The Role of the Type III Secretion System in the Intracellular Lifestyle of Enteric Pathogens

Insights into virulence: structure classification of the Vibrio parahaemolyticus RIMD mobilome

IMPORTANCE

The pandemic Vpar strain RIMD causes seafood-borne illness worldwide. Previous comparative genomic studies have revealed pathogenicity islands in RIMD that contribute to the success of the strain in infection. However, not all virulence determinants have been identified, and many of the proteins encoded in known pathogenicity islands are of unknown function. Based on the EOCD database, we used evolution-based classification of structure models for the RIMD proteome to improve our functional understanding of virulence determinants acquired by the pandemic strain. We further identify and classify previously unknown mobile protein domains as well as fast evolving residue positions in structure models that contribute to virulence and adaptation with respect to a pre-pandemic strain. Our work highlights key contributions of phage in mediating seafood born illness, suggesting this strain balances its avoidance of phage predators with its successful colonization of human hosts.

Salmonella T3SS effector SseK1 arginine-glycosylates the two-component response regulator OmpR to alter bile salt resistance

Type III secretion system (T3SS) effector proteins are primarily recognized for binding host proteins to subvert host immune response during infection. Besides their known host target proteins, several T3SS effectors also interact with endogenous bacterial proteins. Here we demonstrate that the Salmonella T3SS effector glycosyltransferase SseK1 glycosylates the bacterial two-component response regulator OmpR on two arginine residues, R15 and R122. Arg-glycosylation of OmpR results in reduced expression of ompF, a major outer membrane porin gene. Glycosylated OmpR has reduced affinity to the ompF promoter region, as compared to the unglycosylated form of OmpR. Additionally, the Salmonella ΔsseK1 mutant strain had higher bile salt resistance and increased capacity to form biofilms, as compared to WT Salmonella, thus linking OmpR glycosylation to several important aspects of bacterial physiology.

Membrane-localized expression, production and assembly of Vibrio parahaemolyticus T3SS2 provides evidence for transertion

It has been proposed that bacterial membrane proteins may be synthesized and inserted into the membrane by a process known as transertion, which involves membrane association of their encoding genes, followed by coupled transcription, translation and membrane insertion. Here, we provide evidence supporting that the pathogen Vibrio parahaemolyticus uses transertion to assemble its type III secretion system (T3SS2), to inject virulence factors into host cells. We propose a two-step transertion process where the membrane-bound co-component receptor (VtrA/VtrC) is first activated by bile acids, leading to membrane association and expression of its target gene, vtrB, located in the T3SS2 pathogenicity island. VtrB, the transmembrane transcriptional activator of T3SS2, then induces the localized expression and membrane assembly of the T3SS2 structural components and its effectors. We hypothesize that the proposed transertion process may be used by other enteric bacteria for efficient assembly of membrane-bound molecular complexes in response to extracellular signals.

Fis Connects Two Sensory Pathways, Quorum Sensing and Surface Sensing, to Control Motility in Vibrio parahaemolyticus

Factor for inversion stimulation (Fis) is a global regulator that is highly expressed during exponential phase growth and undetectable in stationary phase growth. Quorum sensing (QS) is a global regulatory mechanism that controls gene expression in response to changes in cell density and growth phase. In Vibrio parahaemolyticus, a marine species and a significant human pathogen, the QS regulatory sRNAs, Qrr1 to Qrr5, are expressed during exponential growth and negatively regulate the high cell density QS master regulator OpaR. OpaR is a positive regulator of capsule polysaccharide (CPS) formation, which is required for biofilm formation, and is a repressor of lateral flagella required for swarming motility. In V. parahaemolyticus, we show that Fis is a positive regulator of the qrr sRNAs expression. In an in-frame fis deletion mutant, qrr expression was repressed and opaR expression was induced. The Δfis mutant produced CPS and biofilm, but swarming motility was abolished. Also, the fis deletion mutant was more sensitive to polymyxin B. Swarming motility requires expression of both the surface sensing scrABC operon and lateral flagella laf operon. Our data showed that in the Δfis mutant both laf and scrABC genes were repressed. Fis controlled swarming motility indirectly through the QS pathway and directly through the surface sensing pathway. To determine the effects of Fis on cellular metabolism, we performed in vitro growth competition assays, and found that Δfis was outcompeted by wild type in minimal media supplemented with intestinal mucus as a sole nutrient source. The data showed that Fis positively modulated mucus components L-arabinose, D-gluconate and N-acetyl-D-glucosamine catabolism gene expression. In an in vivo colonization competition assay, Δfis was outcompeted by wild type, indicating Fis is required for fitness. Overall, these data demonstrate a global regulatory role for Fis in V. parahaemolyticus that includes QS, motility, and metabolism.

Recent Progress in Shigella and Burkholderia pseudomallei Vaccines

Significant advancement has been made in the development of vaccines against bacterial pathogens. However, several roadblocks have been found during the evaluation of vaccines against intracellular bacterial pathogens. Therefore, new lessons could be learned from different vaccines developed against unrelated intracellular pathogens. Bacillary dysentery and melioidosis are important causes of morbidity and mortality in developing nations, which are caused by the intracellular bacteria Shigella and Burkholderia pseudomallei, respectively. Although the mechanisms of bacterial infection, dissemination, and route of infection do not provide clues about the commonalities of the pathogenic infectious processes of these bacteria, a wide variety of vaccine platforms recently evaluated suggest that in addition to the stimulation of antibodies, identifying protective antigens and inducing T cell responses are some additional required elements to induce effective protection. In this review, we perform a comparative evaluation of recent candidate vaccines used to combat these two infectious agents, emphasizing the common strategies that can help investigators advance effective and protective vaccines to clinical trials.

Adaptations of Vibrio parahaemolyticus to Stress During Environmental Survival, Host Colonization, and Infection

Vibrio parahaemolyticus (Vp) is an aquatic Gram-negative bacterium that may infect humans and cause gastroenteritis and wound infections. The first pandemic of Vp associated infection was caused by the serovar O3:K6 and epidemics caused by the other serovars are increasingly reported. The two major virulence factors, thermostable direct hemolysin (TDH) and/or TDH-related hemolysin (TRH), are associated with hemolysis and cytotoxicity. Vp strains lacking tdh and/or trh are avirulent and able to colonize in the human gut and cause infection using other unknown factors. This pathogen is well adapted to survive in the environment and human host using several genetic mechanisms. The presence of prophages in Vp contributes to the emergence of pathogenic strains from the marine environment. Vp has two putative type-III and type-VI secretion systems (T3SS and T6SS, respectively) located on both the chromosomes. T3SS play a crucial role during the infection process by causing cytotoxicity and enterotoxicity. T6SS contribute to adhesion, virulence associated with interbacterial competition in the gut milieu. Due to differential expression, type III secretion system 2 (encoded on chromosome-2, T3SS2) and other genes are activated and transcribed by interaction with bile salts within the host. Chromosome-1 encoded T6SS1 has been predominantly identified in clinical isolates. Acquisition of genomic islands by horizontal gene transfer provides enhanced tolerance of Vp toward several antibiotics and heavy metals. Vp consists of evolutionarily conserved targets of GTPases and kinases. Expression of these genes is responsible for the survival of Vp in the host and biochemical changes during its survival. Advanced genomic analysis has revealed that various genes are encoded in Vp pathogenicity island that control and expression of virulence in the host. In the environment, the biofilm gene expression has been positively correlated to tolerance toward aerobic, anaerobic, and micro-aerobic conditions. The genetic similarity analysis of toxin/antitoxin systems of Escherichia coli with VP genome has shown a function that could induce a viable non-culturable state by preventing cell division. A better interpretation of the Vp virulence and other mechanisms that support its environmental fitness are important for diagnosis, treatment, prevention and spread of infections. This review identifies some of the common regulatory pathways of Vp in response to different stresses that influence its survival, gut colonization and virulence.

Dynamic relocalization of cytosolic type III secretion system components prevents premature protein secretion at low external pH

Many bacterial pathogens use a type III secretion system (T3SS) to manipulate host cells. Protein secretion by the T3SS injectisome is activated upon contact to any host cell, and it has been unclear how premature secretion is prevented during infection. Here we report that in the gastrointestinal pathogens Yersinia enterocolitica and Shigella flexneri, cytosolic injectisome components are temporarily released from the proximal interface of the injectisome at low external pH, preventing protein secretion in acidic environments, such as the stomach. We show that in Yersinia enterocolitica, low external pH is detected in the periplasm and leads to a partial dissociation of the inner membrane injectisome component SctD, which in turn causes the dissociation of the cytosolic T3SS components. This effect is reversed upon restoration of neutral pH, allowing a fast activation of the T3SS at the native target regions within the host. These findings indicate that the cytosolic components form an adaptive regulatory interface, which regulates T3SS activity in response to environmental conditions.

Advances in the development of Salmonella-based vaccine strategies for protection against Salmonellosis in humans

Salmonella spp. are important human pathogens globally causing millions of cases of typhoid fever and non‐typhoidal salmonellosis annually. There are only a few vaccines licensed for use in humans which all target Salmonella enterica serovar Typhi. Vaccine development is hampered by antigenic diversity between the thousands of serovars capable of causing infection in humans. However, a number of attenuated candidate vaccine strains are currently being developed. As facultative intracellular pathogens with multiple systems for transporting effector proteins to host cells, attenuated Salmonella strains can also serve as ideal tools for the delivery of foreign antigens to create multivalent live carrier vaccines for simultaneous immunization against several unrelated pathogens. Further, the ease with which Salmonella can be genetically modified and the extensive knowledge of the virulence mechanisms of this pathogen means that this bacterium has often served as a model organism to test new approaches. In this review we focus on (1) recent advances in live attenuated Salmonella vaccine development, (2) improvements in expression of foreign antigens in carrier vaccines and (3) adaptation of attenuated strains as sources of purified antigens and vesicles that can be used for subunit and conjugate vaccines or together with attenuated vaccine strains in heterologous prime‐boosting immunization strategies. These advances have led to the development of new vaccines against Salmonella which have or will soon be tested in clinical trials.

Burkholderia pseudomallei as an Enteric Pathogen: Identification of Virulence Factors Mediating Gastrointestinal Infection

Burkholderia pseudomallei is a Gram-negative bacterium and the causative agent of melioidosis. Despite advances in our understanding of the disease, B. pseudomallei poses a significant health risk, especially in regions of endemicity, where treatment requires prolonged antibiotic therapy. Even though the respiratory and percutaneous routes are well documented and considered the main ways to acquire the pathogen, the gastrointestinal tract is believed to be an underreported and underrecognized route of infection. In the present study, we describe the development of in vitro and in vivo models to study B. pseudomallei gastrointestinal infection. Further, we report that the type 6 secretion system (T6SS) and type 1 fimbriae are important virulence factors required for gastrointestinal infection. Using a human intestinal epithelial cell line and mouse primary intestinal epithelial cells (IECs), we demonstrated that B. pseudomallei adheres, invades, and forms multinucleated giant cells, ultimately leading to cell toxicity. We demonstrated that mannose-sensitive type 1 fimbria is involved in the initial adherence of B. pseudomallei to IECs, although the impact on full virulence was limited. Finally, we also showed that B. pseudomallei requires a functional T6SS for full virulence, bacterial dissemination, and lethality in mice infected by the intragastric route. Overall, we showed that B. pseudomallei is an enteric pathogen and that type 1 fimbria is important for B. pseudomallei intestinal adherence, and we identify a new role for T6SS as a key virulence factor in gastrointestinal infection. These studies highlight the importance of gastrointestinal melioidosis as an understudied route of infection and open a new avenue for the pathogenesis of B. pseudomallei.

The interaction of Escherichia coli O157 :H7 and Salmonella Typhimurium flagella with host cell membranes and cytoskeletal components

Bacterial flagella have many established roles beyond swimming motility. Despite clear evidence of flagella-dependent adherence, the specificity of the ligands and mechanisms of binding are still debated. In this study, the molecular basis of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium flagella binding to epithelial cell cultures was investigated. Flagella interactions with host cell surfaces were intimate and crossed cellular boundaries as demarcated by actin and membrane labelling. Scanning electron microscopy revealed flagella disappearing into cellular surfaces and transmission electron microscopy of S. Typhiumurium indicated host membrane deformation and disruption in proximity to flagella. Motor mutants of E. coli O157:H7 and S. Typhimurium caused reduced haemolysis compared to wild-type, indicating that membrane disruption was in part due to flagella rotation. Flagella from E. coli O157 (H7), EPEC O127 (H6) and S. Typhimurium (P1 and P2 flagella) were shown to bind to purified intracellular components of the actin cytoskeleton and directly increase in vitro actin polymerization rates. We propose that flagella interactions with host cell membranes and cytoskeletal components may help prime intimate attachment and invasion for E. coli O157:H7 and S. Typhimurium, respectively.

Vibrio deploys type 2 secreted lipase to esterify cholesterol with host fatty acids and mediate cell egress

Pathogens find diverse niches for survival including inside a host cell where replication occurs in a relatively protective environment. Vibrio parahaemolyticus is a facultative intracellular pathogen that uses its type 3 secretion system 2 (T3SS2) to invade and replicate inside host cells. Analysis of the T3SS2 pathogenicity island encoding the T3SS2 appeared to lack a mechanism for egress of this bacterium from the invaded host cell. Using a combination of molecular tools, we found that VPA0226, a constitutively secreted lipase, is required for escape of V. parahaemolyticus from the host cells. This lipase must be delivered into the host cytoplasm where it preferentially uses fatty acids associated with innate immune response to esterify cholesterol, weakening the plasma membrane and allowing egress of the bacteria. This study reveals the resourcefulness of microbes and the interplay between virulence systems and host cell resources to evolve an ingenious scheme for survival and escape.

Host-microbiota interactions in inflammatory bowel disease

The mammalian intestine is colonized by trillions of microorganisms that have co-evolved with the host in a symbiotic relationship. The presence of large numbers of symbionts near the epithelial surface of the intestine poses an enormous challenge to the host because it must avoid the activation of harmful inflammatory responses to the microorganisms while preserving its ability to mount robust immune responses to invading pathogens. In patients with inflammatory bowel disease, there is a breakdown of the multiple strategies that the immune system has evolved to promote the separation between symbiotic microorganisms and the intestinal epithelium and the effective killing of penetrant microorganisms, while suppressing the activation of inappropriate T cell responses to resident microorganisms. Understanding the complex interactions between intestinal microorganisms and the host may provide crucial insight into the pathogenesis of inflammatory bowel disease as well as new avenues to prevent and treat the disease.