Isolation and characterization of conditional adherent and non-type 1 fimbriated Salmonella typhimurium mutants

Salmonella secretion systems: Differential roles in pathogen-host interactions

The bacterial genus Salmonella includes a large group of food-borne pathogens that cause a variety of gastrointestinal or systemic diseases in hosts. Salmonella use several secretion devices to inject various effectors targeting eukaryotic hosts, or bacteria. In the past few years, considerable progress has been made towards understanding the structural features and molecular mechanisms of the secretion systems of Salmonella, particularly regarding their roles in host-pathogen interactions. In this review, we summarize the current advances about the main characteristics of the Salmonella secretion systems. Clarifying the roles of the secretion systems in the process of infecting various hosts will broaden our understanding of the importance of microbial interactions in maintaining human health and will provide information for developing novel therapeutic approaches.

Salmonella enterica serovars Typhimurium and Typhi as model organisms: revealing paradigm of host-pathogen interactions

The lifestyle of intracellular pathogens has always questioned the skill of a microbiologist in the context of finding the permanent cure to the diseases caused by them. The best tool utilized by these pathogens is their ability to reside inside the host cell, which enables them to easily bypass the humoral immunity of the host, such as the complement system. They further escape from the intracellular immunity, such as lysosome and inflammasome, mostly by forming a protective vacuole-bound niche derived from the host itself. Some of the most dreadful diseases are caused by these vacuolar pathogens, for example, tuberculosis by Mycobacterium or typhoid fever by Salmonella. To deal with such successful pathogens therapeutically, the knowledge of a host-pathogen interaction system becomes primarily essential, which further depends on the use of a model system. A well characterized pathogen, namely Salmonella, suits the role of a model for this purpose, which can infect a wide array of hosts causing a variety of diseases. This review focuses on various such aspects of research on Salmonella which are useful for studying the pathogenesis of other intracellular pathogens.

Salmonella enterica serovar Typhimurium binds to HeLa cells via Fim-mediated reversible adhesion and irreversible type three secretion system 1-mediated docking

The food-borne pathogen Salmonella enterica serovar Typhimurium invades mammalian epithelial cells. This multistep process comprises bacterial binding to the host cell, activation of the Salmonella type three secretion system 1 (T1), injection of effector proteins, triggering of host cell actin rearrangements, and S. Typhimurium entry. While the latter steps are well understood, much less is known about the initial binding step. Earlier work had implicated adhesins (but not T1) or T1 (but not other adhesins). We have studied here the Salmonella virulence factors mediating S. Typhimurium binding to HeLa cells. Using an automated microscopy assay and isogenic S. Typhimurium mutants, we analyzed the role of T1 and of several known adhesins (Fim, Pef, Lpf, Agf, and Shd) in host cell binding. In wild-type S. Typhimurium, host cell binding was mostly attributable to T1. However, in the absence of T1, Fim (but not Pef, Lpf, Agf, and Shd) also mediated HeLa cell binding. Furthermore, in the absence of T1 and type I fimbriae (Fim), we still observed residual binding, pointing toward at least one additional, unidentified binding mechanism. Dissociation experiments established that T1-mediated binding was irreversible ("docking"), while Fim-mediated binding was reversible ("reversible adhesion"). Finally, we show that noninvasive bacteria docking via T1 or adhering via Fim can efficiently invade HeLa cells, if actin rearrangements are triggered in trans by a wild-type S. Typhimurium helper strain. Our data show that binding to HeLa cells is mediated by at least two different mechanisms and that both can lead to invasion if actin rearrangements are triggered.

Adhesin-dependent binding and uptake of Salmonella enterica serovar Typhimurium by dendritic cells

Salmonella enterica serovar Typhimurium can be internalized by immature dendritic cells (DCs). The interacting host and bacterial molecules initiating this process remain uncharacterized. The objective of this study was to investigate whether specific fimbriae are involved in the early step of binding and uptake of Salmonella by DCs. Type 1 fimbriated S. enterica serovar Typhimurium or recombinant Escherichia coli expressing the type 1 fimbriae showed a significantly greater ability to attach to murine bone-marrow-derived DCs than non-fimbriated bacteria. The FimH adhesin was required for efficient interactions with DCs, since fimbriated fimH mutants were impaired in both binding and internalization. Finally, the internalization involved a FimH-dependent process but did not require sipB, a gene essential for Salmonella-mediated invasion of mammalian epithelial cells. Collectively, these data suggest that the bacterial interaction of DCs through the type 1 fimbrial adhesin FimH is sufficient to target S. enterica serovar Typhimurium for cellular uptake.

Salmonella enterica serotype Typhimurium fimbrial proteins serve as antigens during infection of mice

The Salmonella enterica serotype Typhimurium genome contains 13 operons with homology to fimbrial gene sequences. Here we investigated the role of 11 serotype Typhimurium fimbrial proteins, including FimA, AgfA (CsgA), BcfA, StbA, SthA, LpfA, PefA, StdA, StcA, StiA, and StfA, as antigens during the infection of genetically resistant mice (CBA). Upon the growth of serotype Typhimurium in standard laboratory broth culture, only the expression of FimA could be detected by Western blot analysis. The infection of mice with serotype Typhimurium grown in broth culture, followed by at least one subsequent infection, resulted in seroconversion of animals to FimA, AgfA, BcfA, StbA, SthA, LpfA, PefA, StdA, StcA, StiA, and StfA positivity. Most animals seroconverted to only a subset of these fimbrial antigens. The immunization of mice with glutathione S-transferase (GST)-FimA, GST-AgfA, GST-BcfA, GST-StbA, GST-SthA, GST-LpfA, GST-PefA, GST-StdA, GST-StcA, GST-StiA, and GST-StfA fusion proteins resulted in reduced fecal shedding of serotype Typhimurium during a challenge compared to that by a control group immunized with purified GST protein. Collectively, these data suggest that the expression of serotype Typhimurium fimbrial antigens is induced during the infection of mice.

Proteome analysis of serovars Typhimurium and Pullorum of Salmonella enterica subspecies I

BACKGROUND

Salmonella enterica subspecies I includes several closely related serovars which differ in host ranges and ability to cause disease. The basis for the diversity in host range and pathogenic potential of the serovars is not well understood, and it is not known how host-restricted variants appeared and what factors were lost or acquired during adaptations to a specific environment. Differences apparent from the genomic data do not necessarily correspond to functional proteins and more importantly differential regulation of otherwise identical gene content may play a role in the diverse phenotypes of the serovars of Salmonella.

RESULTS

In this study a comparative analysis of the cytosolic proteins of serovars Typhimurium and Pullorum was performed using two-dimensional gel electrophoresis and the proteins of interest were identified using mass spectrometry. An annotated reference map was created for serovar Typhimurium containing 233 entries, which included many metabolic enzymes, ribosomal proteins, chaperones and many other proteins characteristic for the growing cell. The comparative analysis of the two serovars revealed a high degree of variation amongst isolates obtained from different sources and, in some cases, the variation was greater between isolates of the same serovar than between isolates with different sero-specificity. However, several serovar-specific proteins, including intermediates in sulphate utilisation and cysteine synthesis, were also found despite the fact that the genes encoding those proteins are present in the genomes of both serovars.

CONCLUSION

Current microbial proteomics are generally based on the use of a single reference or type strain of a species. This study has shown the importance of incorporating a large number of strains of a species, as the diversity of the proteome in the microbial population appears to be significantly greater than expected. The characterisation of a diverse selection of strains revealed parts of the proteome of S. enterica that alter their expression while others remain stable and allowed for the identification of serovar-specific factors that have so far remained undetected by other methods.

The Salmonella enterica serotype Typhimurium lpf, bcf, stb, stc, std, and sth fimbrial operons are required for intestinal persistence in mice

Salmonella enterica serotype Typhimurium causes human infections that can frequently be traced back through the food chain to healthy livestock whose intestine is colonized by the pathogen. Little is known about the genes important for intestinal carriage of S. enterica serotype Typhimurium in vertebrate animals. Here we characterized the role of 10 fimbrial operons, agf, fim, lpf, pef, bcf, stb, stc, std, stf, and sth, using competitive infection experiments performed in genetically susceptible (BALB/c) and resistant (CBA) mice. Deletion of agfAB, fimAICDHF, lpfABCDE, pefABCDI, bcfABCDEFG, stbABCD, stcABCD, stdAB, stfACDEFG, or sthABCDE did not reduce the ability of S. enterica serotype Typhimurium to colonize the spleen and cecum of BALB/c mice 5 days after infection. Similarly, deletion of agfAB, fimAICDHF, pefABCDI, and stfACDEFG did not result in reduced recovery of S. enterica serotype Typhimurium from fecal samples collected from infected CBA mice over a 30-day time period. However, S. enterica serotype Typhimurium strains carrying deletions in lpfABCDE, bcfABCDEFG, stbABCD, stcABCD, stdAB, or sthABCDE were recovered at significantly reduced numbers from the feces of CBA mice. There was a good correlation (R(2) = 0.9626) between competitive indices in the cecum and fecal samples of CBA mice at 30 days after infection, suggesting that the recovery of S. enterica serotype Typhimurium from fecal samples closely reflected its ability to colonize the cecum. Collectively, these data show that six fimbrial operons (lpf, bcf, stb, stc, std, and sth) contribute to long-term intestinal carriage of S. enterica serotype Typhimurium in genetically resistant mice.

Expression of Escherichia coli antigens in Salmonella typhimurium as a vaccine to prevent airsacculitis in chickens

Avian pathogenic Escherichia coli strains are associated with a variety of extraintestinal poultry diseases, including airsacculitis, colisepticemia, and cellulitis. A number of E. coli serotypes are associated with these diseases, although the most prevalent serotype is O78. Fimbrial proteins expressed by these strains appear to be important virulence factors, including type 1 fimbriae, P fimbriae, and curli. We have been working to develop an effective vaccine to protect chickens against these diseases. We have previously shown that an attenuated Salmonella typhimurium strain expressing O78 lipopolysaccharide provides protection against challenge with an O78 avian pathogenic E. coli strain. In this work, we have constructed an attenuated S. typhimurium that expresses both the O78 lipopolysaccharide and E. coli-derived type 1 fimbriae. In these studies, chickens were vaccinated at day of hatch and again at 2 wk of age. Birds were challenged at 4 wk of age. We found that the vaccine candidate provided significant protection against airsacculitis as compared to untreated controls or birds vaccinated with an attenuated S. typhimurium that did not express any E. coli antigens. In a separate experiment, challenged vaccinates showed significant weight gain compared to challenged nonvaccinates. We were not able to demonstrate protection against E. coli O1 or O2 serotype challenge, nor against challenge with wild-type S. typhimurium.

The use of flow cytometry to detect expression of subunits encoded by 11 Salmonella enterica serotype Typhimurium fimbrial operons

The Salmonella enterica serotype Typhimurium (S. Typhimurium) genome contains 13 putative fimbrial operons termed agf (csg), fim, pef, lpf, bcf, saf, stb, stc, std, stf, sth, sti and stj. Evidence for in vitro expression of fimbrial proteins encoded by these operons is currently only available for agf, fim and pef. We raised antisera against putative major fimbrial subunits of S. Typhimurium, including AgfA, FimA, PefA, LpfA, BcfA, StbA, StcA, StdA, StfA, SthA and StiA. Elaboration of StcA on the bacterial surface could be detected by flow cytometry and immunoelectron microscopy after expression of the cloned stcABCD operon from a heterologous T7 promoter in Escherichia coli. To study the expression of fimbrial antigens in S. Typhimurium by flow cytometry, we constructed strains carrying deletions of agfAB, pefBACDI, lpfABCDE, bcfABCDEFG, stbABCD, stcABC, stdAB, stfACDEFG, sthABCDE or stiABCDE. Using these deletion mutants for gating, expression of fimbrial antigens was measured by flow cytometry in cultures grown in vitro or in samples recovered 8 h after infection of bovine ligated ileal loops with S. Typhimurium. FimA was the only fimbrial antigen expressed by S. Typhimurium after static growth in Luria-Bertani (LB) broth. Injection of static LB broth cultures of S. Typhimurium into bovine ligated ileal loops resulted in the expression of BcfA, FimA, LpfA, PefA, StbA, StcA, StdA, StfA and StiA. These data show that in vivo growth conditions drastically alter the repertoire of fimbrial antigens expressed in S. Typhimurium.

Yersinia pseudotuberculosis produces a cytotoxic necrotizing factor

Cell extracts from Yersinia pseudotuberculosis induced multinucleation in HEp-2 cells in a manner similar to the effect caused by Escherichia coli cytotoxic necrotizing factor (CNF). The activity was not dependent on the Yersinia 70-kb virulence plasmid, and the activity was not inhibited by antibodies capable of neutralizing E. coli CNF type 1. The nucleotide sequence of the Yersinia cnf gene was 65.1% identical to the E. coli cnf gene.

M cells as ports of entry for enteroinvasive pathogens: mechanisms of interaction, consequences for the disease process

M cells are key sites of antigen sampling for the mucosal-associated lymphoid system (MALT) and consequently are essential components of the structures serving as inductive sites for mucosal immunity. In addition, they have recently been recognized as major sites of adherence and major ports of entry for enteric pathogens. In the case of enteroinvasive pathogens, such as Salmonella, Yersinia and Shigella, few clinical evidences, but lots of experimental data indicate that, at least at the early stage of infection, M cells of the follicular associated epithelium transport the pathogens. This has significantly altered our view on the pathogenesis of enteroinvasive infections. Crossing the epithelial barrier seems an achievable task for these bacteria which express adherence and invasion mechanisms which have often been well characterized in epithelial cell lines. These systems seem to be also used for entering and crossing M cells, although reproducible in vitro assays for M cell infection are now required. Having crossed the epithelial lining, the bacteria face phagocytic cells, particularly the macrophages that are present in the follicle dome. Depending on the capacity to survive in the presence of macrophages, and how this survival is achieved by a given invasive species, the outcome of infection can be dramatically affected. In consequence, M cells can be considered as pathogen translocators toward immunocompetent areas of the gut, thus opening the possibility to harness this property in order to design new mucosal vaccines or vaccine vectors.

Molecular basis for the enterocyte tropism exhibited by Salmonella typhimurium type 1 fimbriae

Salmonella typhimurium exhibits a distinct tropism for mouse enterocytes that is linked to their expression of type 1 fimbriae. The distinct binding traits of Salmonella type 1 fimbriae is also reflected in their binding to selected mannosylated proteins and in their ability to promote secondary bacterial aggregation on enterocyte surfaces. The determinant of binding in Salmonella type 1 fimbriae is a 35-kDa structurally distinct fimbrial subunit, FimHS, because inactivation of fimHS abolished binding activity in the resulting mutant without any apparent effect on fimbrial expression. Surprisingly, when expressed in the absence of other fimbrial components and as a translational fusion protein with MalE, FimHS failed to demonstrate any specific binding tropism and bound equally to all cells and mannosylated proteins tested. To determine if the binding specificity of Salmonella type 1 fimbriae was determined by the fimbrial shaft that is intimately associated with FimHS, we replaced the amino-terminal half of FimHS with the corresponding sequence from Escherichia coli FimH (FimHE) that contains the receptor binding domain of FimHE. The resulting hybrid fimbriae bearing FimHES on a Salmonella fimbrial shaft exhibited binding traits that resembled that of Salmonella rather than E. coli fimbriae. Apparently, the quaternary constraints imposed by the fimbrial shaft on the adhesin determine the distinct binding traits of S. typhimurium type 1 fimbriae.

Analysis of the type 1 pilin gene cluster fim in Salmonella: its distinct evolutionary histories in the 5' and 3' regions

The type 1 pilin encoded by fim is present in both Escherichia coli and Salmonella natural isolates, but several lines of evidence indicate that similarities at the fim locus may be an example of independent acquisition rather than common ancestry. For example, the fim gene cluster is found at different chromosomal locations and with distinct gene orders in these closely related species. In this work we examined the fim gene cluster of Salmonella, the genes of which show high nucleotide sequence divergence from their E. coli counterparts, as well as a different G+C content and codon usage. DNA hybridization analysis revealed that, among the salmonellae, the fim gene cluster is present in all isolates of S. enterica but is absent from S. bongori. Molecular phylogenetic analyses of the fimA and fimI genes yield an estimate of phylogeny that is in satisfactory congruence with housekeeping and other virulence genes examined in this species. In contrast, phylogenetic analyses of the fimZ, fimY, and fimW genes indicate that horizontal transfer of this region has occurred more than once. There is also size variation in the fimZ, fimY, and fimW intergenic regions in the 3' region, and these genes are absent in isolate S2983 of subspecies IIIa. Interestingly, the G+C contents of the fimZ, fimY, and fimW genes are less than 46%, which is considerably lower than those of the other six genes of the fim cluster. This study demonstrates that horizontal transmission of all or part of the same gene cluster can occur repeatedly, with the result that different regions of a single gene cluster may have different evolutionary histories.

Characterisation of epitopes of type 1 fimbriae of Salmonella using monoclonal antibodies specific for SEF21 fimbriae of Salmonella enteritidis

Monoclonal antibodies (mAbs) were used to identify and characterise epitopes of type 1 (SEF21) fimbriae of Salmonella enteritidis. The distribution of the epitopes among salmonellas and other enterobacteria was investigated, as well as the influence of growth media and temperatures on their expression. At least four different epitope clusters were identified on SEF21 fimbriae of S. enteritidis. Two of these clusters were associated with fimbrial haemagglutinins that were either common to all salmonellae tested, or restricted only to S. enteritidis and S. dublin. The four epitope clusters were identified on type 1 fimbriae of most Salmonella serotypes, as well as non-haemagglutinating type 2 fimbriae of S. pullorum and S. gallinarum, and on many other enterobacterial species. The expression of the epitopes was affected by growth conditions.

Contribution of horizontal gene transfer and deletion events to development of distinctive patterns of fimbrial operons during evolution of Salmonella serotypes

Only certain serotypes of Salmonella represent 99% of all human clinical isolates. We determined whether the phylogenetic distribution of fimbrial operons would account for the host adaptations observed for Salmonella serotypes. We found that three fimbrial operons, fim, lpf, and agf, were present in a lineage ancestral to Salmonella. While the fim and agf fimbrial operons were highly conserved among all Salmonella serotypes, sequence analysis suggested that the lpf operon was lost from many distantly related lineages. As a consequence, the distribution of the lpf operon cannot be explained easily and may be a consequence of positive and negative selection in different hosts for the presence of these genes. Two other fimbrial operons, sef and pef, each entered two distantly related Salmonella lineages and each is present only in a small number of serotypes. These results show that horizontal gene transfer and deletion events have created unique combinations of fimbrial operons among Salmonella serotypes. The presence of sef and pef correlated with serotypes frequently isolated from common domesticated animals.

The location of four fimbrin-encoding genes, agfA, fimA, sefA and sefD, on the Salmonella enteritidis and/or S. typhimurium XbaI-BlnI genomic restriction maps

Four fimbrin-encoding genes, fimA (type-1 or SEF21 fimbriae), agfA (thin aggregative or SEF17 fimbriae), sefA (SEF14 fimbriae and sefD (SEF18 fimbriae) from Salmonella enteritidis (Se) 27655-3b were located onto the XbaI-BlnI genomic restriction maps of Salmonella typhimurium (St) LT2 and Se strains SSU7998 and 27655-3b. The XbaI or BlnI genomic fragments carrying these genes were identified by hybridization with labeled oligodeoxyribonucleotides or fimbrin-encoding genes. The fimbrin-encoding genes were not encoded by the virulence plasmids, but were located on chromosomal DNA fragments. The position of each gene on a given XbaI fragment was determined by hybridization of a series of XbaI-digested genomic DNA samples from previously characterized Tn10 mutants of Se and St with its respective probe. The fimA gene mapped near 13 centisomes (Cs) between purE884::Tn10 at 12.6 Cs (11.8 min) and apeE2::Tn10 at 12.8 Cs (12.3 min) beside the first XbaI site at 13.0 Cs in St or between purE884::Tn10 at 12.6 Cs and the XbaI site at 13.6 Cs in Se. The agfA gene mapped near 26 Cs between putA::Tn10 and pyrC691::Tn10 in St, but near 40 Cs between pncX::Tn10 and the XbaI site at 43.3 Cs in Se. This difference in map position was due to the location of agfA near one end of the 815-kb chromosomal fragment inverted between Se and St. The sefA and sefD genes mapped precisely at 97.6 Cs in Se, but were absent from the genome of St LT2. To verify the mapping procedures used herein, tctC was also mapped in both Salmonella serovars. As expected, tctC mapped near 60 Cs in both St and Se, thereby confirming previous studies.

Genetic map of Salmonella typhimurium, edition VIII

We present edition VIII of the genetic map of Salmonella typhimurium LT2. We list a total of 1,159 genes, 1,080 of which have been located on the circular chromosome and 29 of which are on pSLT, the 90-kb plasmid usually found in LT2 lines. The remaining 50 genes are not yet mapped. The coordinate system used in this edition is neither minutes of transfer time in conjugation crosses nor units representing "phage lengths" of DNA of the transducing phage P22, as used in earlier editions, but centisomes and kilobases based on physical analysis of the lengths of DNA segments between genes. Some of these lengths have been determined by digestion of DNA by rare-cutting endonucleases and separation of fragments by pulsed-field gel electrophoresis. Other lengths have been determined by analysis of DNA sequences in GenBank. We have constructed StySeq1, which incorporates all Salmonella DNA sequence data known to us. StySeq1 comprises over 548 kb of nonredundant chromosomal genomic sequences, representing 11.4% of the chromosome, which is estimated to be just over 4,800 kb in length. Most of these sequences were assigned locations on the chromosome, in some cases by analogy with mapped Escherichia coli sequences.

Identification and sequence analysis of lpfABCDE, a putative fimbrial operon of Salmonella typhimurium

A chromosomal region present in Salmonella typhimurium but absent from related species was identified by hybridization. A DNA probe originating from 78 min on the S. typhimurium chromosome hybridized with DNA from Salmonella enteritidis, Salmonella heidelberg, and Salmonella dublin but not with DNA from Salmonella typhi, Salmonella arizonae, Escherichia coli, and Shigella serotypes. Cloning and sequence analysis revealed that the corresponding region of the S. typhimurium chromosome encodes a fimbrial operon. Long fimbriae inserted at the poles of the bacterium were observed by electron microscopy when this fimbrial operon was introduced into a nonpiliated E. coli strain. The genes encoding these fimbriae were therefore termed lpfABCDE, for long polar fimbriae. Genetically, the lpf operon was found to be most closely related to the fim operon of S. typhimurium, both in gene order and in conservation of the deduced amino acid sequences.

Identification of ancillary fim genes affecting fimA expression in Salmonella typhimurium

Regulation of the gene, fimA, encoding the major fimbrial subunit of S. typhimurium S6704 was examined by using a lambda fimA-lacZ lysogen. Transformation of the lambda fimA-lacZ lysogen with various derivatives of the recombinant plasmid that encodes type 1 fimbrial expression, pISF101, indicated that two regions of this plasmid alter beta-galactosidase production. One plasmid is a deletion resulting in the loss of a 28-kDa polypeptide downstream of fimA, while the other plasmid encodes a 24- and a 27-kDa polypeptide. Northern (RNA) blot analyses indicated that the steady-state fimA mRNA levels of these transformants were high. In addition, phenotypic expression of type 1 fimbriae by agar-grown cultures is observed only in those transformants bearing plasmids which show increased beta-galactosidase and fimA mRNA levels.