotic cell, the bacterial vacuole undergoes maturation characterized by changes in localization, composition and morphology. In particular, membrane tubules stretching over the microtubule cytoskeleton are formed from the bacterial vacuole. Although these tubules do not occur in all infected cells, they are functionally important and promote intracellular replication. This review focuses on the role and significance of membrane compartment remodeling observed in infected cells and the bacterial and host cell pathways involved.

Quantitative proteomic screen identifies annexin A2 as a host target for Salmonella pathogenicity island-2 effectors SopD2 and PipB2

Intracellular pathogens need to establish an intracellular replicative niche to promote survival and replication within the hostile environment inside the host cell. Salmonella enterica serovar Typhimurium (S. Typhimurium) initiates formation of the unique Salmonella-containing vacuole and an extensive network of Salmonella-induced tubules in order to survive and thrive within host cells. At least six effectors secreted by the type III secretion system encoded within Salmonella pathogenicity island-2 (SPI-2), namely SifA, SopD2, PipB2, SteA, SseJ, and SseF, purportedly manipulate host cell intracellular trafficking and establish the intracellular replicative niche for S. Typhimurium. The phenotypes of these effectors are both subtle and complex, complicating elucidation of the mechanism underpinning host cell manipulation by S. Typhimurium. In this work we used stable isotope labeling of amino acids in cell culture (SILAC) and a S. Typhimurium mutant that secretes increased amounts of effectors to identify cognate effector binding partners during infection. Using this method, we identified the host protein annexin A2 (AnxA2) as a binding partner for both SopD2 and PipB2 and were able to confirm its binding to SopD2 and PipB2 by reciprocal pull down, although there was a low level of non-specific binding of SopD2-2HA and PipB2-2HA to the Ni-Sepharose beads present. We further showed that knockdown of AnxA2 altered the intracellular positioning of the Salmonella containing vacuole (SCV). This suggests that AnxA2 plays a role in the subcellular positioning of the SCV which could potentially be mediated through protein–protein interactions with either SopD2 or PipB2. This demonstrates the value of studying effector interactions using proteomic techniques and natural effector delivery during infection rather than transfection.

The Salmonella effector SifA initiates a kinesin-1 and kinesin-3 recruitment process mirroring that mediated by Arl8a/b

When intracellular, pathogenic Salmonella reside in a membrane compartment composed of interconnected vacuoles and tubules, the formation of which depends on the translocation of bacterial effectors into the host cell. Cytoskeletons and their molecular motors are prime targets for these effectors. In this study, we show that the microtubule molecular motor KIF1Bß, a member of the kinesin-3 family, is a key element for the establishment of the Salmonella replication niche as its absence is detrimental to the stability of bacterial vacuoles and the formation of associated tubules. Kinesin-3 interacts with the Salmonella effector SifA but also with SKIP, a host protein complexed to SifA. The interaction with SifA is essential for the recruitment of kinesin-3 on Salmonella vacuoles while that with SKIP is incidental. In the non-infectious context, however, the interaction with SKIP is essential for the recruitment and activity of kinesin-3 on a part of lysosomes. Finally, our results show that in infected cells, the presence of SifA establishes a kinesin-1 and kinesin-3 recruitment pathway that is analogous to and functions independently of that mediated by the Arl8a/b GTPases.

Genetic Determinants of Stress Resistance in Desiccated Salmonella enterica

Enteric pathogens, including Salmonella, are capable of long-term survival after desiccation and resist heat treatments that are lethal to hydrated cells. The mechanisms of dry-heat resistance differ from those of wet-heat resistance. To elucidate the mechanisms of dry-heat resistance in Salmonella, screening of the dry-heat resistance of 108 Salmonella strains, representing 39 serotypes, identified the 22 most resistant and the 8 most sensitive strains for comparative genome analysis. A total of 289 genes of the accessory genome were differently distributed between resistant and sensitive strains. Among these genes, 28 proteins with a putative relationship to stress resistance were selected for to quantify relative gene expression before and after desiccation and expression by solid-state cultures on agar plates relative to cultures growing in liquid culture media. Of these 28 genes, 15 genes were upregulated (P < 0.05) after desiccation or by solid-state cultures on agar plates. These 15 genes were cloned into the low-copy-number vector pRK767 under the control of the lacZ promoter. The expression of 6 of these 15 genes increased (P < 0.05) resistance to dry heat and to treatment with pressure of 500 MPa. Our finding extends the knowledge of mechanisms of stress resistance in desiccated Salmonella to improve control of this bacterium in dry food. IMPORTANCE This study directly targeted an increasing threat to food safety and developed knowledge and targeted strategies that can be used by the food industry to help reduce the risk of foodborne illness in their dry products and thereby reduce the overall burden of foodborne illness. Genomic and physiological analyses have elucidated mechanisms of bacterial resistance to many food preservation technologies, including heat, pressure, disinfection chemicals, and UV light; however, information on bacterial mechanisms of resistance to dry heat is scarce. Mechanisms of tolerance to desiccation likely also contribute to resistance to dry heat, but this assumption has not been verified experimentally. It remains unclear how mechanisms of resistance to wet heat relate to dry-heat resistance. Thus, this study will fill a knowledge gap to improve the safety of dry foods.

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 effector SopD promotes plasma membrane scission by inhibiting Rab10

Salmonella utilizes translocated virulence proteins (termed effectors) to promote host cell invasion. The effector SopD contributes to invasion by promoting scission of the plasma membrane, generating Salmonella-containing vacuoles. SopD is expressed in all Salmonella lineages and plays important roles in animal models of infection, but its host cell targets are unknown. Here we show that SopD can bind to and inhibit the small GTPase Rab10, through a C-terminal GTPase activating protein (GAP) domain. During infection, Rab10 and its effectors MICAL-L1 and EHBP1 are recruited to invasion sites. By inhibiting Rab10, SopD promotes removal of Rab10 and recruitment of Dynamin-2 to drive scission of the plasma membrane. Together, our study uncovers an important role for Rab10 in regulating plasma membrane scission and identifies the mechanism used by a bacterial pathogen to manipulate this function during infection.

Small Noncoding RNAs and Their Role in the Pathogenesis of Mycobacterium tuberculosis Infection

Mycobacterium tuberculosis possesses a significant arsenal of strategies to combat immune defense of the host organism. Small noncoding RNAs, which constitute the largest group of regulatory RNAs, play an important role in the host–pathogen interactions and represent one of the levels of the regulation of interactions of microbial cells with their environment. The regulatory role of small RNAs in pathogenic bacteria is essential when rapid adaptation to the changing environmental conditions with further synchronization of metabolic reactions are required to ensure microbial survival and infection progression. During the past few years, eight small RNAs from M. tuberculosis have been functionally characterized, and targets for four of them have been identified. Small RNAs from M. tuberculosis and other pathogenic microorganisms were found to be one of the most important functional factors in the adaptive response to changing environmental conditions.

SopD from Salmonella specifically inactivates Rab8

Salmonella outer protein D (SopD) is secreted into a host during the first stages of the Salmonella infection and contributes to the systemic virulence of the bacterium. SopD2 is a SopD homolog and possesses GTPase activating protein (GAP) activity towards Rab32. Here, we identified Rab-proteins as putative SopD-targets using a yeast two-hybrid approach. In vitro investigations subsequently revealed Rab8a as an exclusive SopD substrate in contrast to SopD2, which has a broader specificity targeting Rab29, Rab32 and Rab38 in vitro. Additionally, we determined the catalytic efficiencies of SopD and SopD2 towards their physiologically relevant substrates. Moreover, mutagenesis studies provided insights into possible key residues of the Rab-protein and the GAP involved in the conversion of active to inactive GTPase. In conclusion, we demonstrate that Salmonella SopD and SopD2 act as RabGAPs and can inactivate Rab signaling.

Proteomics of host-bacterial interactions: new insights from dual perspectives

Mass-spectrometry (MS)-based proteomics is a powerful and robust platform for studying the interactions between biological systems during health and disease. Bacterial infections represent a significant threat to global health and drive the pursuit of novel therapeutic strategies to combat emerging and resistant pathogens. During infection, the interplay between a host and pathogen determines the ability of the microbe to survive in a hostile environment and promotes an immune response by the host as a protective measure. It is the protein-level changes from either biological system that define the outcome of infection, and MS-based proteomics provides a rapid and effective platform to identify such changes. In particular, proteomics detects alterations in protein abundance, quantifies protein secretion and (or) release, measures an array of post-translational modifications that influence signaling cascades, and profiles protein-protein interactions through protein complex and (or) network formation. Such information provides new insight into the role of known and novel bacterial effectors, as well as the outcome of host cell activation. In this Review, we highlight the diverse applications of MS-based proteomics in profiling the relationship between bacterial pathogens and the host. Our work identifies a plethora of strategies for exploring mechanisms of infection from dual perspectives (i.e., host and pathogen), and we suggest opportunities to extrapolate the current knowledgebase to other biological systems for applications in therapeutic discovery.

Multiple Salmonella-pathogenicity island 2 effectors are required to facilitate bacterial establishment of its intracellular niche and virulence

The pathogenesis of Salmonella Typhimurium depends on the bacterium’s ability to survive and replicate within host cells. The formation and maintenance of a unique membrane-bound compartment, termed the Salmonella-containing vacuole (SCV), is essential for S. Typhimurium pathogenesis. SCV-bound S. Typhimurium induces formation of filamentous tubules that radiate outwards from the SCV, termed Salmonella-induced filaments (SIFs). SIF formation is concomitant with the onset of replication within host epithelial cells. SIF biogenesis, formation and maintenance of the SCV, and the intracellular positioning of the SCV within the host cell requires translocation of bacterial proteins (effectors) into the host cell. Effectors secreted by the type III secretion system encoded on Salmonella pathogenicity island 2 (T3SS2) function to interfere with host cellular processes and promote both intracellular survival and replication of S. Typhimurium. Seven T3SS2-secreted effectors, SifA, SopD2, PipB2, SteA, SseJ, SseF, and SseG have previously been implicated to play complementary, redundant, and/or antagonistic roles with respect to SIF biogenesis, intracellular positioning of the SCV, and SCV membrane dynamics modulation during infection. We undertook a systematic study to delineate the contribution of each effector to these processes by (i) deleting all seven of these effectors in a single S. Typhimurium strain; and (ii) deleting combinations of multiple effectors based on putative effector function. Using this deletion mutant library, we show that each of SIF biogenesis, intracellular SCV localization, intramacrophage replication, colonization, and virulence depends on the activities of multiple effectors. Together, our data demonstrates the complex interplay between these seven effectors and highlights the necessity to study T3SS2-secreted effectors as groups, rather than studies of individual effectors.

Global priority pathogens: virulence, antimicrobial resistance and prospective treatment options

Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp. and Salmonella spp. are part of a group of pathogens that pose a major threat to human health due to the emergence of multidrug-resistant strains. Moreover, these bacteria have several virulence factors that allow them to successfully colonize their hosts, such as toxins and the ability to produce biofilms, resulting in an urgent need to develop new strategies to fight these pathogens. In this review, we compile the most up-to-date information on the epidemiology, virulence and resistance of these clinically important microorganisms. Additionally, we address new therapeutic alternatives, with a focus on molecules with antivirulence activity, which are considered promising to combat multidrug-resistant bacteria.

BioID screen of Salmonella type 3 secreted effectors reveals host factors involved in vacuole positioning and stability during infection

Many bacterial pathogens express virulence proteins that are translocated into host cells (herein referred to as effectors), where they can interact with target proteins to manipulate host cell processes. These effector-host protein interactions are often dynamic and transient in nature, making them difficult to identify using traditional interaction-based methods. Here, we performed a systematic comparison between proximity-dependent biotin labelling (BioID) and immunoprecipitation coupled with mass spectrometry to investigate a series of Salmonella type 3 secreted effectors that manipulate host intracellular trafficking (SifA, PipB2, SseF, SseG and SopD2). Using BioID, we identified 632 candidate interactions with 381 unique human proteins, collectively enriched for roles in vesicular trafficking, cytoskeleton components and transport activities. From the subset of proteins exclusively identified by BioID, we report that SifA interacts with BLOC-2, a protein complex that regulates dynein motor activity. We demonstrate that the BLOC-2 complex is necessary for SifA-mediated positioning of Salmonella-containing vacuoles, and affects stability of the vacuoles during infection. Our study provides insight into the coordinated activities of Salmonella type 3 secreted effectors and demonstrates the utility of BioID as a powerful, complementary tool to characterize effector-host protein interactions.

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.

Targeted inactivation of Salmonella Agona metabolic genes by group II introns and in vivo assessment of pathogenicity and anti-tumour activity in mouse model

The fight against cancer has been a never-ending battle. Limitations of conventional therapies include lack of selectivity, poor penetration and highly toxic to the host. Using genetically modified bacteria as a tumour therapy agent has gained the interest of scientist from the past few decades. Low virulence and highly tolerability of Salmonella spp. in animals and humans make it as the most studied pathogen with regards to anti-tumour therapy. The present study aims to construct a genetically modified S. Agona auxotroph as an anti-tumour agent. LeuB and ArgD metabolic genes in ΔSopBΔSopD double knockout S. Agona were successfully knocked out using a Targetron gene knockout system. The knockout was confirmed by colony PCR and the strains were characterized in vitro and in vivo. The knockout of metabolic genes causes significant growth defect in M9 minimal media. Quadruple knockout ΔSopBΔSopDΔLeuBΔArgD (BDLA) exhibited lowest virulence among all of the strains in all parameters including bacterial load, immunity profile and histopathology studies. In vivo anti-tumour study on colorectal tumour bearing-BALB/c mice revealed that all strains of S. Agona were able to suppress the growth of the large solid tumour as compared with negative control and ΔLeuBΔArgD (LA) and BDLA auxotroph showed better efficacy. Interestingly, higher level of tumour growth suppression was noticed in large tumour. However, multiple administration of bacteria dosage did not increase the tumour suppression efficacy. In this study, the virulence of BDLA knockout strain was slightly reduced and tumour growth suppression efficacy was successfully enhanced, which provide a valuable starting point for the development of S. Agona as anti-tumour agent.

Novel Effector Protein EspY3 of Type III Secretion System from Enterohemorrhagic Escherichia coli Is Localized in Actin Pedestals

Enterohemorrhagic Escherichia coli (EHEC) and enteropathogenic Escherichia coli (EPEC) are attaching and effacing (A/E) pathogens, which translocate effector proteins to intestinal enterocytes through a type III secretion system (T3SS). T3SS and most of its effector proteins are encoded in a pathogenicity island called LEE. Recently, new effectors have been located outside the LEE. This study aimed to characterize EspY3, a novel non-LEE encoded T3SS effector of EHEC. EspY3 shares homology with SopD and PipB2 effector proteins of Salmonella's T3SS-1 and T3SS-2, respectively. The presence of recombinant EspY3 in the supernatant samples demonstrated that EspY3 was secreted by the T3SS of EHEC and EPEC. Through infection assays, we demonstrated the translocation of EspY3 into Caco-2 cells by T3SS of EPEC. The subcellular localization of EspY3 was determined in the pedestal region, where its presence generates a significant increase in the size of the pedestals area. The EspY3 effector induced the elongation of polymerized actin pedestals in infected Caco-2 by EPEC. This study confirmed that EspY3 is part of the repertoire of T3SS effectors of EHEC O157:H7, and that it participates in modeling cellular actin during the infection.

Quantitative Yeast Genetic Interaction Profiling of Bacterial Effector Proteins Uncovers a Role for the Human Retromer in Salmonella Infection

Intracellular bacterial pathogens secrete a repertoire of effector proteins into host cells that are required to hijack cellular pathways and cause disease. Despite decades of research, the molecular functions of most bacterial effectors remain unclear. To address this gap, we generated quantitative genetic interaction profiles between 36 validated and putative effectors from three evolutionarily divergent human bacterial pathogens and 4,190 yeast deletion strains. Correlating effector-generated profiles with those of yeast mutants, we recapitulated known biology for several effectors with remarkable specificity and predicted previously unknown functions for others. Biochemical and functional validation in human cells revealed a role for an uncharacterized component of the Salmonella SPI-2 translocon, SseC, in regulating maintenance of the Salmonella vacuole through interactions with components of the host retromer complex. These results exhibit the power of genetic interaction profiling to discover and dissect complex biology at the host-pathogen interface.

General response of Salmonella enterica serovar Typhimurium to desiccation: A new role for the virulence factors sopD and sseD in survival

Salmonella can survive for long periods under extreme desiccation conditions. This stress tolerance poses a risk for food safety, but relatively little is known about the molecular and cellular regulation of this adaptation mechanism. To determine the genetic components involved in Salmonella’s cellular response to desiccation, we performed a global transcriptomic analysis comparing S. enterica serovar Typhimurium cells equilibrated to low water activity (a_w 0.11) and cells equilibrated to high water activity (a_w 1.0). The analysis revealed that 719 genes were differentially regulated between the two conditions, of which 290 genes were up-regulated at a_w 0.11. Most of these genes were involved in metabolic pathways, transporter regulation, DNA replication/repair, transcription and translation, and, more importantly, virulence genes. Among these, we decided to focus on the role of sopD and sseD. Deletion mutants were created and their ability to survive desiccation and exposure to a_w 0.11 was compared to the wild-type strain and to an E. coli O157:H7 strain. The sopD and sseD mutants exhibited significant cell viability reductions of 2.5 and 1.3 Log (CFU/g), respectively, compared to the wild-type after desiccation for 4 days on glass beads. Additional viability differences of the mutants were observed after exposure to a_w 0.11 for 7 days. E. coli O157:H7 lost viability similarly to the mutants. Scanning electron microscopy showed that both mutants displayed a different morphology compared to the wild-type and differences in production of the extracellular matrix under the same conditions. These findings suggested that sopD and sseD are required for Salmonella’s survival during desiccation.

Identification of a Novel Salmonella Type III Effector by Quantitative Secretome Profiling

Salmonella enterica serovar Typhimurium is arguably one of the most studied bacterial pathogens and successful infection requires the delivery of its virulence factors (effectors) directly into host cells via the type III secretion systems (T3SSs). Central to Salmonella pathogenesis, these effector proteins have been subjected to extensive studies over the years. Nevertheless, whether additional effectors exist remains unclear. Here we report the identification of a novel Salmonella T3SS effector STM1239 (which we renamed SopF) via quantitative secretome profiling. Immunoblotting and β-lactamase reporter assays confirmed the secretion and translocation of SopF in a T3SS-dependent manner. Moreover, ectopic expression of SopF caused significant toxicity in yeast cells. Importantly, genetic ablation of sopF led to Salmonella strains defective in intracellular replication within macrophages and the mutant were also markedly attenuated in a mouse model of infection. Our study underscores the use of quantitative secretome profiling in identifying novel virulence factors for bacterial pathogens.

A second wave of Salmonella T3SS1 activity prolongs the lifespan of infected epithelial cells

Type III secretion system 1 (T3SS1) is used by the enteropathogen Salmonella enterica serovar Typhimurium to establish infection in the gut. Effector proteins translocated by this system across the plasma membrane facilitate invasion of intestinal epithelial cells. One such effector, the inositol phosphatase SopB, contributes to invasion and mediates activation of the pro-survival kinase Akt. Following internalization, some bacteria escape from the Salmonella-containing vacuole into the cytosol and there is evidence suggesting that T3SS1 is expressed in this subpopulation. Here, we investigated the post-invasion role of T3SS1, using SopB as a model effector. In cultured epithelial cells, SopB-dependent Akt phosphorylation was observed at two distinct stages of infection: during and immediately after invasion, and later during peak cytosolic replication. Single cell analysis revealed that cytosolic Salmonella deliver SopB via T3SS1. Although intracellular replication was unaffected in a SopB deletion mutant, cells infected with ΔsopB demonstrated a lack of Akt phosphorylation, earlier time to death, and increased lysis. When SopB expression was induced specifically in cytosolic Salmonella, these effects were restored to levels observed in WT infected cells, indicating that the second wave of SopB protects this infected population against cell death via Akt activation. Thus, T3SS1 has two, temporally distinct roles during epithelial cell colonization. Additionally, we found that delivery of SopB by cytosolic bacteria was translocon-independent, in contrast to canonical effector translocation across eukaryotic membranes, which requires formation of a translocon pore. This mechanism was also observed for another T3SS1 effector, SipA. These findings reveal the functional and mechanistic adaptability of a T3SS that can be harnessed in different microenvironments.

Salmonella effector SopD2 interferes with Rab34 function

Many intracellular pathogens have evolved highly specialized mechanisms to isolate themselves from the host cell's innate immune response while still obtaining the necessary nutrients to survive. Salmonella utilizes type 3 secretion systems (T3SSs) to deliver bacterial proteins called effectors, across the encompassing Salmonella Containing vacuole (SCV) membrane, to subvert the host's membrane trafficking pathways and alter other cellular processes. The Salmonella Pathogenicity Island (SPI)-2 effector SopD2 has recently been demonstrated to modulate multiple members of the Rab GTPase family such as Rab7, Rab8, Rab10, and Rab32 (D'Costa et al., , Cell Reports, 12:1508-18; Spano et al., , Cell Host & Microbe, 19:216-26). Here, we demonstrate the additional capacity of SopD2 to bind Rab34 and modulate its function. Our data indicate that depletion of Rab34 delays maturation of the SCV, and consequently, inhibits intracellular Salmonella enterica serotype typhimurium (S. typhimurium) growth. Interestingly, intracellular growth of the S. typhimurium lacking SopD2 was severely impaired in Rab34-depleted cells, suggesting a compounding virulence effect. Overall this study reveals an additional member of the Rab GTPase family, Rab34, that is modulated by SopD2 and provides insight into its role in Salmonella biology.

Effector proteins support the asymmetric apportioning of Salmonella during cytokinesis

Salmonella-infected cells are characterized by the presence of intra-cellular membranous tubules that emerge from bacterial vacuoles and extend along microtubules. The formation of Salmonella-induced tubules depends on the Salmonella pathogenicity island 2-encoded type III secretion system (T3SS-2) that translocates bacterial effector proteins inside host cells. Effector proteins have enzymatic activities or allow for hijacking of cellular functions. The role of Salmonella-induced tubules in virulence remains unclear but their absence is correlated with virulence defects. This study describes the presence of inter-cellular tubules that arise between daughter cells during cytokinesis. Inter-cellular tubules connect bacterial vacuoles originally present in the parent cell and that have been apportioned between daughters. Their formation requires a functional T3SS-2 and effector proteins. Our data establish a correlation between the formation of inter-cellular tubules and the asymmetric distribution of bacterial vacuoles in daughters. Thus, by manipulating the distribution of bacteria in cytokinetic cells, Salmonella T3SS-2 effector proteins may increase bacterial spreading and the systemic character of the infection.

Small RNAs Regulate Primary and Secondary Metabolism in Gram-negative Bacteria

Over the last decade, small (often noncoding) RNA molecules have been discovered as important regulators influencing myriad aspects of bacterial physiology and virulence. In particular, small RNAs (sRNAs) have been implicated in control of both primary and secondary metabolic pathways in many bacterial species. This chapter describes characteristics of the major classes of sRNA regulators, and highlights what is known regarding their mechanisms of action. Specific examples of sRNAs that regulate metabolism in gram-negative bacteria are discussed, with a focus on those that regulate gene expression by base pairing with mRNA targets to control their translation and stability.

Salmonellae interactions with host processes

Salmonellae invasion and intracellular replication within host cells result in a range of diseases, including gastroenteritis, bacteraemia, enteric fever and focal infections. In recent years, considerable progress has been made in our understanding of the molecular mechanisms that salmonellae use to alter host cell physiology; through the delivery of effector proteins with specific activities and through the modulation of defence and stress response pathways. In this Review, we summarize our current knowledge of the complex interplay between bacterial and host factors that leads to inflammation, disease and, in most cases, control of the infection by its animal hosts, with a particular focus on Salmonella enterica subsp. enterica serovar Typhimurium. We also highlight gaps in our knowledge of the contributions of salmonellae and the host to disease pathogenesis, and we suggest future avenues for further study.

Small RNA functions in carbon metabolism and virulence of enteric pathogens

Enteric pathogens often cycle between virulent and saprophytic lifestyles. To endure these frequent changes in nutrient availability and composition bacteria possess an arsenal of regulatory and metabolic genes allowing rapid adaptation and high flexibility. While numerous proteins have been characterized with regard to metabolic control in pathogenic bacteria, small non-coding RNAs have emerged as additional regulators of metabolism. Recent advances in sequencing technology have vastly increased the number of candidate regulatory RNAs and several of them have been found to act at the interface of bacterial metabolism and virulence factor expression. Importantly, studying these riboregulators has not only provided insight into their metabolic control functions but also revealed new mechanisms of post-transcriptional gene control. This review will focus on the recent advances in this area of host-microbe interaction and discuss how regulatory small RNAs may help coordinate metabolism and virulence of enteric pathogens.

The small RNA SgrS: roles in metabolism and pathogenesis of enteric bacteria

Bacteria adapt to ever-changing habitats through specific responses to internal and external stimuli that result in changes in gene regulation and metabolism. One internal metabolic cue affecting such changes in Escherichia coli and related enteric species is cytoplasmic accumulation of phosphorylated sugars such as glucose-6-phosphate or the non-metabolizable analog α-methylglucoside-6-phosphate. This “glucose-phosphate stress” triggers a dedicated stress response in γ-proteobacteria including several enteric pathogens. The major effector of this stress response is a small RNA (sRNA), SgrS. In E. coli and Salmonella, SgrS regulates numerous mRNA targets via base pairing interactions that result in alterations in mRNA translation and stability. Regulation of target mRNAs allows cells to reduce import of additional sugars and increase sugar efflux. SgrS is an unusual sRNA in that it also encodes a small protein, SgrT, which inhibits activity of the major glucose transporter. The two functions of SgrS, base pairing and production of SgrT, reduce accumulation of phosphorylated sugars and thereby relieve stress and promote growth. Examination of SgrS homologs in many enteric species suggests that SgrS has evolved to regulate distinct targets in different organisms. For example, in Salmonella, SgrS base pairs with sopD mRNA and represses production of the encoded effector protein, suggesting that SgrS may have a specific role in the pathogenesis of some γ-proteobacteria. In this review, we outline molecular mechanisms involved in SgrS regulation of its target mRNAs. We also discuss the response to glucose-phosphate stress in terms of its impact on metabolism, growth physiology, and pathogenesis.

Salmonella enterica serovar Typhimurium skills to succeed in the host: virulence and regulation

Salmonella enterica serovar Typhimurium is a primary enteric pathogen infecting both humans and animals. Infection begins with the ingestion of contaminated food or water so that salmonellae reach the intestinal epithelium and trigger gastrointestinal disease. In some patients the infection spreads upon invasion of the intestinal epithelium, internalization within phagocytes, and subsequent dissemination. In that case, antimicrobial therapy, based on fluoroquinolones and expanded-spectrum cephalosporins as the current drugs of choice, is indicated. To accomplish the pathogenic process, the Salmonella chromosome comprises several virulence mechanisms. The most important virulence genes are those located within the so-called Salmonella pathogenicity islands (SPIs). Thus far, five SPIs have been reported to have a major contribution to pathogenesis. Nonetheless, further virulence traits, such as the pSLT virulence plasmid, adhesins, flagella, and biofilm-related proteins, also contribute to success within the host. Several regulatory mechanisms which synchronize all these elements in order to guarantee bacterial survival have been described. These mechanisms govern the transitions from the different pathogenic stages and drive the pathogen to achieve maximal efficiency inside the host. This review focuses primarily on the virulence armamentarium of this pathogen and the extremely complicated regulatory network controlling its success.

Diverse secreted effectors are required for Salmonella persistence in a mouse infection model

Salmonella enterica serovar Typhimurium causes typhoid-like disease in mice and is a model of typhoid fever in humans. One of the hallmarks of typhoid is persistence, the ability of the bacteria to survive in the host weeks after infection. Virulence factors called effectors facilitate this process by direct transfer to the cytoplasm of infected cells thereby subverting cellular processes. Secretion of effectors to the cell cytoplasm takes place through multiple routes, including two separate type III secretion (T3SS) apparati as well as outer membrane vesicles. The two T3SS are encoded on separate pathogenicity islands, SPI-1 and -2, with SPI-1 more strongly associated with the intestinal phase of infection, and SPI-2 with the systemic phase. Both T3SS are required for persistence, but the effectors required have not been systematically evaluated. In this study, mutations in 48 described effectors were tested for persistence. We replaced each effector with a specific DNA barcode sequence by allelic exchange and co-infected with a wild-type reference to calculate the ratio of wild-type parent to mutant at different times after infection. The competitive index (CI) was determined by quantitative PCR in which primers that correspond to the barcode were used for amplification. Mutations in all but seven effectors reduced persistence demonstrating that most effectors were required. One exception was CigR, a recently discovered effector that is widely conserved throughout enteric bacteria. Deletion of cigR increased lethality, suggesting that it may be an anti-virulence factor. The fact that almost all Salmonella effectors are required for persistence argues against redundant functions. This is different from effector repertoires in other intracellular pathogens such as Legionella.

Identification of salmonella pathogenicity island-2 type III secretion system effectors involved in intramacrophage replication of S. enterica serovar typhimurium: implications for rational vaccine design

Salmonella enterica serovars cause severe diseases in humans, such as gastroenteritis and typhoid fever. The development of systemic disease is dependent on a type III secretion system (T3SS) encoded by Salmonella pathogenicity island-2 (SPI-2). Translocation of effector proteins across the Salmonella-containing vacuole, via the SPI-2 T3SS, enables bacterial replication within host cells, including macrophages. Here, we investigated the contribution of these effectors to intramacrophage replication of Salmonella enterica serovar Typhimurium using Fluorescence Dilution, a dual-fluorescence tool which allows direct measurement of bacterial replication. Of 32 strains, each carrying single mutations in genes encoding effectors, 10 (lacking sifA, sseJ, sopD2, sseG, sseF, srfH, sseL, spvD, cigR, or steD) were attenuated in replication in mouse bone marrow-derived macrophages. The replication profiles of strains combining deletions in effector genes were also investigated: a strain lacking the genes sseG, sopD2, and srfH showed an increased replication defect compared to single-mutation strains and was very similar to SPI-2 T3SS-deficient bacteria with respect to its replication defect. This strain was substantially attenuated in virulence in vivo and yet retained intracellular vacuole integrity and a functional SPI-2 T3SS. Moreover, this strain was capable of SPI-2 T3SS-mediated delivery of a model antigen for major histocompatibility complex (MHC) class I-dependent T-cell activation. This work establishes a basis for the use of a poly-effector mutant strain as an attenuated vaccine carrier for delivery of heterologous antigens directly into the cytoplasm of host cells.

Live attenuated strains of Salmonella enterica serotype Typhi have generated much interest in the search for improved vaccines against typhoid fever and as vaccine vectors for the delivery of heterologous antigens. A promising vaccine candidate is the ΔaroC ΔssaV S. Typhi strain, which owes its attenuation mainly to lack of a type III secretion system (SPI-2 T3SS). The SPI-2 T3SS is important for bacterial proliferation inside macrophages, but not all of the effectors involved in this process have been identified. Here, we show that 10 effectors of the related strain S. Typhimurium contribute to intracellular replication in macrophages. Moreover, we establish that a poly-effector mutant strain of S. Typhimurium can have a severe replication defect and maintain a functional SPI-2 T3SS, which can be exploited for delivery of heterologous antigens.

Impact of Salmonella enterica Type III Secretion System Effectors on the Eukaryotic Host Cell

Type III secretion systems are molecular machines used by many Gram-negative bacterial pathogens to inject proteins, known as effectors, directly into eukaryotic host cells. These proteins manipulate host signal transduction pathways and cellular processes to the pathogen’s advantage. Salmonella enterica possesses two virulence-related type III secretion systems that deliver more than forty effectors. This paper reviews our current knowledge about the functions, biochemical activities, host targets, and impact on host cells of these effectors. First, the concerted action of effectors at the cellular level in relevant aspects of the interaction between Salmonella and its hosts is analyzed. Then, particular issues that will drive research in the field in the near future are discussed. Finally, detailed information about each individual effector is provided.

The streptomycin mouse model for Salmonella diarrhea: functional analysis of the microbiota, the pathogen's virulence factors, and the host's mucosal immune response

The mammalian intestine is colonized by a dense microbial community, the microbiota. Homeostatic and symbiotic interactions facilitate the peaceful co-existence between the microbiota and the host, and inhibit colonization by most incoming pathogens ('colonization resistance'). However, if pathogenic intruders overcome colonization resistance, a fierce, innate inflammatory defense can be mounted within hours, the adaptive arm of the immune system is initiated, and the pathogen is fought back. The molecular nature of the homeostatic interactions, the pathogen's ability to overcome colonization resistance, and the triggering of native and adaptive mucosal immune responses are still poorly understood. To study these mechanisms, the streptomycin mouse model for Salmonella diarrhea is of great value. Here, we review how S. Typhimurium triggers mucosal immune responses by active (virulence factor elicited) and passive (MyD88-dependent) mechanisms and introduce the S. Typhimurium mutants available for focusing on either response. Interestingly, mucosal defense turns out to be a double-edged sword, limiting pathogen burdens in the gut tissue but enhancing pathogen growth in the gut lumen. This model allows not only studying the molecular pathogenesis of Salmonella diarrhea but also is ideally suited for analyzing innate defenses, microbe handling by mucosal phagocytes, adaptive secretory immunoglobulin A responses, probing microbiota function, and homeostatic microbiota-host interactions. Finally, we discuss the general need for defined assay conditions when using animal models for enteric infections and the central importance of littermate controls.

The ancestral SgrS RNA discriminates horizontally acquired Salmonella mRNAs through a single G-U wobble pair

SgrS RNA is a model for the large class of Hfq-associated small RNAs that act to posttranscriptionally regulate bacterial mRNAs. The function of SgrS is well-characterized in nonpathogenic Escherichia coli, where it was originally shown to counteract glucose-phosphate stress by acting as a repressor of the ptsG mRNA, which encodes the major glucose transporter. We have discovered additional SgrS targets in Salmonella Typhimurium, a pathogen related to E. coli that recently acquired one-quarter of all genes by horizontal gene transfer. We show that the conserved short seed region of SgrS that recognizes ptsG was recruited to target the Salmonella-specific sopD mRNA of a secreted virulence protein. The SgrS-sopD interaction is exceptionally selective; we find that sopD2 mRNA, whose gene arose from sopD duplication during Salmonella evolution, is deaf to SgrS because of a nonproductive G-U pair in the potential SgrS-sopD2 RNA duplex vs. G-C in SgrS-sopD. In other words, SgrS discriminates the two virulence factor mRNAs at the level of a single hydrogen bond. Our study suggests that bacterial pathogens use their large suites of conserved Hfq-associated regulators to integrate horizontally acquired genes into existing posttranscriptional networks, just as conserved transcription factors are recruited to tame foreign genes at the DNA level. The results graphically illustrate the importance of the seed regions of bacterial small RNAs to select new targets with high fidelity and suggest that target predictions must consider all or none decisions by individual seed nucleotides.

Salmonella effectors: important players modulating host cell function during infection

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a Gram-negative facultative food-borne pathogen that causes gastroenteritis in humans. This bacterium has evolved a sophisticated machinery to alter host cell function critical to its virulence capabilities. Central to S. Typhimurium pathogenesis are two Type III secretion systems (T3SS) encoded within pathogenicity islands SPI-1 and SPI-2 that are responsible for the secretion and translocation of a set of bacterial proteins termed effectors into host cells with the intention of altering host cell physiology for bacterial entry and survival. Thus, once delivered by the T3SS, the secreted effectors play critical roles in manipulating the host cell to allow for bacteria invasion, induction of inflammatory responses, and the assembly of an intracellular protective niche created for bacterial survival and replication. Emerging evidence indicates that these effectors are modular proteins consisting of distinct functional domains/motifs that are utilized by the bacteria to activate intracellular signalling pathways modifying host cell function. Also, recently reported are the dual functionality of secreted effectors and the concept of 'terminal reassortment'. Herein, we highlight some of the nascent concepts regarding Salmonella effectors in the context of infection.

The deubiquitinase activity of the Salmonella pathogenicity island 2 effector, SseL, prevents accumulation of cellular lipid droplets

To cause disease, Salmonella enterica serovar Typhimurium requires two type III secretion systems that are encoded by Salmonella pathogenicity islands 1 and 2 (SPI-1 and -2). These secretion systems serve to deliver specialized proteins (effectors) into the host cell cytosol. While the importance of these effectors to promote colonization and replication within the host has been established, the specific roles of individual secreted effectors in the disease process are not well understood. In this study, we used an in vivo gallbladder epithelial cell infection model to study the function of the SPI-2-encoded type III effector, SseL. The deletion of the sseL gene resulted in bacterial filamentation and elongation and the unusual localization of Salmonella within infected epithelial cells. Infection with the ΔsseL strain also caused dramatic changes in host cell lipid metabolism and led to the massive accumulation of lipid droplets in infected cells. This phenotype was directly attributable to the deubiquitinase activity of SseL, as a Salmonella strain carrying a single point mutation in the catalytic cysteine also resulted in extensive lipid droplet accumulation. The excessive buildup of lipids due to the absence of a functional sseL gene also was observed in murine livers during S. Typhimurium infection. These results suggest that SseL alters host lipid metabolism in infected epithelial cells by modifying the ubiquitination patterns of cellular targets.

Reduced invasion to human epithelial cell lines of Salmonella enterica serovar Typhi carrying S. Typhimurium sopD2

Salmonella enterica serovar Typhi and Typhimurium are closely related serovars. However, S. Typhi, a human-specific pathogen, has 5% of genes as pseudogenes, far more than S. Typhimurium, which only has 1%. One of these pseudogenes corresponds to sopD2, which in S. Typhimurium encodes an effector protein involved in Salmonella-containing vacuole biogenesis in human epithelial cell lines, which is needed for full virulence of the pathogen. We investigated whether S. Typhi trans-complemented with the functional sopD2 gene from S. Typhimurium (sopD2(STM) ) would reduce the invasion of human epithelial cell lines. Our results showed that the presence of sopD2(STM) in S. Typhi significantly modified the bacterial ability to alter cellular permeability and decrease the CFUs recovered after cell invasion of human epithelial cell line. These results add to mounting evidence that pseudogenes contribute to S. Typhi adaptation to humans.

Dynamic modification of microtubule-dependent transport by effector proteins of intracellular Salmonella enterica

Intracellular Salmonella enterica translocate effector proteins that modify microtubule-dependent transport processes of the host cell and modulate the biogenesis of the Salmonella-containing vacuole (SCV). One functional consequence is the induction of tubular aggregates of endosomal membranes, termed Salmonella-induced filaments or SIFs, and further tubular membrane compartments have recently been described. SIFs are unique, highly dynamic compartments that form by modification of vesicular transport on microtubules. The molecular mechanism of the interference of intracellular Salmonella with host cell vesicular transport is still elusive, but recent studies demonstrate the complexity of pathogenic activities and the intricacy of manipulating host cell functions.

Salmonella typhimurium diarrhea: switching the mucosal epithelium from homeostasis to defense

The mammalian intestine is a complex biological system composed of the epithelium, the gut associated immune system, a commensal microbial community of approx. 10(10) cells per gram of content ('microbiota') and an occasional onslaught by pathogens. The mechanisms governing homeostasis and immune defense are of great importance, but incompletely understood. This is explained by the system's sheer complexity. So far, no single study has considered all relevant parameters, that is (i) innate and adaptive mucosal immune responses; (ii) mucosa cell gene expression; (iii) community composition of the microbiota; (iv) microbiota gene expression; (v) genetic profiling of the host; (vi) the virulence complement expressed by the pathogen in vivo. This exquisite complexity explains why simplified model systems have fuelled much recent progress on the system's regulating principles. Here, we focus on one particular model, the streptomycin pretreated mouse model for Salmonella diarrhea, to illustrate novel concepts in microbe-mucosa interaction, that is how this system switches from homeostasis to disease.

Salmonella Interaction with and Passage through the Intestinal Mucosa: Through the Lens of the Organism

Salmonella enterica serotypes are invasive enteric pathogens spread through fecal contamination of food and water sources, and represent a constant public health threat around the world. The symptoms associated with salmonellosis and typhoid disease are largely due to the host response to invading Salmonella, and to the mechanisms these bacteria employ to survive in the presence of, and invade through the intestinal mucosal epithelia. Surmounting this barrier is required for survival within the host, as well as for further dissemination throughout the body, and subsequent systemic disease. In this review, we highlight some of the major hurdles Salmonella must overcome upon encountering the intestinal mucosal epithelial barrier, and examine how these bacteria surmount and exploit host defense mechanisms.

Salmonella phage ST64B encodes a member of the SseK/NleB effector family

Salmonella enterica is a species of bacteria that is a major cause of enteritis across the globe, while certain serovars cause typhoid, a more serious disease associated with a significant mortality rate. Type III secreted effectors are major contributors to the pathogenesis of Salmonella infections. Genes encoding effectors are acquired via horizontal gene transfer, and a subset are encoded within active phage lysogens. Because the acquisition of effectors is in flux, the complement of effectors possessed by various Salmonella strains frequently differs. By comparing the genome sequences of S. enterica serovar Typhimurium strain SL1344 with LT2, we identified a gene with significant similarity to SseK/NleB type III secreted effector proteins within a phage ST64B lysogen that is absent from LT2. We have named this gene sseK3. SseK3 was co-regulated with the SPI-2 type III secretion system in vitro and inside host cells, and was also injected into infected host cells. While no role for SseK3 in virulence could be identified, a role for the other family members in murine typhoid was found. SseK3 and other phage-encoded effectors were found to have a significant but sparse distribution in the available Salmonella genome sequences, indicating the potential for more uncharacterised effectors to be present in less studied serovars. These phage-encoded effectors may be principle subjects of contemporary selective processes shaping Salmonella-host interactions.