TTSS2-deficient hha mutant of Salmonella Typhimurium exhibits significant systemic attenuation in immunocompromised hosts

Manipulation of host immune defenses by effector proteins delivered from multiple secretion systems of Salmonella and its application in vaccine research

Salmonella is an important zoonotic bacterial species and hazardous for the health of human beings and livestock globally. Depending on the host, Salmonella can cause diseases ranging from gastroenteritis to life-threatening systemic infection. In this review, we discuss the effector proteins used by Salmonella to evade or manipulate four different levels of host immune defenses: commensal flora, intestinal epithelial-mucosal barrier, innate and adaptive immunity. At present, Salmonella has evolved a variety of strategies against host defense mechanisms, among which various effector proteins delivered by the secretory systems play a key role. During its passage through the digestive system, Salmonella has to face the intact intestinal epithelial barrier as well as competition with commensal flora. After invasion of host cells, Salmonella manipulates inflammatory pathways, ubiquitination and autophagy processes with the help of effector proteins. Finally, Salmonella evades the adaptive immune system by interfering the migration of dendritic cells and interacting with T and B lymphocytes. In conclusion, Salmonella can manipulate multiple aspects of host defense to promote its replication in the host.

Effects of colonization-associated gene yqiC on global transcriptome, cellular respiration, and oxidative stress in Salmonella Typhimurium

BACKGROUND

yqiC is required for colonizing the Salmonella enterica serovar Typhimurium (S. Typhimurium) in human cells; however, how yqiC regulates nontyphoidal Salmonella (NTS) genes to influence bacteria-host interactions remains unclear.

METHODS

The global transcriptomes of S. Typhimurium yqiC-deleted mutant (ΔyqiC) and its wild-type strain SL1344 after 2 h of in vitro infection with Caco-2 cells were obtained through RNA sequencing to conduct comparisons and identify major yqiC-regulated genes, particularly those involved in Salmonella pathogenicity islands (SPIs), ubiquinone and menaquinone biosynthesis, electron transportation chains (ETCs), and carbohydrate/energy metabolism. A Seahorse XFp Analyzer and assays of NADH/NAD⁺ and H₂O₂ were used to compare oxygen consumption and extracellular acidification, glycolysis parameters, adenosine triphosphate (ATP) generation, NADH/NAD⁺ ratios, and H₂O₂ production between ΔyqiC and SL1344.

RESULTS

After S. Typhimurium interacts with Caco-2 cells, yqiC represses gene upregulation in aspartate carbamoyl transferase, type 1 fimbriae, and iron-sulfur assembly, and it is required for expressing ilvB operon, flagellin, tdcABCD, and dmsAB. Furthermore, yqiC is required for expressing mainly SPI-1 genes and specific SPI-4, SPI-5, and SPI-6 genes; however, it diversely regulates SPI-2 and SPI-3 gene expression. yqiC significantly contributes to menD expression in menaquinone biosynthesis. A Kyoto Encyclopedia of Genes and Genomes analysis revealed the extensive association of yqiC with carbohydrate and energy metabolism. yqiC contributes to ATP generation, and the analyzer results demonstrate that yqiC is required for maintaining cellular respiration and metabolic potential under energy stress and for achieving glycolysis, glycolytic capacity, and glycolytic reserve. yqiC is also required for expressing ndh, cydA, nuoE, and sdhB but suppresses cyoC upregulation in the ETC of aerobically and anaerobically grown S. Typhimurium; priming with Caco-2 cells caused a reversed regulation of yiqC toward upregulation in these ETC complex genes. Furthermore, yqiC is required for maintaining NADH/NAD⁺ redox status and H₂O₂ production.

CONCLUSIONS

Specific unreported genes that were considerably regulated by the colonization-associated gene yqiC in NTS were identified, and the key role and tentative mechanisms of yqiC in the extensive modulation of virulence factors, SPIs, ubiquinone and menaquinone biosynthesis, ETCs, glycolysis, and oxidative stress were discovered.

Cross-Talk Between the Intestinal Epithelium and Salmonella Typhimurium

Salmonella enterica serovars are invasive gram-negative bacteria, causing a wide range of diseases from gastroenteritis to typhoid fever, representing a public health threat around the world. Salmonella gains access to the intestinal lumen after oral ingestion of contaminated food or water. The crucial initial step to establish infection is the interaction with the intestinal epithelium. Human-adapted serovars such as S. Typhi or S. Paratyphi disseminate to systemic organs and induce life-threatening disease known as typhoid fever, whereas broad-host serovars such as S. Typhimurium usually are limited to the intestine and responsible for gastroenteritis in humans. To overcome intestinal epithelial barrier, Salmonella developed mechanisms to induce cellular invasion, intracellular replication and to face host defence mechanisms. Depending on the serovar and the respective host organism, disease symptoms differ and are linked to the ability of the bacteria to manipulate the epithelial barrier for its own profit and cross the intestinal epithelium.

This review will focus on S. Typhimurium (STm). To better understand STm pathogenesis, it is crucial to characterize the crosstalk between STm and the intestinal epithelium and decipher the mechanisms and epithelial cell types involved. Thus, the purpose of this review is to summarize our current knowledge on the molecular dialogue between STm and the various cell types constituting the intestinal epithelium with a focus on the mechanisms developed by STm to cross the intestinal epithelium and access to subepithelial or systemic sites and survive host defense mechanisms.

Molecular determinants of peaceful coexistence versus invasiveness of non-Typhoidal Salmonella: Implications in long-term side-effects

The genus Salmonella represents a wide range of strains including Typhoidal and Non-Typhoidal Salmonella (NTS) isolates that exhibit illnesses of varied pathophysiologies. The more frequent NTS ensues a self-limiting enterocolitis with rare occasions of bacteremia or systemic infections. These self-limiting Salmonella strains are capable of subverting and dampening the host immune system to achieve a more prolonged survival inside the host system thus leading to chronic manifestations. Notably, emergence of new invasive NTS isolates known as invasive Non-Typhoidal Salmonella (iNTS) have worsened the disease burden significantly in some parts of the world. NTS strains adapt to attain persister phenotype intracellularly and cause relapsing infections. These chronic infections, in susceptible hosts, are also capable of causing diseases like IBS, IBD, reactive arthritis, gallbladder cancer and colorectal cancer. The present understanding of molecular mechanism of how these chronic infections are manifested is quite limited. The current work is an effort to review the prevailing knowledge emanating from a large volume of research focusing on various forms of NTS infections including those that cause localized, systemic and persistent disease. The review will further dwell into the understanding of how this pathogen contributes to the associated long term sequelae.

A ROD9 island encoded gene in Salmonella Enteritidis plays an important role in acid tolerance response and helps in systemic infection in mice

Salmonella, like other pathogenic bacteria has undergone multiple genomic alterations to adapt itself into specific host environments executing varied degrees of virulence through evolution. Such variations in genome content have been assumed to lead the closely related non-typhoidal serovars, S. Enteritidis, and S. Typhimurium to exhibit Type Three Secretion System -2 (T3SS-2) based diverse colonization and inflammation kinetics. Mutually exclusive genes present in either of the serovars are recently being studied and in our currentwork, we focused on a particular island ROD9, present in S. Enteritidis but not in S. Typhimurium. Earlier reports have identified a few genes from this island to be responsible for virulence in vitro as well as in vivo. In this study, we have identified another gene, SEN1008 from the same island encoding a hypothetical protein to be a potential virulence determinant showing systemic attenuation upon mutation in C57BL/6 mice infection model. The isogenic mutant strain displayed reduced adhesion to epithelial cells in vitro as well as was highly immotile. It was also deficient in intracellular replication in vitro, with a highly suppressed SPI-2and failed to cause acute colitis at 72-h p.i.in vivo. Moreover, on acid exposure, SEN1008 showed 17 folds and 2 fold up-regulations during adaptation and challenge phases,respectively and ΔSEN1008 failed to survive during ATR assay, indicating its role under acid stress. Together, our findings suggested ΔSEN1008 to be significantly attenuated and we propose this gene to be a potent factor responsible for S. Enteritidis pathogenesis.

Salmonella enterica Serovar Typhimurium 14028s Genomic Regions Required for Colonization of Lettuce Leaves

Contamination of edible produce leaves with human bacterial pathogens has been associated with serious disease outbreaks and has become a major public health concern affecting all aspects of the market, from farmers to consumers. While pathogen populations residing on the surface of ready-to-eat produce can be potentially removed through thorough washing, there is no disinfection technology available that effectively eliminates internal bacterial populations. By screening 303 multi-gene deletion (MGD) mutants of Salmonella enterica serovar Typhimurium (STm) 14028s, we were able to identify ten genomic regions that play a role in opening the stomatal pore of lettuce leaves. The major metabolic functions of the deleted regions are associated with sensing the environment, bacterium movement, transport through the bacterial membrane, and biosynthesis of surface appendages. Interestingly, at 21 days post inoculation, seven of these mutants showed increased population titers inside the leaf, two mutants showed similar titers as the wild type bacterium, whereas one mutant with a large deletion that includes the Salmonella pathogenicity island 2 (SPI-2) showed significantly impaired persistence in the leaf apoplast. These findings suggest that not all the genomic regions required for initiation of leaf colonization (i.e., epiphytic behavior and tissue penetration) are essential for continuing bacterial survival as an endophyte. We also observed that mutants lacking either SPI-1 (Mut3) or SPI-2 (Mut9) induce callose deposition levels comparable to those of the wild type STm 14028s; therefore, these islands do not seem to affect this lettuce defense mechanism. However, the growth of Mut9, but not Mut3, was significantly impaired in the leaf apoplastic wash fluid (AWF) suggesting that the STm persistence in the apoplast may be linked to nutrient acquisition capabilities or overall bacterial fitness in this niche, which are dependent on the gene(s) deleted in the Mut9 strain. The genetic basis of STm colonization of leaves investigated in this study provides a foundation from which to develop mitigation tactics to enhance food safety.

Identification of a novel gene in ROD9 island of Salmonella Enteritidis involved in the alteration of virulence-associated genes expression

Salmonella enterica subsp. I serovar Enteritidis (S. Enteritidis), one of the causative agents for non-typhoidal gastrointestinal diseases in humans is an intracellular bacterium and mechanism for its invasion into host cells is critical to cause infection. The virulence of the pathogen is explained by the expression of genes located on its pathogenicity islands, mostly encoded under SPI-1 and SPI-2. However, S. Typhimurium SL1344, despite sharing ∼98% of its genome with S. Enteritidis P125109, lacks few regions of differences (ROD) that are hypothesized to impart virulence potential to S. Enteritidis. In this study, we created different mutants in the ROD9 island of S. Enteritidis, also referred as SPI-19 and identified a novel locus, SEN1005, encoding a hypothetical protein that is involved in its pathogenesis. ΔSEN1005 displayed significantly reduced entry into cultured epithelial cells as well as uptake by macrophages and failed to cause acute colitis in C57BL/6 mice at day 3 post-infection (p.i.). Additionally, the global transcriptome analysis revealed a highly repressed SPI-1 and other down-regulated genes responsible for flagellar assembly, chemotaxis and motility in the mutant which correlated with decreased invasion and abated inflammation as compared to the wild-type. Therefore, our findings revealed that ΔSEN1005 was attenuated in vitro as well as in vivo and we propose this hypothetical protein to play a role in altering the expression of genes involved in Salmonella virulence.

The Hha-TomB Toxin-Antitoxin System Shows Conditional Toxicity and Promotes Persister Cell Formation by Inhibiting Apoptosis-Like Death in S. Typhimurium

Toxin-antitoxin (TA) modules are two component “addictive” genetic elements found on either plasmid or bacterial chromosome, sometimes on both. TA systems perform a wide range of functions like biofilm formation, persistence, programmed cell death, phage abortive infection etc. Salmonella has been reported to contain several such TA systems. However, the hemolysin expression modulating protein (Hha) and its adjacent uncharacterized hypothetical protein TomB (previously known as YbaJ), have not been listed as a TA module in Salmonella. In this study we established that Hha and TomB form a bonafide TA system where Hha serves as a toxin while TomB functions as an antitoxin. Interestingly, the toxicity of Hha was conditional causing cell death under acid stress. The antitoxin attenuated the toxicity of Hha by forming a TA complex through stable interactions. The Hha-TomB TA system was found to increase persistence and inhibit programmed cell death under antibiotic stress where a phenotypically diverse population expressing differential level of TA components was observed. Therefore we propose that Hha and TomB prevent cells from committing suicide thereby promoting persister cell formation.

Altered virulence potential of Salmonella Enteritidis cultured in different foods: A cumulative effect of differential gene expression and immunomodulation

Salmonella enterica serovars Enteritidis (S. Enteritidis) is one of the most common causes of food borne illness. Bacterial growth environment plays an important role in regulating gene expression thereby affecting the virulence profile of the bacteria. Different foods present diverse growth conditions which may affect the pathogenic potential of the bacteria. In the present study, the effect of food environments on the pathogenic potential of S. Enteritidis has been evaluated. S. Enteritidis was grown in different foods e.g. egg white, peanut butter and milk, and virulent phenotypes were compared to those grown in Luria Bertani broth. In-vivo experiments in C57BL/6 mice revealed S. Enteritidis grown in egg white did not induce significant (p<0.001) production of proinflammatory cytokines in mice and were unable to cause colitis despite efficient colonization in cecum, mesenteric lymph node, spleen and liver. Further studies revealed that bacteria grown in LB activated MAP Kinase and NFκB pathways efficiently, while those grown in egg white poorly activated the above pathways which can account for the decreased production of proinflammatory cytokines. qRT PCR analysis revealed SPI-1 effectors were downregulated in bacteria grown in egg white. Interestingly, bacteria grown in egg white showed reversal of phenotype upon change in growth media to LB. Additionally, bacteria grown in milk and peanut butter showed different degrees of virulence in mice as compared to those grown in LB media. Thus, the present study demonstrates that, S. Enteritidis grown in egg white colonizes systemic sites without causing colitis in a mouse model, while bacteria grown in milk and peanut butter show different pathogenicity profiles suggesting that food environments significantly affect the pathogenicity of S. Enteritidis.

Animal models to study acute and chronic intestinal inflammation in mammals

Acute and chronic inflammatory diseases of the intestine impart a significant and negative impact on the health and well-being of human and non-human mammalian animals. Understanding the underlying mechanisms of inflammatory disease is mandatory to develop effective treatment and prevention strategies. As inflammatory disease etiologies are multifactorial, the use of appropriate animal models and associated metrics of disease are essential. In this regard, animal models used alone or in combination to study acute and chronic inflammatory disease of the mammalian intestine paired with commonly used inflammation-inducing agents are reviewed. This includes both chemical and biological incitants of inflammation, and both non-mammalian (i.e. nematodes, insects, and fish) and mammalian (i.e. rodents, rabbits, pigs, ruminants, dogs, and non-human primates) models of intestinal inflammation including germ-free, gnotobiotic, as well as surgical, and genetically modified animals. Importantly, chemical and biological incitants induce inflammation via a multitude of mechanisms, and intestinal inflammation and injury can vary greatly according to the incitant and animal model used, allowing studies to ascertain both long-term and short-term effects of inflammation. Thus, researchers and clinicians should be aware of the relative strengths and limitations of the various animal models used to study acute and chronic inflammatory diseases of the mammalian intestine, and the scope and relevance of outcomes achievable based on this knowledge. The ability to induce inflammation to mimic common human diseases is an important factor of a successful animal model, however other mechanisms of disease such as the amount of infective agent to induce disease, invasion mechanisms, and the effect various physiologic changes can have on inducing damage are also important features. In many cases, the use of multiple animal models in combination with both chemical and biological incitants is necessary to answer the specific question being addressed regarding intestinal disease. Some incitants can induce acute responses in certain animal models while others can be used to induce chronic responses; this review aims to illustrate the strengths and weaknesses in each animal model and to guide the choice of an appropriate acute or chronic incitant to facilitate intestinal disease.

The O-antigen negative ∆wbaV mutant of Salmonella enterica serovar Enteritidis shows adaptive resistance to antimicrobial peptides and elicits colitis in streptomycin pretreated mouse model

BACKGROUND

Salmonella enterica serovar Enteritidis, the most common cause of human gastroenteritis, employs several virulence factors including lipopolysaccharide (LPS) for infection and establishment of disease inside the host. The LPS of S. enterica serovar Enteritidis consists of lipid A, core oligosaccharide and O-antigen (OAg). The OAg consists of repeating units containing different sugars. The sugars of OAg are synthesized and assembled by a set of enzymes encoded by genes organized into clusters. Present study focuses on the effect of deletion of genes involved in biosynthesis of OAg repeating units on resistance to antimicrobial peptides and virulence in mice.

METHODS

In the present study, the OAg biosynthesis was impaired by deleting tyv, prt and wbaV genes involved in tyvelose biosynthesis and its transfer to OAg. The virulence phenotype of resulting mutants was evaluated by assessing resistance to antimicrobial peptides, serum complement, adhesion, invasion and in vivo colonization.

RESULTS

Deletion of the above three genes resulted in the production of OAg-negative LPS. All the OAg-negative mutants showed phenotype reported for rough strains. Interestingly, ΔwbaV mutant showed increased resistance against antimicrobial peptides and normal human serum. In addition, the ΔwbaV mutant also showed increased adhesion and invasion as compared to the other two O-Ag negative mutants Δtyv and Δprt. In vivo experiments also confirmed the increased virulent phenotype of ΔwbaV mutant as compared to Δprt mutant.

CONCLUSION

OAg-negative mutants are known to be avirulent; however, this study demonstrates that certain OAg negative mutants e.g. ∆wbaV may also show resistance to antimicrobial peptides and cause colitis in Streptomyces pretreated mouse model.