Identification of a new Salmonella enterica serovar Enteritidis locus involved in cell invasion and in the colonisation of chicks

TolC facilitates the intracellular survival and immunomodulation of Salmonella Typhi in human host cells

Salmonella enterica serovar Typhi (S. Typhi) causes typhoid fever, a systemic infection that affects millions of people worldwide. S. Typhi can invade and survive within host cells, such as intestinal epithelial cells and macrophages, by modulating their immune responses. However, the immunomodulatory capability of S. Typhi in relation to TolC-facilitated efflux pump function remains unclear. The role of TolC, an outer membrane protein that facilitates efflux pump function, in the invasion and immunomodulation of S. Typhi, was studied in human intestinal epithelial cells and macrophages. The tolC deletion mutant of S. Typhi was compared with the wild-type and its complemented strain in terms of their ability to invade epithelial cells, survive and induce cytotoxicity in macrophages, and elicit proinflammatory cytokine production in macrophages. The tolC mutant, which has a defective outer membrane, was impaired in invading epithelial cells compared to the wild-type strain, but the intracellular presence of the tolC mutant exhibited greater cytotoxicity and induced higher levels of proinflammatory cytokines (IL-1β and IL-8) in macrophages compared to the wild-type strain. These effects were reversed by complementing the tolC mutant with a functional tolC gene. Our results suggest that TolC plays a role in S. Typhi to efficiently invade epithelial cells and suppress host immune responses during infection. TolC may be a potential target for the development of novel therapeutics against typhoid fever.

Deletion of Salmonella enterica serovar Typhi tolC reduces bacterial adhesion and invasion toward host cells

Background

S. Typhi is a Gram-negative bacterium that causes typhoid fever in humans. Its virulence depends on the TolC outer membrane pump, which expels toxic compounds and antibiotics. However, the role of TolC in the host cell adhesion and invasion by S. Typhi is unclear.

Objective

We aimed to investigate how deleting the tolC affects the adhesion and invasion of HT-29 epithelial and THP-1 macrophage cells by S. Typhi in vitro.

Methods

We compared the adhesion and invasion rates of the wild-type and the tolC mutant strains of S. Typhi using in vitro adhesion and invasion assays. We also measured the expression levels of SPI-1 genes (invF, sipA, sipC, and sipD) using quantitative PCR.

Results

We found that the tolC mutant showed a significant reduction in adhesion and invasion compared to the wild-type strain in both cell types. We also observed that the expression of SPI-1 genes was downregulated in the tolC mutant.

Discussion

Our results suggest that TolC modulates the expression of SPI-1 genes and facilitates the adhesion and invasion of host cells by S. Typhi. Our study provides new insights into the molecular mechanisms of S. Typhi pathogenesis and antibiotic resistance. However, our study is limited by the use of in vitro models and does not reflect the complex interactions between S. Typhi and host cells in vivo.

Pathomorphological changes in the organs of chickens infected spontaneously by the species Salmonella pullorum on private farms in Chernivtsi region

Commercial poultry production growth and the increase in the number of small farms specializing in raising broiler chickens, laying hens, quails, and other poultry contribute to the spread of infectious diseases. Non-compliance with the principles of biosafety during incubation and breeding results in mass deaths of poultry and, consequently, significant economic losses for farmers. Salmonellosis is one of the most dangerous anthropozoonotic diseases of poultry, which is most often registered in private farms. Age analysis of the poultry salmonellosis in the EU countries indicates the infection of adult laying hens most often, young poultry to a lesser extent, and chickens aged up to 10 days less frequently. Although the program for the prevention and elimination of poultry salmonellosis has been approved at the legislative level in Ukraine, monitoring studies are not carefully conducted. This is especially true for private homesteads and small farms, which greatly complicates the epizootiological situation in some regions of the country. The article describes the pathological and histological changes in the liver, heart, lungs, kidneys, and spleen. The changes were detected in chickens aged 10 and 14 days infected with microorganisms of the species Salmonella pullorum. Eggs for incubation were obtained from different family flocks, and incubation was performed in a single incubator. The initial clinical signs of the disease appeared in chickens aged 7 days and included diarrhea, increased water consumption, lameness, mass concentration of chickens near heat sources, nervous phenomena in the form of circle walking, and partial blindness in some cases. The pathological autopsy revealed hepatomegaly with sharp change in the organ colour, diapedetic hemorrhage under Glisson’s capsule, and diffuse miliary necrosis. Greyish-white nodular lesions of the lungs and heart, dystrophic changes in the kidneys, and deposition of uric acid salts in the ureters were also characteristic features. Diffuse coagulation necrosis, massive perivascular infiltration by heterophilic lymphocytes and stasis were observed in the liver. Changes in the heart were characterized by significant infiltration by mononuclear cells and heterophiles, which led to atrophy, necrosis, and replacement of cardiomyocytes by connective tissue cellular elements. Delymphatization and necrosis of the lymph nodes were pronounced in the spleen.

Immune Suppression Induced by Gallibacterium anatis GtxA During Interaction with Chicken Macrophage-Like HD11 Cells

The RTX toxin GtxA expressed by Gallibacterium anatis biovar haemolytica has been proposed a major virulence factor during disease manifestations in the natural host, the chicken. To better understand the role of GtxA in the pathogenesis of G. anatis, we compared the GtxA expressing wildtype strain with its isogenic ∆gtxA mutant that was unable to express GtxA during exposure to chicken macrophage-like HD11 cells. From adhesion and invasion assays, we showed that GtxA appears to promote adhesion and invasion of HD11 cells. By using quantitative RT-PCR, we also demonstrated that the G. anatis expressing GtxA induced a mainly anti-inflammatory (IL-10) host cell response as opposed to the pro-inflammatory (IL-1β, IL-6 and TNF-α) response induced by the GtxA deletion mutant. Interestingly, these results, at least partly, resemble recent responses observed from spleen tissue of chickens infected with the same two bacterial strains. The effect of the GtxA toxin on the type of cell death was less clear. While GtxA clearly induced cell death, our efforts to characterize whether this was due to primarily necrosis or apoptosis through expression analysis of a broad range of apoptosis genes did not reveal clear answers.

Envelope Stress and Regulation of the Salmonella Pathogenicity Island 1 Type III Secretion System

Salmonella enterica serovar Typhimurium uses a type three secretion system (T3SS) encoded on the Salmonella pathogenicity island 1 (SPI1) to invade intestinal epithelial cells and induce inflammatory diarrhea. The SPI1 T3SS is regulated by numerous environmental and physiological signals, integrated to either activate or repress invasion. Transcription of hilA, encoding the transcriptional activator of the SPI1 structural genes, is activated by three AraC-like regulators, HilD, HilC, and RtsA, that act in a complex feed-forward loop. Deletion of bamB, encoding a component of the β-barrel assembly machinery, causes a dramatic repression of SPI1, but the mechanism was unknown. Here, we show that partially defective β-barrel assembly activates the RcsCDB regulon, leading to decreased hilA transcription. This regulation is independent of RpoE activation. Though Rcs has been previously shown to repress SPI1 when disulfide bond formation is impaired, we show that activation of Rcs in a bamB background is dependent on the sensor protein RcsF, whereas disulfide bond status is sensed independently. Rcs decreases transcription of the flagellar regulon, including fliZ, the product of which indirectly activates HilD protein activity. Rcs also represses hilD, hilC, and rtsA promoters by an unknown mechanism. Both dsbA and bamB mutants have motility defects, though this is simply regulatory in a bamB background; motility is restored in the absence of Rcs. Effector secretion assays show that repression of SPI1 in a bamB background is also regulatory; if expressed, the SPI1 T3SS is functional in a bamB background. This emphasizes the sensitivity of SPI1 regulation to overall envelope homeostasis.IMPORTANCE Salmonella causes worldwide foodborne illness, leading to massive disease burden and an estimated 600,000 deaths per year. Salmonella infects orally and invades intestinal epithelial cells using a type 3 secretion system that directly injects effector proteins into host cells. This first step in invasion is tightly regulated by a variety of inputs. In this work, we demonstrate that Salmonella senses the functionality of outer membrane assembly in determining regulation of invasion machinery, and we show that Salmonella uses distinct mechanisms to detect specific perturbations in envelope assembly.

Maintaining Integrity Under Stress: Envelope Stress Response Regulation of Pathogenesis in Gram-Negative Bacteria

The Gram-negative bacterial envelope is an essential interface between the intracellular and harsh extracellular environment. Envelope stress responses (ESRs) are crucial to the maintenance of this barrier and function to detect and respond to perturbations in the envelope, caused by environmental stresses. Pathogenic bacteria are exposed to an array of challenging and stressful conditions during their lifecycle and, in particular, during infection of a host. As such, maintenance of envelope homeostasis is essential to their ability to successfully cause infection. This review will discuss our current understanding of the σ^E- and Cpx-regulated ESRs, with a specific focus on their role in the virulence of a number of model pathogens.

Role of systemic infection, cross contaminations and super-shedders in Salmonella carrier state in chicken

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.

Genetic Basis of Salmonella Enteritidis Pathogenesis in Chickens

Salmonella Enteritidis (SE) is the predominant cause of the food-borne salmonellosis in humans, in part because this serotype has the unique ability to contaminate chicken eggs without causing discernible illness in the infected birds. Attempts to develop effective vaccines and eradicate SE from chickens are undermined by significant limitations in our current understanding of the genetic basis of pathogenesis of SE in this reservoir host. In this chapter, we summarize the infection kinetics and provide an overview of the current understanding of genetic factors underlying SE infection in the chicken host. We also discuss the important knowledge gaps that, if addressed, will improve our understanding of the complex biology of SE in young chickens and in egg laying hens.

Novel antimicrobial peptides with promising activity against multidrug resistant Salmonella enterica serovar Choleraesuis and its stress response mechanism

AIMS

To evaluate the antibacterial efficacy of novel antimicrobial peptides (AMPs) against multidrug-resistant (MDR) Salmonella enterica serovar Choleraesuis (Salm. Choleraesuis) and to delineate the AMP-responsive mechanisms of wild-type (WT) and MDR strains.

METHODS AND RESULTS

Proteomic approaches were performed based on two-dimensional gel electrophoresis and liquid chromatography-electrospray ionization-quadrupole- time-of-flight tandem mass spectrometry to analyse the protein profiles of these two strains upon exposure to AMP GW-Q6. Quantitative real-time PCR was conducted to determine the mRNA expression level of the target genes. Furthermore, lipopolysaccharide (LPS) competition analysis was used to verify whether LPS may serve as the potential binding target when AMP approach and adhere to the bacterial surface.

CONCLUSIONS

The minimal inhibitory concentration assay revealed that our AMPs were even more effective against the MDR strains (4-32 μg ml(-1) ), compared with those for the WT (8-64 μg ml(-1) ). LPS dose-dependently suppressed the GW-Q6 antimicrobial activity. Clusters of orthologous groups analysis showed that the majority of the AMP-responsive proteins were involved in cell envelope biogenesis and outer membrane, translation and chaperones.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results indicated that the novel AMP GW-Q6 serves as a potential candidate for antimicrobial drug development against MDR strains. These findings will also be helpful for expanding our knowledge on the molecular mechanisms of AMP-microbe interaction and the pathogenicity of salmonellosis caused by MDR strains of Salm. Choleraesuis.

The Klebsiella pneumoniae YfgL (BamB) lipoprotein contributes to outer membrane protein biogenesis, type-1 fimbriae expression, anti-phagocytosis, and in vivo virulence

Klebsiella pneumoniae is an opportunistic pathogen that causes several kinds of infections, including pneumonia, bacteremia, urinary tract infection and community-acquired pyogenic liver abscess (PLA). Adhesion is the critical first step in the infection process. Our previous work demonstrated that the transcellular translocation is exploited by K. pneumoniae strains to migrate from the gut flora into other tissues, resulting in systemic infections. However, the initial stages of K. pneumoniae infection remain unclear. In this study, we demonstrated that a K. pneumoniae strain deleted for yfgL (bamB) exhibited reduced adherence to and invasion of host cells; changed biogenesis of major β-barrel outer membrane proteins; decreased transcriptional expression of type-1 fimbriae; and increased susceptibility to vancomycin and erythromycin. The yfgL deletion mutant also had reduced ability to against neutrophil phagocytosis; exhibited decreased induction of host IL-6 production; and was profoundly attenuated for virulence in a K. pneumoniae model of bacteremia. Thus, the K. pneumoniae YfgL lipoprotein mediates in outer membrane proteins biogenesis and is crucial for anti-phagocytosis and survival in vivo. These data provide a new insight for K. pneumoniae attachment and such knowledge could facilitate preventive therapies or alternative therapies against K. pneumoniae.

Assembly of Outer Membrane β-Barrel Proteins: the Bam Complex

The major class of integral proteins found in the outer membrane (OM) of E. coli and Salmonella adopt a β-barrel conformation (OMPs). OMPs are synthesized in the cytoplasm with a typical signal sequence at the amino terminus, which directs them to the secretion machinery (SecYEG) located in the inner membrane for translocation to the periplasm. Chaperones such as SurA, or DegP and Skp, escort these proteins across the aqueous periplasm protecting them from aggregation. The chaperones then deliver OMPs to a highly conserved outer membrane assembly site termed the Bam complex. In E. coli, the Bam complex is composed of an essential OMP, BamA, and four associated OM lipoproteins, BamBCDE, one of which, BamD, is also essential. Here we provide an overview of what we know about the process of OMP assembly and outline the various hypotheses that have been proposed to explain how proteins might be integrated into the asymmetric OM lipid bilayer in an environment that lacks obvious energy sources. In addition, we describe the envelope stress responses that ensure the fidelity of OM biogenesis and how factors, such as phage and certain toxins, have coopted this essential machine to gain entry into the cell.

A Novel Periplasmic Protein, VrpA, Contributes to Efficient Protein Secretion by the Type III Secretion System in Xanthomonas spp

Efficient secretion of type III effector proteins from the bacterial cytoplasm to host cell cytosol via a type III secretion system (T3SS) is crucial for virulence of plant-pathogenic bacterium. Our previous study revealed a conserved hypothetical protein, virulence-related periplasm protein A (VrpA), which was identified as a critical virulence factor for Xanthomonas citri subsp. citri. In this study, we demonstrate that mutation of vrpA compromises X. citri subsp. citri virulence and hypersensitive response induction. This deficiency is also observed in the X. campestris pv. campestris strain, suggesting a functional conservation of VrpA in Xanthomonas spp. Our study indicates that VrpA is required for efficient protein secretion via T3SS, which is supported by multiple lines of evidence. A CyaA reporter assay shows that VrpA is involved in type III effector secretion; quantitative reverse-transcription polymerase chain reaction analysis suggests that the vrpA mutant fails to activate citrus-canker-susceptible gene CsLOB1, which is transcriptionally activated by transcription activator-like effector PthA4; in vitro secretion study reveals that VrpA plays an important role in secretion of T3SS pilus, translocon, and effector proteins. Our data also indicate that VrpA in X. citri subsp. citri localizes to bacterial periplasmic space and the periplasmic localization is required for full function of VrpA and X. citri subsp. citri virulence. Protein-protein interaction studies show that VrpA physically interacts with periplasmic T3SS components HrcJ and HrcC. However, the mutation of VrpA does not affect T3SS gene expression. Additionally, VrpA is involved in X. citri subsp. citri tolerance of oxidative stress. Our data contribute to the mechanical understanding of an important periplasmic protein VrpA in Xanthomonas spp.

Deciphering the roles of BamB and its interaction with BamA in outer membrane biogenesis, T3SS expression and virulence in Salmonella

The folding and insertion of β-barrel proteins in the outer membrane of Gram-negative bacteria is mediated by the BAM complex, which is composed of the outer membrane protein BamA and four lipoproteins BamB to BamE. In Escherichia coli and/or Salmonella, the BamB lipoprotein is involved in (i) β-barrel protein assembly in the outer membrane, (ii) outer membrane permeability to antibiotics, (iii) the control of the expression of T3SS which are major virulence factors and (iv) the virulence of Salmonella. In E. coli, this protein has been shown to interact directly with BamA. In this study, we investigated the structure-function relationship of BamB in order to assess whether the roles of BamB in these phenotypes were inter-related and whether they require the interaction of BamB with BamA. For this purpose, recombinant plasmids harbouring point mutations in bamB were introduced in a ΔSalmonella bamB mutant. We demonstrated that the residues L173, L175 and R176 are crucial for all the roles of BamB and for the interaction of BamB with BamA. Moreover, the results obtained with a D229A BamB variant, which is unable to immunoprecipitate BamA, suggest that the interaction of BamB with BamA is not absolutely necessary for BamB function in outer-membrane protein assembly, T3SS expression and virulence. Finally, we showed that the virulence defect of the ΔbamB mutant is not related to its increased susceptibility to antimicrobials, as the D227A BamB variant fully restored the virulence of the mutant while having a similar antibiotic susceptibility to the ΔbamB strain. Overall, this study demonstrates that the different roles of BamB are not all inter-related and that L173, L175 and R176 amino-acids are privileged sites for the design of BamB inhibitors that could be used as alternative therapeutics to antibiotics, at least against Salmonella.

Heterogeneity of persistence of Salmonella enterica serotype Senftenberg strains could explain the emergence of this serotype in poultry flocks

Salmonella enterica serotype Senftenberg (S. Senftenberg) has recently become more frequent in poultry flocks. Moreover some strains have been implicated in severe clinical cases. To explain the causes of this emergence in farm animals, 134 S. Senftenberg isolates from hatcheries, poultry farms and human clinical cases were analyzed. Persistent and non-persistent strains were identified in chicks. The non-persistent strains disappeared from ceca a few weeks post inoculation. This lack of persistence could be related to the disappearance of this serotype from poultry farms in the past. In contrast, persistent S. Senftenberg strains induced an intestinal asymptomatic carrier state in chicks similar to S. Enteritidis, but a weaker systemic infection than S. Enteritidis in chicks and mice. An in vitro analysis showed that the low infectivity of S. Senftenberg is in part related to its low capacity to invade enterocytes and thus to translocate the intestinal barrier. The higher capacity of persistent than non-persistent strains to colonize and persist in the ceca of chickens could explain the increased persistence of S. Senftenberg in poultry flocks. This trait might thus present a human health risk as these bacteria could be present in animals before slaughter and during food processing.

Reduction of Salmonella enterica serovar enteritidis colonization in 20-day-old broiler chickens by the plant-derived compounds trans-cinnamaldehyde and eugenol

The efficacies of trans-cinnamaldehyde (TC) and eugenol (EG) for reducing Salmonella enterica serovar Enteritidis colonization in broiler chickens were investigated. In three experiments for each compound, 1-day-old chicks (n = 75/experiment) were randomly assigned to five treatment groups (n = 15/treatment group): negative control (-ve S. Enteritidis, -ve TC, or EG), compound control (-ve S. Enteritidis, +ve 0.75% [vol/wt] TC or 1% [vol/wt] EG), positive control (+ve S. Enteritidis, -ve TC, or EG), low-dose treatment (+ve S. Enteritidis, +ve 0.5% TC, or 0.75% EG), and high-dose treatment (+ve S. Enteritidis, +ve 0.75% TC, or 1% EG). On day 0, birds were tested for the presence of any inherent Salmonella (n = 5/experiment). On day 8, birds were inoculated with ∼8.0 log(10) CFU S. Enteritidis, and cecal colonization by S. Enteritidis was ascertained (n = 10 chicks/experiment) after 24 h (day 9). Six birds from each treatment group were euthanized on days 7 and 10 after inoculation, and cecal S. Enteritidis numbers were determined. TC at 0.5 or 0.75% and EG at 0.75 or 1% consistently reduced (P < 0.05) S. Enteritidis in the cecum (≥3 log(10) CFU/g) after 10 days of infection in all experiments. Feed intake and body weight were not different for TC treatments (P > 0.05); however, EG supplementation led to significantly lower (P < 0.05) body weights. Follow-up in vitro experiments revealed that the subinhibitory concentrations (SICs, the concentrations that did not inhibit Salmonella growth) of TC and EG reduced the motility and invasive abilities of S. Enteritidis and downregulated expression of the motility genes flhC and motA and invasion genes hilA, hilD, and invF. The results suggest that supplementation with TC and EG through feed can reduce S. Enteritidis colonization in chickens.

The universally conserved prokaryotic GTPases

Members of the large superclass of P-loop GTPases share a core domain with a conserved three-dimensional structure. In eukaryotes, these proteins are implicated in various crucial cellular processes, including translation, membrane trafficking, cell cycle progression, and membrane signaling. As targets of mutation and toxins, GTPases are involved in the pathogenesis of cancer and infectious diseases. In prokaryotes also, it is hard to overestimate the importance of GTPases in cell physiology. Numerous papers have shed new light on the role of bacterial GTPases in cell cycle regulation, ribosome assembly, the stress response, and other cellular processes. Moreover, bacterial GTPases have been identified as high-potential drug targets. A key paper published over 2 decades ago stated that, "It may never again be possible to capture [GTPases] in a family portrait" (H. R. Bourne, D. A. Sanders, and F. McCormick, Nature 348:125-132, 1990) and indeed, the last 20 years have seen a tremendous increase in publications on the subject. Sequence analysis identified 13 bacterial GTPases that are conserved in at least 75% of all bacterial species. We here provide an overview of these 13 protein subfamilies, covering their cellular functions as well as cellular localization and expression levels, three-dimensional structures, biochemical properties, and gene organization. Conserved roles in eukaryotic homologs will be discussed as well. A comprehensive overview summarizing current knowledge on prokaryotic GTPases will aid in further elucidating the function of these important proteins.

Infection of mice by Salmonella enterica serovar Enteritidis involves additional genes that are absent in the genome of serovar Typhimurium

Salmonella enterica serovar Enteritidis causes a systemic, typhoid-like infection in newly hatched poultry and mice. In the present study, a library of 54,000 transposon mutants of S. Enteritidis phage type 4 (PT4) strain P125109 was screened for mutants deficient in the in vivo colonization of the BALB/c mouse model using a microarray-based negative-selection screening. Mutants in genes known to contribute to systemic infection (e.g., Salmonella pathogenicity island 2 [SPI-2], aro, rfa, rfb, phoP, and phoQ) and enteric infection (e.g., SPI-1 and SPI-5) in this and other Salmonella serovars displayed colonization defects in our assay. In addition, a strong attenuation was observed for mutants in genes and genomic islands that are not present in S. Typhimurium or in most other Salmonella serovars. These genes include a type I restriction/modification system (SEN4290 to SEN4292), the peg fimbrial operon (SEN2144A to SEN2145B), a putative pathogenicity island (SEN1970 to SEN1999), and a type VI secretion system remnant SEN1001, encoding a hypothetical protein containing a lysin motif (LysM) domain associated with peptidoglycan binding. Proliferation defects for mutants in these individual genes and in exemplar genes for each of these clusters were confirmed in competitive infections with wild-type S. Enteritidis. A ΔSEN1001 mutant was defective for survival within RAW264.7 murine macrophages in vitro. Complementation assays directly linked the SEN1001 gene to phenotypes observed in vivo and in vitro. The genes identified here may perform novel virulence functions not characterized in previous Salmonella models.

Expression of Toll-like receptor 4 and downstream effectors in selected cecal cell subpopulations of chicks resistant or susceptible to Salmonella carrier state

Toll-like receptor 4 (TLR4), which recognizes lipopolysaccharide from Gram-negative bacteria, plays a major role in resistance of mice and humans to Salmonella infection. In chickens, Salmonella may establish a carrier state whereby bacteria are able to persist in the host organism without triggering clinical signs. Based on cellular morphological parameters, we developed a method, without using antibodies, to separate three cecal cell subpopulations: lymphocytes, enterocytes, and a population encompassing multiple cell types. We analyzed the mRNA expression of TLR4, interleukin-1β (IL-1β), IL-8, IL-12, and lipopolysaccharide-induced tumor necrosis factor alpha factor (LITAF) in cecal subpopulations of chicks from inbred lines resistant or susceptible to the carrier state infected with Salmonella enterica serovar Enteritidis. The results showed that resistance to the carrier state in chicks is associated with a larger percentage of lymphocytes and with higher levels of expression of TLR4 and IL-8 at homeostasis in the three cell subpopulations, as well as with a higher level of expression of LITAF in lymphocytes during the carrier state. In contrast to the early phase of infection, the carrier state is characterized by no major cell recruitment differences between infected and noninfected animals and no significant modification in terms of TLR4, IL-1β, IL-8, IL-12, and LITAF expression in all cell subpopulations measured. However, TLR4 expression increased in the lymphocytes of chicks from the susceptible line, reaching the same level as that in infected chicks from the resistant line. These observations suggest that the carrier state is characterized by a lack of immune activation and highlight the interest of working at the level of the cell population rather than that of the organ.

Cell invasion of poultry-associated Salmonella enterica serovar Enteritidis isolates is associated with pathogenicity, motility and proteins secreted by the type III secretion system

Salmonella enterica serovar Enteritidis (S. Enteritidis) is a major cause of food-borne gastroenteritis in humans worldwide. Poultry and poultry products are considered the major vehicles of transmission to humans. Using cell invasiveness as a surrogate marker for pathogenicity, we tested the invasiveness of 53 poultry-associated isolates of S. Enteritidis in a well-differentiated intestinal epithelial cell model (Caco-2). The method allowed classification of the isolates into low (n = 7), medium (n = 18) and high (n = 30) invasiveness categories. Cell invasiveness of the isolates did not correlate with the presence of the virulence-associated gene spvB or the ability of the isolates to form biofilms. Testing of representative isolates with high and low invasiveness in a mouse model revealed that the former were more invasive in vivo and caused more and earlier mortalities, whereas the latter were significantly less invasive in vivo, causing few or no mortalities. Further characterization of representative isolates with low and high invasiveness showed that most of the isolates with low invasiveness had impaired motility and impaired secretion of either flagella-associated proteins (FlgK, FljB and FlgL) or type III secretion system (TTSS)-secreted proteins (SipA and SipD) encoded on Salmonella pathogenicity island-1. In addition, isolates with low invasiveness had impaired ability to invade and/or survive within chicken macrophages. These data suggest that not all isolates of S. Enteritidis recovered from poultry may be equally pathogenic, and that the pathogenicity of S. Enteritidis isolates is associated, in part, with both motility and secretion of TTSS effector proteins.

Crystal structure of Escherichia coli BamB, a lipoprotein component of the β-barrel assembly machinery complex

In Gram-negative bacteria, the BAM (β-barrel assembly machinery) complex catalyzes the essential process of assembling outer membrane proteins. The BAM complex in Escherichia coli consists of five proteins: one β-barrel membrane protein, BamA, and four lipoproteins, BamB, BamC, BamD, and BamE. Despite their role in outer membrane protein biogenesis, there is currently a lack of functional and structural information on the lipoprotein components of the BAM complex. Here, we report the first crystal structure of BamB, the largest and most functionally characterized lipoprotein component of the BAM complex. The crystal structure shows that BamB has an eight-bladed β-propeller structure, with four β-strands making up each blade. Mapping onto the structure the residues previously shown to be important for BamA interaction reveals that these residues, despite being far apart in the amino acid sequence, are localized to form a continuous solvent-exposed surface on one side of the β-propeller. Found on the same side of the β-propeller is a cluster of residues conserved among BamB homologs. Interestingly, our structural comparison study suggests that other proteins with a BamB-like fold often participate in protein or ligand binding, and that the binding interface on these proteins is located on the surface that is topologically equivalent to where the conserved residues and the residues that are important for BamA interaction are found on BamB. Our structural and bioinformatic analyses, together with previous biochemical data, provide clues to where the BamA and possibly a substrate interaction interface may be located on BamB.

The periplasmic chaperone Skp is required for successful Salmonella Typhimurium infection in a murine typhoid model

The alternative sigma factor σ(E) (rpoE) is essential for survival in vivo of Salmonella Typhimurium but is dispensable during growth in the laboratory. We have been identifying σ(E)-regulated genes and studying their regulation and function to elucidate their potential role in the severe attenuation of S. Typhimurium rpoE mutants. In this study we identify five promoters that control the rseP, yaeT (bamA), skp region. A confirmed σ(E)-dependent promoter, yaeTp1, and a second downstream promoter, yaeTp2, are located within the upstream gene rseP and direct expression of the downstream genes. The only known function of RseP is σ(E) activation, and it is therefore not expected to be essential for S. Typhimurium in vitro. However, it proved impossible to delete the entire rseP gene due to the presence of internal promoters that regulate the essential gene yaeT. We could inactivate rseP by deleting the first third of the gene, leaving the yaeT promoters intact. Like the rpoE mutant, the rseP mutant exhibited severe attenuation in vivo. We were able to delete the entire coding sequence of skp, which encodes a periplasmic chaperone involved in targeting misfolded outer-membrane proteins to the β-barrel assembly machinery. The skp mutant was attenuated in mice after oral and parenteral infection. Virulence could be complemented by providing skp in trans but only by linking it to a heterologous σ(E)-regulated promoter. The reason the skp mutant is attenuated is currently enigmatic, but we know it is not through increased sensitivity to a variety of RpoE-activating host stresses, such as H(2)O(2), polymyxin B and high temperature, or through altered secretion of effector proteins by either the Salmonella pathogenicity island (SPI)-1 or the SPI-2 type III secretion system.

Investigation of the role of the BAM complex and SurA chaperone in outer-membrane protein biogenesis and type III secretion system expression in Salmonella

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 ΔbamB 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 ΔbamC and ΔdegP 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 ΔsurA and ΔbamE 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.

Importance of subunit vaccine antigen of major Fli C antigenic site of Salmonella enteritidis II: a challenge trial

Salmonella enterica subsp. enterica serovar Enteritidis (SE) infection in chickens shows a mild pathogenicity except for young ages, compared with other animals, and laying hens sometimes produce SE-contaminated eggs leading to public health concerns. To reduce the problem, SE bacterin in poultry farms has been applied. We previously demonstrated that a subunit antigen, g.m. part polypeptide in SE-Fli C (SEp 9), could be a candidate subunit antigen of SE vaccine which may show less side effects in chickens. In this study, we used SEp 9 along with an adjuvant to inoculate chickens, then the chickens were orally challenged with SE, and suppression of the SE count in the cecum was investigated. Chickens inoculated with a commercial SE vaccine were prepared as positive controls (vaccine group), and those with physiological saline (control group) for comparison of the bacterial count after challenge. Employing two types of antibody-detection ELISA coated with either de-flagellated SE or SEp 9, specific antibody levels in blood and the intestine were determined. The bacterial count was significantly lower 1 and 3 weeks after challenge in the SEp 9 than in the control group. Specific antibody only against SEp 9 in blood but not the intestine of these birds in the SEp 9 group was detected. This study confirmed that SEp 9 antigen is a major effective antigen in SE inactivated vaccine, and it is suggested that only the subunit vaccine antigen SEp 9 is needed to effectively suppress colonization in the chicken intestine, without the need for other SE component antigens.

A longitudinal study of environmental Salmonella contamination in caged and free-range layer flocks

The environmental contamination by salmonella was examined over a 12-month period in 74 commercial layer flocks from eight farms in the UK, which previously had been identified as being contaminated with salmonella. Samples of faeces, dust, litter, egg belt spillage and wildlife vectors were taken, plus swabs of cages, feeders, drinkers, floors, egg belts and boots. Some sampling was performed in each month of the year. Numerous serovars were detected but Salmonella enterica serotype Enteritidis was the only persistent serotype found among single-age flocks. There was a significant correlation between qualitative environmental samples and semi-quantitative faeces samples. The level of environmental contamination increased significantly over time. There were significant temperature and seasonal effects upon contamination. Wildlife vectors proved to be sensitive samples for the detection of salmonella. The efficacy of cleaning and disinfection upon residual salmonella contamination, and upon subsequent flock contamination, was highly variable between and within premises. The variability between detected prevalences over time and between flocks indicates a need for regular, sensitive monitoring of flocks for salmonella to permit targeting of control measures aimed at eliminating contamination of the layer environment by salmonella. There is substantial scope for improvement of cleaning and disinfection procedures.

Semiquantitative assessment of the distribution of Salmonella in the environment of caged layer flocks

AIMS

To evaluate a semiquantitative technique for the enumeration of Salmonella in the environment of layer flocks and to compare findings with those of a standard qualitative technique.

METHODS AND RESULTS

Samples were taken from faeces, floor dust, dust on cages, feeders and egg belts. After mixing with buffered peptone water, serial dilutions were prepared and culture was performed using pre-enrichment, then plating on semisolid selective and solid isolation media. Comparison with a qualitative pre-enrichment technique indicated a similar sensitivity for both methods despite smaller sample sizes. The numbers of Salmonella detected for a site or sample type did not correlate closely with the prevalence of positive samples.

CONCLUSIONS

The sensitive detection and quantification of Salmonella in the flock environment is practicable with the technique described. Quantitative data in many cases do not correlate with qualitative findings.

SIGNIFICANCE AND IMPACT OF THE STUDY

The significance of certain environmental factors and interventions in the maintenance and dissemination of Salmonella in poultry houses may be over- or under-represented by prevalence data alone. The technique described allows the issue of poultry house contamination to be examined from a new perspective.

The YfgL lipoprotein is essential for type III secretion system expression and virulence of Salmonella enterica Serovar Enteritidis

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 DeltayfgL 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 Delta 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.

Strong decrease in invasive ability and outer membrane vesicle release in Crohn's disease-associated adherent-invasive Escherichia coli strain LF82 with the yfgL gene deleted

Adherent-invasive Escherichia coli strain LF82 recovered from a chronic lesion of a patient with Crohn's disease is able to invade cultured intestinal epithelial cells. Three mutants with impaired ability to invade epithelial cells had the Tn5phoA transposon inserted in the yfgL gene encoding the YfgL lipoprotein. A yfgL- negative isogenic mutant showed a marked decrease both in its ability to invade Intestine-407 cells and in the amount of the outer membrane proteins OmpA and OmpC in the culture supernatant, as shown by analysis of the culture supernatant protein contents by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization-time of flight mass spectrometry. Transcomplementation of the LF82-DeltayfgL isogenic mutant with the cloned yfgL gene restored invasion ability and outer membrane protein release in the culture supernatant. The outer membrane proteins in the culture supernatant of strain LF82 resulted from the formation of vesicles. This was shown by Western blot analysis of periplasmic and outer membrane fraction markers typically found in outer membrane vesicles and by transmission electron microscopic analysis of ultracentrifuged cell-free LF82 supernatant pellets, indicating the presence of vesicles with a bilayered structure surrounding a central electron-dense core. Thus, deletion of the yfgL gene in strain LF82 resulted in a decreased ability to invade intestinal epithelial cells and a decreased release of outer membrane vesicles.

Precise excision and secondary transposition of TnphoA in non-motile mutants of a Salmonella enterica serovar Enteritidis clinical isolate

Mutagenesis with TnphoA has been widely used in many bacteria. Here, we report the excision and secondary transposition of this transposon in three non-motile (fliC, fliF and motB) mutants of Salmonella enterica serovar Enteritidis (S. Enteritidis). Isolation of motile revertants showed that they were kanamycin resistant and conserved a copy of TnphoA in their genome in an insertion site different from the initial one. They also expressed an intact flagella. Characterization of the motile revertant derived from the fliC mutant showed that TnphoA excised precisely from the fliC gene, resulting in an equivalent amount of FliC secreted protein in the revertant compared to that of the wild-type strain. These results show that TnphoA mutants should be used with care and underline the value of using transposon derivatives lacking the transposase gene.