The RNA chaperone Hfq regulates expression of fimbrial-related genes and virulence of Salmonella enterica serovar Enteritidis

RyhB Regulates Capsular Synthesis for Serum Resistance and Virulence of Avian Pathogenic Escherichia coli

Avian pathogenic Escherichia coli (APEC) causes bloodstream infections mainly by resisting the bactericidal action of host serum. Although various protein and polysaccharide factors involved in serum resistance have been identified, the role of small non-coding RNA (sRNA) in serum resistance has rarely been studied. The sRNA RyhB contributes to serum resistance in APEC, but the regulation mechanism of RyhB to serum resistance-related targets remains unknown. Here, we studied the regulatory mechanism of RyhB on capsule synthesis and how RyhB regulates serum resistance, macrophage phagocytosis resistance, and pathogenicity to natural hosts by regulating capsule synthesis. The results showed that RyhB upregulates capsular synthesis by interacting with the promoter regions of the capsule gene cluster and activating the translation of the capsule. The deletion of ryhB and/or neu reduced the ability of resistance to serum, macrophage phagocytosis, and pathogenicity of APEC in ducks. It can be concluded that RyhB directly upregulates the expression of capsular gene cluster and capsular synthesis and then indirectly promotes resistance to serum and macrophage phagocytosis and pathogenicity to ducks.

Positive regulation of Type III secretion effectors and virulence by RyhB paralogs in Salmonella enterica serovar Enteritidis

Small non-coding RNA RyhB is a key regulator of iron homeostasis in bacteria by sensing iron availability in the environment. Although RyhB is known to influence bacterial virulence by interacting with iron metabolism related regulators, its interaction with virulence genes, especially the Type III secretion system (T3SS), has not been reported. Here, we demonstrate that two RyhB paralogs of Salmonella enterica serovar Enteritidis upregulate Type III secretion system (T3SS) effectors, and consequently affect Salmonella invasion into intestinal epithelial cells. Specifically, we found that RyhB-1 modulate Salmonella response to stress condition of iron deficiency and hypoxia, and stress in simulated intestinal environment (SIE). Under SIE culture conditions, both RyhB-1 and RyhB-2 are drastically induced and directly upregulate the expression of T3SS effector gene sipA by interacting with its 5′ untranslated region (5′ UTR) via an incomplete base-pairing mechanism. In addition, the RyhB paralogs upregulate the expression of T3SS effector gene sopE. By regulating the invasion-related genes, RyhBs in turn affect the ability of S. Enteritidis to adhere to and invade into intestinal epithelial cells. Our findings provide evidence that RyhBs function as critical virulence factors by directly regulating virulence-related gene expression. Thus, inhibition of RyhBs may be a potential strategy to attenuate Salmonella.

Transcription of the Subtilase Cytotoxin Gene subAB 1 in Shiga Toxin-Producing Escherichia coli Is Dependent on hfq and hns

Certain foodborne Shiga toxin-producing Escherichia coli (STEC) strains carry genes encoding the subtilase cytotoxin (SubAB). Although the mode of action of SubAB is under intensive investigation, information about the regulation of subAB gene expression is currently not available. In this study, we investigated the regulation of the chromosomal subAB ₁ gene in laboratory E. coli strain DH5α and STEC O113:H21 strain TS18/08 using a luciferase reporter gene assay. Special emphasis was given to the role of the global regulatory protein genes hfq and hns in subAB ₁ promoter activity. Subsequently, quantitative real-time PCR was performed to analyze the expression of Shiga toxin 2a (Stx2a), SubAB1, and cytolethal distending toxin V (Cdt-V) genes in STEC strain TS18/08 and its isogenic hfq and hns deletion mutants. The deletion of hfq led to a significant increase of up to 2-fold in subAB ₁ expression, especially in the late growth phase, in both strains. However, deletion of hns showed different effects on the promoter activity during the early and late exponential growth phases in both strains. Furthermore, upregulation of stx _2a and cdt-V was demonstrated in hfq and hns deletion mutants in TS18/08. These data showed that the expression of subAB ₁, stx _2a, and cdt-V is integrated in the regulatory network of global regulators Hfq and H-NS in Escherichia coli IMPORTANCE Shiga toxin-producing Escherichia coli (STEC) strains are responsible for outbreaks of foodborne diseases, such as hemorrhagic colitis and the hemolytic uremic syndrome. The pathogenicity of those strains can be attributed to, among other factors, the production of toxins. Recently, the subtilase cytotoxin was detected in locus of enterocyte effacement (LEE)-negative STEC, and it was confirmed that it contributes to the cytotoxicity of those STEC strains. Although the mode of action of SubAB1 is under intensive investigation, the regulation of gene expression is currently not known. The global regulatory proteins H-NS and Hfq have impact on many cellular processes and have been described to regulate virulence factors as well. Here, we investigate the role of hns and hfq in expression of subAB ₁ as well as stx _2a and cdt-V in an E. coli laboratory strain as well as in wild-type STEC strain TS18/08.

Small non-coding RNA STnc640 regulates expression of fimA fimbrial gene and virulence of Salmonella enterica serovar Enteritidis

Background

Small non-coding RNAs (sRNAs) regulate bacterial gene expression at the post-transcriptional level. STnc640 is a type of sRNA that was identified in Salmonella Typhimurium.

Results

In this study, STnc640 in Salmonella Enteritidis was confirmed to be an Hfq-dependent sRNA. TargetRNA software analysis showed that fimbrial genes fimA and bcfA were likely to be the target genes of STnc640. To investigate the target mRNAs and function of STnc640 in pathogenicity, we constructed the deletion mutant strain 50336△stnc640 and the complemented strain 50336△stnc640/pstnc640 in Salmonella Enteritidis 50336. The RT-qPCR results showed that the mRNA level of fimA was decreased, while bcfA was unchanged in 50336△stnc640 compared with that in the wild type (WT) strain. The adhesion ability of 50336△stnc640 to Caco-2 cells was increased compared to the 50336 WT strain. The virulence of 50336△stnc640 was enhanced in a one-day-old chicken model of S. Enteritidis disease as determined by quantifying the 50% lethal dose (LD₅₀) of the bacterial strains.

Conclusions

The results demonstrate that STnc640 contributes to the virulence of Salmonella Enteritidis.

Small RNA Esr41 inversely regulates expression of LEE and flagellar genes in enterohaemorrhagic Escherichia coli

Enterohaemorrhagic Escherichia coli (EHEC) is a life-threatening human pathogen worldwide. The locus of enterocyte effacement (LEE) in EHEC encodes a type three secretion system and effector proteins, all of which are essential for bacterial adherence to host cells. When LEE expression is activated, flagellar gene expression is down-regulated because bacterial flagella induce the immune responses of host cells at the infection stage. Therefore, this inverse regulation is also important for EHEC infection. We report here that a small regulatory RNA (sRNA), Esr41, mediates LEE repression and flagellar gene activation. Multiple copies of esr41 abolished LEE expression by down-regulating the expression of ler and pch, which encode positive regulators of LEE. This regulation led to reduced EHEC adhesion to host cells. Translational gene-reporter fusion experiments revealed that Esr41 regulates ler expression at a post-transcriptional level, and pch transcription, probably via an unknown target of Esr41. Esr41-mediated ler and pch repression was not observed in cells lacking hfq, which encodes an RNA-binding protein essential for most sRNA functions, indicating that Esr41 acts in an Hfq-dependent manner. We previously reported an increase in cell motility induced by Esr41. This motility enhancement was also observed in EHEC lacking ler, showing that Esr41-mediated enhancement of cell motility is in a ler-independent manner. In addition, Esr41 activated the expression of flagellar Class 3 genes by indirectly inducing the transcription of fliA, which encodes the sigma factor for flagellar synthesis. These results suggest that Esr41 plays important roles in the inverse regulation of LEE and flagellar gene expression.

Immunogenicity and protective efficacy of a Salmonella Enteritidis sptP mutant as a live attenuated vaccine candidate

Background

Salmonella enterica serovar Enteritidis (S. Enteritidis) is a highly adaptive pathogen in both humans and animals. As a Salmonella Type III secretion system (T3SS) effector, Salmonella protein tyrosine phosphatase (SptP) is critical for virulence in this genus. To investigate the feasibility of using C50336ΔsptP as a live attenuated oral vaccine in mice, we generated the sptP gene deletion mutant C50336ΔsptP in S. Enteritidis strain C50336 by λ-Red mediated recombination and evaluated the protective ability of the S. Enteritidis sptP mutant strain C50336ΔsptP against mice salmonellosis.

Results

We found that C50336ΔsptP was a highly immunogenic, effective, and safe vaccine in mice. Compared to wild-type C50336, C50336ΔsptP showed reduced virulence as confirmed by the 50% lethal dose (LD₅₀) in orally infected mice. C50336ΔsptP also showed decreased bacterial colonization both in vivo and in vitro. Immunization with C50336ΔsptP had no significant effect on body weight and did not result in obvious clinical symptoms relative to control animals treated with phosphate-buffered saline (PBS), but induced humoral and cellular immune responses at 12 and 26 days post inoculation. Immunization with 1 × 10⁸ colony-forming units (CFU) C50336ΔsptP per mouse provided 100% protection against subsequent challenge with the wild-type C50336 strain, and immunized mice showed mild and temporary clinical symptoms as compared to those of control group.

Conclusions

These results demonstrate that C50336ΔsptP can be a live attenuated oral vaccine for salmonellosis.

Inhibition of Phagocytic Killing of Escherichia coli in Drosophila Hemocytes by RNA Chaperone Hfq

An RNA chaperone of Escherichia coli, called host factor required for phage Qβ RNA replication (Hfq), forms a complex with small noncoding RNAs to facilitate their binding to target mRNA for the alteration of translation efficiency and stability. Although the role of Hfq in the virulence and drug resistance of bacteria has been suggested, how this RNA chaperone controls the infectious state remains unknown. In the present study, we addressed this issue using Drosophila melanogaster as a host for bacterial infection. In an assay for abdominal infection using adult flies, an E. coli strain with mutation in hfq was eliminated earlier, whereas flies survived longer compared with infection with a parental strain. The same was true with flies deficient in humoral responses, but the mutant phenotypes were not observed when a fly line with impaired hemocyte phagocytosis was infected. The results from an assay for phagocytosis in vitro revealed that Hfq inhibits the killing of E. coli by Drosophila phagocytes after engulfment. Furthermore, Hfq seemed to exert this action partly through enhancing the expression of σ(38), a stress-responsive σ factor that was previously shown to be involved in the inhibition of phagocytic killing of E. coli, by a posttranscriptional mechanism. Our study indicates that the RNA chaperone Hfq contributes to the persistent infection of E. coli by maintaining the expression of bacterial genes, including one coding for σ(38), that help bacteria evade host immunity.

Edwardsiella tarda Hfq: impact on host infection and global protein expression

Hfq is an RNA-binding protein that plays an important role in many cellular processes. In this study, we examined the biological effect of the Hfq of Edwardsiella tarda, a severe fish pathogen with a broad host range that includes humans. To facilitate the study, a markerless hfq in-frame deletion wild type, TXhfq, was constructed. Compared to the wild type TX01, TXhfq exhibited (i) retarded planktonic and biofilm growth, (ii) decreased resistance against oxidative stress, (iii) attenuated overall virulence and tissue dissemination and colonization capacity, (iv) impaired ability to replicate in host macrophages and to block host immune response. Introduction of a trans-expressed hfq gene into TXhfq restored the lost virulence of TXhfq. To identify potential Hfq targets, comparative global proteomic analysis was conducted, which revealed that 20 proteins belonging to different functional categories were differentially expressed in TXhfq and TX01. Quantitative real time RT-PCR analysis showed that the mRNA levels of two thirds of the genes of the identified proteins were consistent with the proteomic results. Since TXhfq is dramatically attenuated in virulence, we further examined its potential as a naturally delivered vaccine administered via the immersion route in a flounder model. The results showed that TXhfq induced effective protection against lethal E. tarda challenge. Taken together, our study indicated that Hfq is required for the normal operation of E. tarda in multiple aspects, and that Hfq probably exerts a regulatory effect on a wide range of target genes at both transcription and post-transcription levels.