Diarrhea and colitis in mice require the Salmonella pathogenicity island 2-encoded secretion function but not SifA or Spv effectors

SifA SUMOylation governs Salmonella Typhimurium intracellular survival via modulation of lysosomal function

One of the mechanisms shaping the pathophysiology during the infection of enteric pathogen Salmonella Typhimurium is host PTM machinery utilization by the pathogen encoded effectors. Salmonella Typhimurium (S. Tm) during infection in host cells thrives in a vacuolated compartment, Salmonella containing vacuole (SCV), which sequentially acquires host endosomal and lysosomal markers. Long tubular structures, called as Salmonella induced filaments (SIFs), are further generated by S. Tm, which are known to be required for SCV's nutrient acquisition, membrane maintenance and stability. A tightly coordinated interaction involving prominent effector SifA and various host adapters PLEKHM1, PLEKHM2 and Rab GTPases govern SCV integrity and SIF formation. Here, we report for the first time that the functional regulation of SifA is modulated by PTM SUMOylation at its 11th lysine. S. Tm expressing SUMOylation deficient lysine 11 mutants of SifA (SifAK11R) is defective in intracellular proliferation due to compromised SIF formation and enhanced lysosomal acidification. Furthermore, murine competitive index experiments reveal defective in vivo proliferation and weakened virulence of SifAK11R mutant. Concisely, our data reveal that SifAK11R mutant nearly behaves like a SifA knockout strain which impacts Rab9-MPR mediated lysosomal acidification pathway, the outcome of which culminates in reduced bacterial load in in vitro and in vivo infection model systems. Our results bring forth a novel pathogen-host crosstalk mechanism where the SUMOylation of effector SifA regulated S. Tm intracellular survival.

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.

Alteration of Salmonella enterica Virulence and Host Pathogenesis through Targeting sdiA by Using the CRISPR-Cas9 System

Salmonella enterica is a common cause of many enteric infections worldwide and is successfully engineered to deliver heterologous antigens to be used as vaccines. Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs) RNA-guided Cas9 endonuclease is a promising genome editing tool. In the current study, a CRISPR-Cas9 system was used to target S.enterica sdiA that encodes signal molecule receptor SdiA and responds to the quorum sensing (QS) signaling compounds N-acylhomoserine lactones (AHLs). For this purpose, sdiA was targeted in both S.enterica wild type (WT) and the ΔssaV mutant strain, where SsaV has been reported to be an essential component of SPI2-T3SS. The impact of sdiA mutation on S. enterica virulence was evaluated at both early invasion and later intracellular replication in both the presence and absence of AHL. Additionally, the influence of sdiA mutation on the pathogenesis S. enterica WT and mutants was investigated in vivo, using mice infection model. Finally, the minimum inhibitory concentrations (MICs) of various antibiotics against S. enterica strains were determined. Present findings show that mutation in sdiA significantly affects S.enterica biofilm formation, cell adhesion and invasion. However, sdiA mutation did not affect bacterial intracellular survival. Moreover, in vivo bacterial pathogenesis was markedly lowered in S.enterica ΔsdiA in comparison with the wild-type strain. Significantly, double-mutant sdiA and ssaV attenuated the S. enterica virulence and in vivo pathogenesis. Moreover, mutations in selected genes increased Salmonella susceptibility to tested antibiotics, as revealed by determining the MICs and MBICs of these antibiotics. Altogether, current results clearly highlight the importance of the CRISPR-Cas9 system as a bacterial genome editing tool and the valuable role of SdiA in S.enterica virulence. The present findings extend the understanding of virulence regulation and host pathogenesis of Salmonellaenterica.

Ciprofloxacin interferes with Salmonella Typhimurium intracellular survival and host virulence through repression of Salmonella pathogenicity island-2 (SPI-2) genes expression

Current study aims to characterize the influence of sub-minimum inhibitory concentration (sub-MIC) of ciprofloxacin on Salmonella intracellular survival and host virulence. Herein, Salmonella resistance patterns to various antibiotics were in agreement with those reported in previous studies. Moreover, intracellular survival of both ciprofloxacin-sensitive and -resistant Salmonella was markedly reduced upon treatment with sub-MIC of ciprofloxacin as determined by gentamicin protection assay. These findings were further confirmed using immunostaining indicating an inhibitory effect of sub-MIC of ciprofloxacin on Salmonella intracellular survival. RT-qPCR revealed that expression of genes encoding Salmonella type three secretion system (TTSS) decreased upon bacterial exposure to sub-MIC of ciprofloxacin. Furthermore, bacterial exposure to sub-MIC of ciprofloxacin significantly reduced expression of both sifA and sifB, which are important for Salmonella filaments formation within the host. Treatment of Salmonella with sub-MIC of ciprofloxacin reduced bacterial capacity to kill mice infection models. A lower mortality rate was observed in mice injected with Salmonella treated with sub-MIC of ciprofloxacin as compared with mice inoculated with untreated bacteria. Collectively, current findings indicate that, in addition to its bactericidal potential, sub-MIC of ciprofloxacin could inhibit Salmonella intracellular survival, virulence genes expression as well as host pathogenesis, providing another mechanism for ciprofloxacin in limiting Salmonella host infection.

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.

Salmonella enterica Effectors SifA, SpvB, SseF, SseJ, and SteA Contribute to Type III Secretion System 1-Independent Inflammation in a Streptomycin-Pretreated Mouse Model of Colitis

Salmonella enterica serovar Typhimurium (S. Typhimurium) induces inflammatory changes in the ceca of streptomycin-pretreated mice. In this mouse model of colitis, the type III secretion system 1 (T3SS-1) has been shown to induce rapid inflammatory change in the cecum at early points, 10 to 24 h after infection. Five proteins, SipA, SopA, SopB, SopD, and SopE2, have been identified as effectors involved in eliciting intestinal inflammation within this time range. In contrast, a T3SS-1-deficient strain was shown to exhibit inflammatory changes in the cecum at 72 to 120 h postinfection. However, the effectors eliciting T3SS-1-independent inflammation remain to be clarified. In this study, we focused on two T3SS-2 phenotypes, macrophage proliferation and cytotoxicity, to identify the T3SS-2 effectors involved in T3SS-1-independent inflammation. We identified a mutant strain that could not induce cytotoxicity in a macrophage-like cell line and that reduced intestinal inflammation in streptomycin-pretreated mice. We also identified five T3SS-2 effectors, SifA, SpvB, SseF, SseJ, and SteA, associated with T3SS-1-independent macrophage cytotoxicity. We then constructed a strain lacking T3SS-1 and all the five T3SS-2 effectors, termed T1S5. The S. Typhimurium T1S5 strain significantly reduced cytotoxicity in macrophages in the same manner as a mutant invA spiB strain (T1T2). Finally, the T1S5 strain elicited no inflammatory changes in the ceca of streptomycin-pretreated mice. We conclude that these five T3SS-2 effectors contribute to T3SS-1-independent inflammation.

Retrospective application of transposon-directed insertion-site sequencing to investigate niche-specific virulence of Salmonella Typhimurium in cattle

Background

Salmonella enterica subspecies enterica is an animal and zoonotic pathogen of global importance. Cattle are a significant reservoir of human non-typhoidal salmonellosis and can suffer enteric and systemic disease owing to the ability of Salmonella to survive within the bovine lymphatic system and intestines. Contamination of food can occur due to the incorporation of contaminated peripheral lymph nodes or by direct contamination of carcasses with gut contents. It is essential to understand the mechanisms used by Salmonella to enter and persist within the bovine lymphatic system and how they differ from those required for intestinal colonization to minimize zoonotic infections.

Results

Transposon-directed insertion site sequencing (TraDIS) was applied to pools of mutants recovered from mesenteric lymph nodes (MLNs) draining the distal ileum of calves after oral inoculation with a library of 8550 random S. Typhimurium mini-Tn5Km2 mutants in pools of 475 mutants per calf. A total of 8315 mutants representing 2852 different genes were detected in MLNs and their in vivo fitness was calculated. Using the same improved algorithm for analysis of transposon-flanking sequences, the identity and phenotype of mutants recovered from the distal ileal mucosa of the same calves was also defined, enabling comparison with previously published data and of mutant phenotypes across the tissues. Phenotypes observed for the majority of mutants were highly significantly correlated in the two tissues. However, 32 genes were identified in which transposon insertions consistently resulted in differential fitness in the ileal wall and MLNs, suggesting niche-specific roles for these genes in pathogenesis. Defined null mutations affecting ptsN and spvC were confirmed to result in tissue-specific phenotypes in calves, thus validating the TraDIS dataset.

Conclusions

This validation of the role of thousands of Salmonella genes and identification of genes with niche-specific roles in a key target species will inform the design of control strategies for bovine salmonellosis and zoonotic infections, for which efficacious and cross-protective vaccines are currently lacking.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5319-0) contains supplementary material, which is available to authorized users.

Goblet cell associated antigen passages are inhibited during Salmonella typhimurium infection to prevent pathogen dissemination and limit responses to dietary antigens

Dietary antigen acquisition by lamina propria (LP) dendritic cells (DCs) is crucial to induce oral tolerance and maintain homeostasis. However, encountering innocuous antigens during infection can lead to inflammatory responses, suggesting processes may limit steady-state luminal antigen capture during infection. We observed that goblet cell (GC) associated antigen passages (GAPs), a steady-state pathway delivering luminal antigens to LP-DCs, are inhibited during Salmonella infection. GAP inhibition was mediated by IL-1β. Infection abrogated luminal antigen delivery and antigen-specific T cell proliferation in the mesenteric lymph node (MLN). Antigen-specific T cell proliferation to dietary antigen was restored by overriding GAP suppression; however, this did not restore regulatory T cell induction, but induced inflammatory T cell responses. Salmonella translocation to the MLN required GCs and correlated with GAPs. Genetic manipulations overriding GAP suppression, or antibiotics inducing colonic GAPs, but not antibiotics that do not, increased dissemination and worsened outcomes independent of luminal pathogen burden. Thus, steady-state sampling pathways are suppressed during infection to prevent responses to dietary antigens, limit pathogen entry, and lessen the disease. Moreover, antibiotics may worsen Salmonella infection by means beyond blunting gut microbiota colonization resistance, providing new insight into how precedent antibiotic use aggravates enteric infection.

GUCY2C Signaling Opposes the Acute Radiation-Induced GI Syndrome

High doses of ionizing radiation induce acute damage to epithelial cells of the gastrointestinal (GI) tract, mediating toxicities restricting the therapeutic efficacy of radiation in cancer and morbidity and mortality in nuclear disasters. No approved prophylaxis or therapy exists for these toxicities, in part reflecting an incomplete understanding of mechanisms contributing to the acute radiation-induced GI syndrome (RIGS). Guanylate cyclase C (GUCY2C) and its hormones guanylin and uroguanylin have recently emerged as one paracrine axis defending intestinal mucosal integrity against mutational, chemical, and inflammatory injury. Here, we reveal a role for the GUCY2C paracrine axis in compensatory mechanisms opposing RIGS. Eliminating GUCY2C signaling exacerbated RIGS, amplifying radiation-induced mortality, weight loss, mucosal bleeding, debilitation, and intestinal dysfunction. Durable expression of GUCY2C, guanylin, and uroguanylin mRNA and protein by intestinal epithelial cells was preserved following lethal irradiation inducing RIGS. Oral delivery of the heat-stable enterotoxin (ST), an exogenous GUCY2C ligand, opposed RIGS, a process requiring p53 activation mediated by dissociation from MDM2. In turn, p53 activation prevented cell death by selectively limiting mitotic catastrophe, but not apoptosis. These studies reveal a role for the GUCY2C paracrine hormone axis as a novel compensatory mechanism opposing RIGS, and they highlight the potential of oral GUCY2C agonists (Linzess; Trulance) to prevent and treat RIGS in cancer therapy and nuclear disasters. Cancer Res; 77(18); 5095-106. ©2017 AACR.

Salmonella spv locus suppresses host innate immune responses to bacterial infection

Salmonella enterica serovar typhimurium (S. typhimurium) is globally distributed and causes massive morbidity and mortality in humans and animals. S. typhimurium carries Salmonella plasmid virulence (spv) locus, which is highly conserved and closely related to bacterial pathogenicity, while its exact role in host immune responses during infection remains to be elucidated. To counteract the invaders, the host has evolved numerous strategies, among which the innate immunity and autophagy act as the first defense. Recently, zebrafish has been universally accepted as a valuable and powerful vertebrate model in analyzing bacteria-host interactions. To investigate whether spv locus enhances the virulence of Salmonella by exerting an effect on the host early defense, zebrafish larvae were employed in this study. LD₅₀ of S. typhimurium to zebrafish larvae and bacterial dissemination were analyzed. Sudan black B and neutral red staining were performed to detect the responses of neutrophils and macrophages to Salmonella infection. Autophagy agonist Torin1 and inhibitor Chloroquine were used to interfere in autophagic flux, and the protein level of Lc3 and p62 were measured by western blotting. Results indicated that spv locus could decrease the LD₅₀ of S. typhimurium to zebrafish larvae, accelerate the reproduction and dissemination of bacteria by inhibiting the function of neutrophils and macrophages. Moreover, spv locus restrained the formation of autophagosomes in the earlier stage of autophagy. These findings suggested the virulence of spv locus involving in suppressing host innate immune responses for the first time, which shed new light on the role of spv operon in Salmonella pathogenicity.

Salmonella-induced Diarrhea Occurs in the Absence of IL-8 Receptor (CXCR2)-Dependent Neutrophilic Inflammation

BACKGROUND

Gastroenteritis is the most common manifestation of nontyphoidal Salmonella enterica infections, but little is known about the pathogenesis of diarrhea in this infection

METHODS

To determine whether polymorphonuclear neutrophils (PMNs) are required for diarrhea for Salmonella colitis, we infected kanamycin-pretreated interleukin 8R (IL-8R) mutant mice and controls, both with nonmutant Slc11a1 (Nramp1, ItyR). We compared the 2 mouse strains for increases in fecal water content (diarrhea) 3 days after infection, changes in expression of ion transporters in colonic epithelial cells, proliferation of epithelial cells, and severity of infection as measured by colony-forming units (CFUs).

RESULTS

The IL-8R knockout mice had fewer PMNs in the colon but the other variables we measured were unaffected except for an increase in CFUs in the colon. The pathologic changes in the cecum were similar in both groups except for the lack of PMNs in the IL-8R knockout mice. There was minimal damage to the colon more distally.

CONCLUSIONS

In the early stage of Salmonella colitis, PMNs are not required for diarrhea or for the decrease in expression of colonic epithelial cell apical ion transporters. They contribute to defense against infection in the cecum but not extracolonically at this stage of Salmonella colitis.

Toll-like receptor-deficient mice reveal how innate immune signaling influences Salmonella virulence strategies

Pathogens utilize features of the host response as cues to regulate virulence gene expression. Salmonella enterica serovar Typhimurium (ST) sense Toll-like receptor (TLR)-dependent signals to induce Salmonella Pathogenicity Island 2 (SPI2), a locus required for intracellular replication. To examine pathogenicity in the absence of such cues, we evaluated ST virulence in mice lacking all TLR function (Tlr2(-/-)xTlr4(-/-)xUnc93b1(3d/3d)). When delivered systemically to TLR-deficient mice, ST do not require SPI2 and maintain virulence by replicating extracellularly. In contrast, SPI2 mutant ST are highly attenuated after oral infection of the same mice, revealing a role for SPI2 in the earliest stages of infection, even when intracellular replication is not required. This early requirement for SPI2 is abolished in MyD88(-/-)xTRIF(-/-) mice lacking both TLR- and other MyD88-dependent signaling pathways, a potential consequence of compromised intestinal permeability. These results demonstrate how pathogens use plasticity in virulence strategies to respond to different host immune environments.

Altered expression and localization of ion transporters contribute to diarrhea in mice with Salmonella-induced enteritis

BACKGROUND & AIMS

Salmonella enterica serovar Typhimurium is an enteropathogen that causes self-limiting diarrhea in healthy individuals, but poses a significant health threat to vulnerable populations. Our understanding of the pathogenesis of Salmonella-induced diarrhea has been hampered by the lack of a suitable mouse model. After a dose of oral kanamycin, Salmonella-infected congenic BALB/c.D2(NrampG169) mice, which carry a wild-type Nramp1 gene, develop clear manifestations of diarrhea. We used this model to elucidate the pathophysiology of Salmonella-induced diarrhea.

METHODS

BALB /c.D2(NrampG169) mice were treated with kanamycin and then infected with wild-type or mutant Salmonella by oral gavage. Colon tissues were isolated and Ussing chambers, quantitative polymerase chain reaction, immunoblot, and confocal microscopy analyses were used to study function and expression of ion transporters and cell proliferation.

RESULTS

Studies with Ussing chambers demonstrated reduced basal and/or adenosine 3',5'-cyclic monophosphate-mediated electrogenic ion transport in infected colonic tissues, attributable to changes in chloride or sodium transport, depending on the segment studied. The effects of infection were mediated, at least in part, by effector proteins secreted by the bacterial Salmonella pathogenicity island 1- and Salmonella pathogenicity island-2-encoded virulence systems. Infected tissue showed reduced expression of the chloride-bicarbonate exchanger down-regulated in adenoma in surface colonic epithelial cells. Cystic fibrosis transmembrane conductance regulator was internalized in colonic crypt epithelial cells without a change in overall expression levels. Confocal analyses, densitometry, and quantitative polymerase chain reaction revealed that expression of epithelial sodium channel β was reduced in distal colons of Salmonella-infected mice. The changes in transporter expression, localization, and/or function were accompanied by crypt hyperplasia in Salmonella-infected mice.

CONCLUSIONS

Salmonella infection induces diarrhea by altering expression and/or function of transporters that mediate water absorption in the colon, likely reflecting the fact that epithelial cells have less time to differentiate into surface cells when proliferation rates are increased by infection.

Evaluation of Salmonella enterica serovar Typhimurium TTSS-2 deficient fur mutant as safe live-attenuated vaccine candidate for immunocompromised mice

Salmonella enterica serovar Typhimurium has been extensively exploited as live attenuated vaccines (LAV) which generally confers better protection than killed or subunit vaccines. However, many LAV are limited by their inherent ability to access systemic organs in many of the vaccinated hosts, especially those which are immunocompromised. We evaluated the efficacy of a live-attenuated SPI2-deficient (ΔssaV) S. Typhimurium vaccine candidate (MT13) that additionally devoids the ferric uptake regulator (fur). We used specific pathogen free (SPF) streptomycin-pretreated mouse colitis model that included healthy C57BL/6 and immunocompromised iNos^−/−, IL10^−/− and CD40L^−/− in the background of C57BL/6 mice to assess the efficacy of developed vaccine candidate. In our study, the S. Typhimurium MT13 strain was established as a safe vaccine candidate to be administered in immunocompromised mice as it was found to be systemically attenuated without conferring significant pathological signs and growth defect within the host. In bacterial challenge experiment, the MT13-vaccinated C57BL/6 mice were protected from subsequent wild-type S. Typhimurium infection by inducing proficient mucosal immunity. The MT13 strain elicited efficient O-antigen specific mucosal secretory IgA associated protective response which was comparable with its parental ssaV mutant. Vaccination with MT13 also showed proficient T-cell activation in host mice; which has direct relation with pathogen clearance from host tissues. Collectively, these data implicate the possible application of SPI-2 deficient fur mutant (MT13) as a novel live attenuated vaccine strain with adept immunogenicity and improved safety, even in immunocompromised hosts. Further, this vaccine candidate can be employed to express heterologous antigens targeted against several other diseases, especially related to enterocolitic pathogens.