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Salmonella enterica, like many gram-negative pathogens, uses type three secretion systems (TTSS) to infect its hosts. The three TTSS of Salmonella, namely, TTSS-1, TTSS-2, and flagella, play a major role in the virulence of this bacterium, allowing it to cross the intestinal barrier and to disseminate systemically. Previous data from our laboratory have demonstrated the involvement of the chromosomal region harboring the yfgL, engA, and yfgJ open reading frames in S. enterica serovar Enteritidis virulence. Using microarray analysis and real-time reverse transcription-PCR after growth of bacterial cultures favorable for either TTSS-1 or TTSS-2 expression, we show in this study that the deletion in S. enterica serovar Enteritidis of yfgL, encoding an outer membrane lipoprotein, led to the transcriptional down-regulation of most Salmonella pathogenicity island 1 (SPI-1), SPI-2, and flagellar genes encoding the TTSS structural proteins and effector proteins secreted by these TTSS. In line with these results, the virulence of the ⌬yfgL mutant was greatly attenuated in mice. Moreover, even if YfgL is involved in the assembly of outer membrane proteins, the regulation of TTSS expression observed was not due to an inability of the ⌬yfgL mutant to assemble TTSS in its membrane. Indeed, when we forced the transcription of SPI-1 genes by constitutively expressing HilA, the secretion of the TTSS-1 effector protein SipA was restored in the culture supernatant of the mutant. These results highlight the crucial role of the outer membrane lipoprotein YfgL in the expression of all Salmonella TTSS and, thus, in the virulence of Salmonella. Therefore, this outer membrane protein seems to be a privileged target for fighting Salmonella.Salmonella enterica infections are an important worldwide health problem. Salmonella serovars are responsible for diseases ranging from mild gastroenteritis to life-threatening systemic infections. During the course of infection, these serovars use many virulence factors, among which the type III secretion systems (TTSS) play a major role. TTSS-1, encoded by Salmonella pathogenicity island 1 (SPI-1), mainly allows intestinal epithelial cell invasion (57), thereby allowing the bacteria to cross the intestinal barrier. TTSS-2, encoded by SPI-2, is required for intracellular survival and multiplication (54) and, consequently, is important for systemic dissemination of the bacteria. The virulence phenotypes associated with SPI-1 and SPI-2 are dependent on the ability of the TTSS to deliver effector proteins into the host cell cytosol. Thus, bacteria hijack the eukaryotic cellular machinery for their own profit. The flagella, which share a common architectural design with TTSS, are involved in the motility of the bacteria and favor the interaction with the intestinal epithelium (31, 47). However, their role in Salmonella virulence remains controversial (26,27,47).We previously characterized a Salmonella enterica subsp. enterica serovar Enteritidis mutant which was altered in motility and in invasion of Cac...

Carriage of Salmonella is often associated with a high level of bacterial excretion and generally occurs after a short systemic infection. However, we do not know whether this systemic infection is required or whether the carrier-state corresponds to continuous reinfection or real persistence in caecal tissue. The use of a Salmonella Enteritidis bamB mutant demonstrated that a carrier-state could be obtained in chicken in the absence of systemic infection. The development of a new infection model in isolator showed that a marked decrease in animal reinfection and host-to-host transmission between chicks led to a heterogeneity of S. Enteritidis excretion and colonization contrary to what was observed in cages. This heterogeneity of infection was characterized by the presence of super-shedders, which constantly disseminated Salmonella to the low-shedder chicks, mainly through airborne movements of contaminated dust particles. The presence of super-shedders, in the absence of host-to-host transmission, demonstrated that constant reinfection was not required to induce a carrier-state. Finally, our results suggest that low-shedder chicks do not have a higher capability to destroy Salmonella but instead can block initial Salmonella colonization. This new paradigm opens new avenues to improve understanding of the carrier-state mechanisms and to define new strategies to control Salmonella infections.© 2018 Society for Applied Microbiology and John Wiley & Sons Ltd.

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In Escherichia coli, the assembly of outer-membrane proteins (OMP) requires the BAM complex and periplasmic chaperones, such as SurA or DegP. Previous work has suggested a potential link between OMP assembly and expression of the genes encoding type-III secretion systems. In order to test this hypothesis, we studied the role of the different lipoproteins of the BAM complex (i.e. BamB, BamC, BamD and BamE), and the periplasmic chaperones SurA and DegP, in these two phenotypes in Salmonella. Analysis of the corresponding deletion mutants showed that, as previously described with the DbamB mutant, BamD, SurA and, to a lesser extent, BamE play a role in outer-membrane biogenesis in Salmonella Enteritidis, while the membrane was not notably disturbed in DbamC and DdegP mutants. Interestingly, we found that BamD is not essential in Salmonella, unlike its homologues in Escherichia coli and Neisseria gonorrhoeae. In contrast, BamD was the only protein required for full expression of T3SS-1 and flagella, as demonstrated by transcriptional analysis of the genes involved in the biosynthesis of these T3SSs. In line with this finding, bamD mutants showed a reduced secretion of effector proteins by these T3SSs, and a reduced ability to invade HT-29 cells. As DsurA and DbamE mutants had lower levels of OMPs in their outer membrane, but showed no alteration in T3SS-1 and flagella expression, these results demonstrate the absence of a systematic link between an OMP assembly defect and the downregulation of T3SSs in Salmonella; therefore, this link appears to be related to a more specific mechanism that involves at least BamB and BamD.