The high-adhesive properties of the FimH adhesin of Salmonella enterica serovar Enteritidis are determined by a single F118S substitution

Salmonella adhesion is decreased by hypoxia due to adhesion and motility structure crosstalk

Initial stages of Salmonella Typhimurium infection involve a series of coordinated events aimed at reaching, attaching to, and invading host cells. Virulence factors such as flagella, fimbriae, and secretion systems play crucial roles in these events and are regulated in response to the host environment. The first point of contact between the pathogen and host is the intestinal epithelial layer, which normally serves as a barrier against invading pathogens, but can also be an entry site for pathogens. The integrity of this barrier can be modulated by the hypoxic environment of the intestines, created by the presence of trillions of microbes. Variable oxygen concentrations can strongly affect many functions of the gut, including secretion of cytokines and growth factors from the host site and affect the ability of Salmonella to persist, invade, and replicate. In this study, we investigated the first stages of Salmonella Typhimurium infection under hypoxic conditions in vitro and found that low oxygen levels significantly decreased bacterial adhesion. Using adhesion and motility assays, biofilm formation tests, as well as gene expression and cytokine secretion analysis, we identified a hypoxia-specific cross-talk between the expression of type 1 fimbriae and flagella, suggesting that altered flagellin expression levels affect the motility of bacteria and further impact their adhesion level, biofilm formation ability, and innate immune response. Overall, understanding how Salmonella interacts with its variable host environment provides insights into the virulence mechanisms of the bacterium and information regarding strategies for preventing or treating infections. Further research is required to fully understand the complex interplay between Salmonella and its host environment.

Adhesion of enteropathogenic, enterotoxigenic and commensal Escherichia coli to the Major Zymogen Granule Membrane Glycoprotein 2

Pathogenic bacteria, such as enteropathogenic Escherichia coli (EPEC) and enterotoxigenic E. coli (ETEC), cause diarrhea in mammals. In particular, E. coli colonizes and infects the gastrointestinal tract via type 1 fimbriae (T1F). Here, the major zymogen granule membrane glycoprotein 2 (GP2) acts as a host cell receptor. GP2 is also secreted by the pancreas and various mucous glands, interacting with luminal type 1 fimbriae-positive E. coli. It is unknown whether GP2 isoforms demonstrate specific E. coli pathotype binding. In this study, we investigated interactions of human, porcine, and bovine EPEC and ETEC, as well as commensal E. coli isolates with human, porcine, and bovine GP2. We first defined pathotype- and host-associated FimH variants. Second, we could prove that GP2 isoforms bound to FimH variants to various degrees. However, the GP2-FimH interactions did not seem to be influenced by the host specificity of E. coli. In contrast, soluble GP2 affected ETEC infection and phagocytosis rates of macrophages. Preincubation of the ETEC pathotype with GP2 reduced the infection of cell lines. Furthermore, preincubation of E. coli with GP2 improved the phagocytosis rate of macrophages. Our findings suggest that GP2 plays a role in the defense against E. coli infection and in the corresponding host immune response.

IMPORTANCE Infection by pathogenic bacteria, such as certain Escherichia coli pathotypes, results in diarrhea in mammals. Pathogens, including zoonotic agents, can infect different hosts or show host specificity. There are Escherichia coli strains which are frequently transmitted between humans and animals, whereas other Escherichia coli strains tend to colonize only one host. This host specificity is still not fully understood. We show that glycoprotein 2 is a selective receptor for particular Escherichia coli strains or variants of the adhesin FimH but not a selector for a species-specific Escherichia coli group. We demonstrate that GP2 is involved in the regulation of colonization and infection and thus represents a molecule of interest for the prevention or treatment of disease.

Identification of Natural Mutations Responsible for Altered Infection Phenotypes of Salmonella enterica Clinical Isolates by Using Cell Line Infection Screens

The initial steps of Salmonella pathogenesis involve adhesion to and invasion into host epithelial cells. While well-studied for Salmonella enterica serovar Typhimurium, the factors contributing to this process in other, host-adapted serovars remains unexplored. Here, we screened clinical isolates of serovars Gallinarum, Dublin, Choleraesuis, Typhimurium, and Enteritidis for adhesion to and invasion into intestinal epithelial cell lines of human, porcine, and chicken origins. Thirty isolates with altered infectivity were used for genomic analyses, and 14 genes and novel mutations associated with high or low infectivity were identified. The functions of candidate genes included virulence gene expression regulation and cell wall or membrane synthesis and components. The role of several of these genes in Salmonella adhesion to and invasion into cells has not previously been investigated. The genes dksA (encoding a stringent response regulator) and sanA (encoding a vancomycin high-temperature exclusion protein) were selected for further analyses, and we confirmed their roles in adhesion to and invasion into host cells. Furthermore, transcriptomic analyses were performed for S Enteritidis and S Typhimurium, with two highly infective and two marginally infective isolates for each serovar. Expression profiles for the isolates with altered infection phenotypes revealed the importance of type 3 secretion system expression levels in the determination of an isolate's infection phenotype. Taken together, these data indicate a new role in cell host infection for genes or gene variants previously not associated with adhesion to and invasion into the epithelial cells.IMPORTANCESalmonella is a foodborne pathogen affecting over 200 million people and resulting in over 200,000 fatal cases per year. Its adhesion to and invasion into intestinal epithelial cells represent one of the first and key steps in the pathogenesis of salmonellosis. Still, around 35 to 40% of bacterial genes have no experimentally validated function, and their contribution to bacterial virulence, including adhesion and invasion, remains largely unknown. Therefore, the significance of this study is in the identification of new genes or gene allelic variants previously not associated with adhesion and invasion. It is well established that blocking adhesion and/or invasion would stop or hamper bacterial infection; therefore, the new findings from this study could be used in future developments of anti-Salmonella therapy targeting genes involved in these key processes. Such treatment could be a valuable alternative, as the prevalence of antibiotic-resistant bacteria is increasing very rapidly.

Construction and application of carbohydrate microarrays to detect foodborne bacteria

The rapid detection of pathogenic bacteria is vital for the prevention of outbreaks of infectious diseases, including infections by the common foodborne bacteria E.coli and Salmonella Carbohydrate microarrays have been developed as a powerful method to investigate carbohydrate-protein interaction with only very small amounts of glycans, which show great potential for detect the carbohydrate mediated interaction with pathogens. Here, different mannose-coated microarrays were constructed and tested with E.coli (K-12 and BL-21) and Salmonella enterica strains (ATCC9184 and ATCC31685) exhibiting different mannose binding affinities. The optimized carbohydrate microarray was then applied to test the binding of 12 Salmonella enterica and 9 E.coli isolates from local patients for the first time and showed strong binding with certain serovars or subtypes. The results showed that microarray probed with the single mannose structure is not enough for the detection of bacteria with various serovars or subtypes, which contain a high degree of allelic variation in adhesin. We suggest that a complex carbohydrate microarray containing different glycan conformation may be needed for detection of different bacteria isolates.

Oligosaccharides from Palm Kernel Cake Enhances Adherence Inhibition and Intracellular Clearance of Salmonella enterica Serovar Enteritidis In Vitro

Salmonella enterica serovar (ser.) Enteritidis (S. Enteritidis) is a foodborne pathogen often associated with contaminated poultry products. This study evaluated the anti-adherence and intracellular clearance capability of oligosaccharides extracted from palm kernel cake (PKC), a by-product of the palm oil industry, and compared its efficacy with commercial prebiotics- fructooligosaccharide (FOS) and mannanoligosaccharide (MOS)-against S. Enteritidis in vitro. Based on the degree of polymerization (DP), PKC oligosaccharides were further divided into 'Small' (DP ≤ 6) and 'Big' (DP > 6) fractions. Results showed that the Small and Big PKC fractions were able to reduce (p < 0.05) S. Enteritidis adherence to Cancer coli-2 (Caco-2) cells at 0.1 mg/ mL while MOS and FOS showed significant reduction at 1.0 mg/mL and 10.0 mg/mL, respectively. In terms of S. Enteritidis clearance, oligosaccharide-treated macrophages showed better S. Enteritidis clearance over time at 50 µg/mL for Small, Big and MOS, while FOS required a concentration of 500 µg/mL for a similar effect. This data highlights that oligosaccharides from PKC, particularly those of lower DP, were more effective than MOS and FOS at reducing S. Enteritidis adherence and enhancing S. Enteritidis clearance in a cell culture model.

The effects of coarse corn and refined functional carbohydrates on the live performance and cecal Salmonella prevalence in coccidiosis-vaccinated broilers1

The interaction between corn particle size and feed additives as it pertains to broiler live performance has been overlooked. This study evaluated the effects of corn particle size and refined-functional carbohydrates (RFC; 100 g/MT) on live performance and Salmonella prevalence in coccidiosis-vaccinated broilers. The following treatments were applied: fine corn (FC), coarse corn (CC), FC+RFC, CC+RFC, and CC+SAL (salinomycin). A natural, non-experimental necrotic enteritis (NE) outbreak began at 12 D of age, and mortality was impacted by dietary treatments. The use of RFC was observed to increase NE-associated mortality compared to broilers fed CC+SAL (P ≤ 0.10). At 19 D, greater than 50% of all broilers were found to be Salmonella-positive; however, at 48 D the use of RFC was shown to decrease cecal Salmonella prevalence. Although differences in early mortality were observed, coccidiosis-vaccinated broilers fed CC or CC+RFC exhibited similar BW and FCR as broilers fed CC+SAL at 48 D (P ≤ 0.05). These data suggested that CC use after 10 D may provide value in a production system free of antibiotic growth promoters and coccidiostats by ameliorating live performance losses associated with coccidiosis vaccination. Further research is warranted to determine how RFC and CC specifically affect Eimeria cycling and the immune response following coccidiosis vaccination and an NE challenge.

Everything You Always Wanted to Know About Salmonella Type 1 Fimbriae, but Were Afraid to Ask

Initial attachment to host intestinal mucosa after oral infection is one of the most important stages during bacterial pathogenesis. Adhesive structures, widely present on the bacterial surface, are mainly responsible for the first contact with host cells and of host-pathogen interactions. Among dozens of different bacterial adhesins, type 1 fimbriae (T1F) are one of the most common adhesive organelles in the members of the Enterobacteriaceae family, including Salmonella spp., and are important virulence factors. Those long, thin structures, composed mainly of FimA proteins, are responsible for recognizing and binding high-mannose oligosaccharides, which are carried by various glycoproteins and expressed at the host cell surface, via FimH adhesin, which is presented at the top of T1F. In this review, we discuss investigations into the functions of T1F, from the earliest work published in 1958 to operon organization, organelle structure, T1F biogenesis, and the various functions of T1F in Salmonella-host interactions. We give special attention to regulation of T1F expression and their role in binding of Salmonella to cells, cell lines, organ explants, and other surfaces with emphasis on biofilm formation and discuss T1F role as virulence factors based on work using animal models. We also discuss the importance of allelic variation in fimH to Salmonella pathogenesis, as well as role of FimH in Salmonella host specificity.

Adhesion of Salmonella to Pancreatic Secretory Granule Membrane Major Glycoprotein GP2 of Human and Porcine Origin Depends on FimH Sequence Variation

Bacterial host tropism is a primary determinant of the range of host organisms they can infect. Salmonella serotypes are differentiated into host-restricted and host-adapted specialists, and host-unrestricted generalists. In order to elucidate the underlying molecular mechanisms of host specificity in Salmonella infection, we investigated the role of the intestinal host cell receptor zymogen granule membrane glycoprotein 2 (GP2), which is recognized by FimH adhesin of type 1 fimbriae found in Enterobacteriaceae. We compared four human and two porcine GP2 isoforms. Isoforms were expressed in Sf9 cells as well as in one human (HEp-2) and one porcine (IPEC-J2) cell line. FimH genes of 128 Salmonella isolates were sequenced and the 10 identified FimH variants were compared regarding adhesion (static adhesion assay) and infection (cell line assay) using an isogenic model. We expressed and characterized two functional porcine GP2 isoforms differing in their amino acid sequence to human isoforms by approximately 25%. By comparing all isoforms in the static adhesion assay, FimH variants were assigned to high, low or no-binding phenotypes. This FimH variant-dependent binding was neither specific for one GP2 isoform nor for GP2 in general. However, cell line infection assays revealed fundamental differences: using HEp-2 cells, infection was also FimH variant-specific but mainly independent of human GP2. In contrast, this FimH variant dependency was not obvious using IPEC-J2 cells. Here, we propose an alternative GP2 adhesion/infection mechanism whereby porcine GP2 is not a receptor that determined host-specificity of Salmonella. Salmonella specialists as well as generalists demonstrated similar binding to GP2. Future studies should focus on spatial distribution of GP2 isoforms in the human and porcine intestine, especially comparing health and disease.

Conformational switch of the bacterial adhesin FimH in the absence of the regulatory domain: Engineering a minimalistic allosteric system

For many biological processes such as ligand binding, enzymatic catalysis, or protein folding, allosteric regulation of protein conformation and dynamics is fundamentally important. One example is the bacterial adhesin FimH, where the C-terminal pilin domain exerts negative allosteric control over binding of the N-terminal lectin domain to mannosylated ligands on host cells. When the lectin and pilin domains are separated under shear stress, the FimH-ligand interaction switches in a so-called catch-bond mechanism from the low- to high-affinity state. So far, it has been assumed that the pilin domain is essential for the allosteric propagation within the lectin domain that would otherwise be conformationally rigid. To test this hypothesis, we generated mutants of the isolated FimH lectin domain and characterized their thermodynamic, kinetic, and structural properties using isothermal titration calorimetry, surface plasmon resonance, nuclear magnetic resonance, and X-ray techniques. Intriguingly, some of the mutants mimicked the conformational and kinetic behaviors of the full-length protein and, even in absence of the pilin domain, conducted the cross-talk between allosteric sites and the mannoside-binding pocket. Thus, these mutants represent a minimalistic allosteric system of FimH, useful for further mechanistic studies and antagonist design.

The Novel Type 1 Fimbriae FimH Receptor Calreticulin Plays a Role in Salmonella Host Specificity

It was suggested that minor differences in the structure of FimH are most likely associated with differences in its adhesion specificities and may determine the tropism of various Salmonella serovars to different species and tissues. We have recently shown that FimH adhesins from host-adapted serovars, e.g., Salmonella Choleraesuis (SCh), bind to other glycoprotein receptors compared to FimH from host-unrestricted Salmonella Enteritidis (SE). Here we identify porcine calreticulin expressed by swine intestinal cells as a host-specific receptor for SCh FimH adhesin, suggesting that such an interaction may contribute to SCh host specificity. Calreticulin was identified by 2D electrophoresis and mass spectrometry as a glycoprotein that was bound specifically by recombinant SCh FimH protein, but not by FimH from SE. The functionality of calreticulin as a specific receptor of SCh FimH adhesin was further confirmed by adhesion and invasion of mutated strains of SCh carrying different variants of FimH proteins to IPEC-J2 cells with overexpression and silenced expression of calreticulin. It was found that SCh carrying the active variant of FimH adhered and invaded IPEC-J2 cells with calreticulin overexpression at significantly higher numbers than those of SCh expressing the non-active variant or SE variant of FimH. Moreover, binding of SCh carrying the active variant of FimH to IPEC-J2 with silenced calreticulin expression was significantly weaker. Furthermore, we observed that SCh infection induces translocation of calreticulin to cell membrane. All of the aforementioned results lead to the general conclusion that Salmonella host specificity requires not only special mechanisms and proteins expressed by the pathogen but also specifically recognized receptors expressed by a specific host.

Changes in the Salmonella enterica Enteritidis phenotypes in presence of acyl homoserine lactone quorum sensing signals

Quorum sensing is used by bacteria to coordinate gene expression in response to population density and involves the production, detection and response to extracellular signaling molecules known as autoinducers (AIs). Salmonella does not synthesize the AI-1, acyl homoserine lactone (AHL) common to gram-negative bacteria; however, it has a receptor for AI-1, the SdiA protein. The effect of SdiA in modulating phenotypes of Salmonella has not been elucidated. In this report, we provide evidence that the AIs-1 affect Salmonella enterica serovar Enteritidis behavior by enhancing the biofilm formation and expression of virulence genes under anaerobic conditions. Biofilm formation by Salmonella was detected by the crystal violet method and by scanning electron microscopy. The presence of AHLs, particularly C12-HSL, increased biofilm formation and promoted expression of biofilm formation genes (lpfA, fimF, fliF, glgC) and virulence genes (hilA, invA, invF). Our results demonstrated that AHLs produced by other organisms played an important role in virulence phenotypes of Salmonella Enteritidis.

Allelic variation contributes to bacterial host specificity

Understanding the molecular parameters that regulate cross-species transmission and host adaptation of potential pathogens is crucial to control emerging infectious disease. Although microbial pathotype diversity is conventionally associated with gene gain or loss, the role of pathoadaptive nonsynonymous single-nucleotide polymorphisms (nsSNPs) has not been systematically evaluated. Here, our genome-wide analysis of core genes within Salmonella enterica serovar Typhimurium genomes reveals a high degree of allelic variation in surface-exposed molecules, including adhesins that promote host colonization. Subsequent multinomial logistic regression, MultiPhen and Random Forest analyses of known/suspected adhesins from 580 independent Typhimurium isolates identifies distinct host-specific nsSNP signatures. Moreover, population and functional analyses of host-associated nsSNPs for FimH, the type 1 fimbrial adhesin, highlights the role of key allelic residues in host-specific adherence in vitro. Together, our data provide the first concrete evidence that functional differences between allelic variants of bacterial proteins likely contribute to pathoadaption to diverse hosts.

One of the key aspects for controlling infectious diseases is understanding how pathogens cross host species. Here the authors conduct a genome-wide analysis of Salmonella and show a high degree of variation, enabling host-adapted colonization among Salmonella intestinal and systemic serovars.

Type 1 fimbriae are important factors limiting the dissemination and colonization of mice by Salmonella Enteritidis and contribute to the induction of intestinal inflammation during Salmonella invasion

We have recently shown that Salmonella Gallinarum type 1 fimbriae with endogenous mannose-resistant (MR) variant of the FimH protein increase systemic dissemination of S. Gallinarum and colonization of internal organs in comparison to the S. Gallinarum fimH knockout strain or the mutant expressing mannose-sensitive (MS) FimH variant from S. Enteritidis. Elaborating from these studies, we proposed that MS variants of FimH are advantageous in gastrointestinal infections, in contrast to MR FimH variants which decrease intestinal colonization and promote their systemic spreading. To support our hypothesis, we carried out in vivo studies using mice infected with wild-type S. Enteritidis and its fimH knockout strain (S. Enteritidis), which was characterized by significantly lower adhesion and invasiveness of murine ICE-1 intestinal cells. Using bioluminescence imaging, we observed that the loss of MS FimH adhesin correlates well with the highly increased colonization of mice by these bacteria. The appearance of the mutant strain was observed much earlier than wild-type Salmonella, and mice infected with 10⁴–10⁷S. Enteritidis fimH::kan CFUs had significantly (P < 0.05) shorter infection-free time than animals inoculated with wild-type S. Enteritidis. Infections caused by non-typhoid Salmonella, such as S. Enteritidis, are associated with massive inflammation of the lamina propria and lymph nodes in the intestinal tract. Therefore, we evaluated the role of MS type 1 fimbriae in the induction of cytokine expression and secretion, using murine ICE-1 intestinal cells. We showed that the expression, as well as secretion, of Il-1b, Il-6, Il-10, and Il-12b was significantly higher in cells infected with wild-type S. Enteritidis compared to cells infected with the mutant strain. Based on our results, we propose that type 1 fimbriae may play an important role in the pathogenicity of S. Enteritidis and may contribute to an intestinal inflammatory response.

FimH adhesin from host unrestricted Salmonella Enteritidis binds to different glycoprotein ligands expressed by enterocytes from sheep, pig and cattle than FimH adhesins from host restricted Salmonella Abortus-ovis, Salmonella Choleraesuis and Salmonella Dublin

Adhesion to gut tissues and colonization of the alimentary tract, two important stages in the pathogenesis of Salmonella, are mediated by FimH adhesin of type 1 fimbriae. It was suggested that minor differences in the structure of FimH are most likely associated with differences in adhesion specificities, and may determine the tropism of various Salmonella serovars to different species and tissues. We investigated this hypothesis by comparing the binding properties of FimH proteins from three Salmonella enterica serovars with limited (Choleraesuis, Dublin) or restricted (Abortusovis) host ranges to FimH from broad host range S. Enteritidis and mannose inactive FimH from S. Gallinarum. Although all active variants of FimH protein were able to bind mannose-rich glycoproteins (RNase B, HRP and Man-BSA) with comparable affinity measured by surface plasmon resonance, there were significant differences in the binding profiles of the FimH proteins from host restricted serovars and host unrestricted serovar Enteritidis, to glycoproteins from enterocyte cell lines established in vitro and derived from sheep, pig and cattle. When low-binding FimH adhesin from S. Enteritidis was subjected to Western blot analysis, it bound to surface membrane protein of about 130 kDa, and high-binding FimH adhesins from S. Abortusovis, S. Choleraesuis and S. Dublin bound to surface membrane protein of about 55 kDa present in each cell line. Differential binding of FimH proteins from host-restricted and broad-host-range Salmonella to intestinal receptors was confirmed using mutant FimH adhesins obtained by site-directed mutagenesis. It was found that the low-binding variant of FimH from S. Choleraesuis with mutation Leu57Pro lost the ability to bind protein band of 55 kDa, but instead interacted with glycoprotein of about 130 kDa. On the other hand, the high-binding variant of FimH adhesin from S. Enteritids with mutation Asn101Ser did not bind to its receptor of 130 kDa, but instead it interacted with glycoprotein ligand of 55 kDa. These results suggest that FimH adhesins of type 1 fimbriae are one of the factors responsible for different host-specificities of these Salmonella serovars.

Microfluidic PCR combined with pyrosequencing for identification of allelic variants with phenotypic associations among targeted Salmonella genes

A novel targeted massive parallel sequencing approach identified genetic variation in eight known or predicted fimbrial adhesins for 46 Salmonella strains. The results highlight associations between specific adhesin alleles, host species, and antimicrobial resistance. The differentiation of allelic variants has potential applications for diagnostic microbiology and epidemiological investigations.

Diversification of the Salmonella fimbriae: a model of macro- and microevolution

Bacteria of the genus Salmonella comprise a large and evolutionary related population of zoonotic pathogens that can infect mammals, including humans and domestic animals, birds, reptiles and amphibians. Salmonella carries a plethora of virulence genes, including fimbrial adhesins, some of them known to participate in mammalian or avian host colonization. Each type of fimbria has its structural subunit and biogenesis genes encoded by one fimbrial gene cluster (FGC). The accumulation of new genomic information offered a timely opportunity to better evaluate the number and types of FGCs in the Salmonella pangenome, to test the use of current classifications based on phylogeny, and to infer potential correlations between FGC evolution in various Salmonella serovars and host niches. This study focused on the FGCs of the currently deciphered 90 genomes and 60 plasmids of Salmonella. The analysis highlighted a fimbriome consisting of 35 different FGCs, of which 16 were new, each strain carrying between 5 and 14 FGCs. The Salmonella fimbriome was extremely diverse with FGC representatives in 8 out of 9 previously categorized fimbrial clades and subclades. Phylogenetic analysis of Salmonella suggested macroevolutionary shifts detectable by extensive FGC deletion and acquisition. In addition, microevolutionary drifts were best depicted by the high level of allelic variation in predicted or known adhesins, such as the type 1 fimbrial adhesin FimH for which 67 different natural alleles were identified in S. enterica subsp. I. Together with strain-specific collections of FGCs, allelic variation among adhesins attested to the pathoadaptive evolution of Salmonella towards specific hosts and tissues, potentially modulating host range, strain virulence, disease progression, and transmission efficiency. Further understanding of how each Salmonella strain utilizes its panel of FGCs and specific adhesin alleles for survival and infection will support the development of new approaches for the control of Salmonellosis.