Cloning and molecular characterization of a gene involved in Salmonella adherence and invasion of cultured epithelial cells

Development of recombinant subunit vaccine targeting InvH protein of Salmonella Typhimurium and evaluation of its immunoprotective efficacy against salmonellosis

Salmonella Typhimurium is the most prevalent non-host specific Salmonella serovars and a major concern for both human and animal health systems worldwide contributing to significant economic loss. Type 3 secretion system (T3SS) of Salmonella plays an important role in bacterial adherence and entry into the host epithelial cells. The product of invH gene of Salmonella is an important component of the needle complex of the type 3 secretion system. Hence, the present study was undertaken to clone and express the 15 kDa InvH surface protein of Salmonella Typhimurium in an E. coli host and to evaluate its immune potency in mice. The purified recombinant InvH (r-InvH) protein provoked a significant (p < 0.01) rise in IgG in the inoculated mice. The immunized mice were completely (100%) protected against the challenge dose of 107.5 LD50, while protection against challenge with the same dose of heterologous serovars was 90%. The bacterin-vaccinated group showed homologous protection of 60% against all three serovars. Findings in this study suggest the potential of the r-InvH protein of S. Typhimurium as an effective vaccine candidate against Salmonella infections.

Bioinformatics design of recombinant chimeric protein containing SipD and LptD immunogens and evaluation of its immunogenicity against Salmonella Typhimurium

The growing emergence of resistant bacteria is the current global concern for the humans and animals. Vaccination could be the desirable method to preventing such infectious diseases. Safe and effective vaccines are urgently needed to manage and prevent Salmonella contamination. Subunit vaccines are safe approaches for the protection against Salmonella spp. The bioinformatics methods were performed to determine the gene structure. Gene cassette (rLPSI) was ordered in pET28a (+), and cloned into E.coli BL21 (DE3), and the recombinant protein was expressed using IPTG (1 mM). Mice were immunized by subcutaneous administration of recombinant protein. Then, the mice were challenged by oral administration of 100LD₅₀ of live S. Typhimurium. The Codon adaptation index of the chimeric gene was multiplied by 0.92. Validation results showed that >90% of residues lie in the desired or extra allowed area of the Ramachandran plot. The recombinant protein (65.9 kDa) was expressed in E.coli. Antibody titers in vaccinated mice were significantly different from those in the control groups. Recombinant protein immunization of the mice provided 90% and 70% protection against 10LD₅₀ and 100LD₅₀ of S. Typhimurium, respectively. In general, the results showed the high efficiency of rLPSI chimeric protein as a protective antigen against S. Typhimurium infection. The rLPSI chimeric protein could be an effective recombinant vaccine candidate against S. Typhimurium infection, but more research is needed.

Differential expression of virulence genes and role of gyrA mutations in quinolone resistant and susceptible strains of Salmonella Weltevreden and Newport isolated from seafood

AIMS

To investigate the differential expression of virulence genes and role of gyrA mutations in quinolone resistant and susceptible strains of Salmonella isolated from seafood.

METHODS AND RESULTS

Forty Salmonella isolates from seafood were tested for antibiotic sensitivity. Minimal inhibitory concentration (MIC) was determined and two nalidixic acid-resistant isolates, viz Salmonella Weltevreden (SW9) and Salmonella Newport (SN36) were selected for identifying the mechanism of resistance. SW9 showed mutation in the gyrA gene at codon 83 (Ser to Tyr) while SN36 presented at codon 87 (Asp to Asn). Experimental induction of resistance to a sensitive Salm. Newport (SN71) showed point mutation at codon 87 (Asp to Gly) in the gyrA gene, and was designated SN71R. All the isolates resistant to nalidixic acid had a single mutation at different positions in the gyrA gene. However, induction of resistance to a sensitive Salm. Weltevreden (SW30) was exceptional in that it did not show any mutation in the gyrA region. Use of Phe-Arg-β-naphthylamide (PAβN) also could not reduce MIC below the Clinical and Laboratory Standards Institute guidelines revealing the absence of efflux mediated resistance. Thus, the resistance mechanism in SW30R is unknown. The growth rate of quinolone resistant isolates was slower than the susceptible ones. The resistant isolates showed decreased epithelial cell invasion and intracellular replication. The mRNA expression levels of some of the genes were significantly (P < 0·005) reduced in SN71R compared to the sensitive strain (SN71).

CONCLUSIONS

Nalidixic acid-resistant Salmonella strains are associated with lower virulence and pathogenicity than the sensitive strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provided valuable information on the difference in the growth, cytotoxicity, infectivity and expression of virulence genes in resistant and susceptible strains. Furthermore, the gyrA mutation was shown to be the main mechanism of quinolone resistance in Salmonella other than the overexpression of efflux pumps or the presence of plasmid mediated quinolone resistance genes.

Assembly and structure of the T3SS

The Type III Secretion System (T3SS) is a multi-mega Dalton apparatus assembled from more than twenty components and is found in many species of animal and plant bacterial pathogens. The T3SS creates a contiguous channel through the bacterial and host membranes, allowing injection of specialized bacterial effector proteins directly to the host cell. In this review, we discuss our current understanding of T3SS assembly and structure, as well as highlight structurally characterized Salmonella effectors. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Host specificity of bacterial pathogens

Most pathogens are able to infect multiple hosts but some are highly adapted to a single-host species. A detailed understanding of the basis of host specificity can provide important insights into molecular pathogenesis, the evolution of pathogenic microbes, and the potential for pathogens to cross the species barrier to infect new hosts. Comparative genomics and the development of humanized mouse models have provided important new tools with which to explore the basis of generalism and specialism. This review will examine host specificity of bacterial pathogens with a focus on generalist and specialist serovars of Salmonella enterica.

DsbB is required for the pathogenesis process of Xanthomonas campestris pv. campestris

The DsbA/DsbB oxidation pathway is one of the two pathways that catalyze disulfide bond formation of proteins in the periplasm of gram-negative bacteria. It has been demonstrated that DsbA is essential for multiple virulence factors of several animal bacterial pathogens. In this article, we present genetic evidence to show that the open reading frame XC_3314 encodes a DsbB protein that is involved in disulfide bond formation in periplasm of Xanthomonas campestris pv. campestris, the causative agent of crucifer black rot disease. The dsbB mutant of X. campestris pv. campestris exhibited attenuation in virulence, hypersensitive response, cell motility, and bacterial growth in planta. Furthermore, mutation in the dsbB gene resulted in ineffective type II and type III secretion systems as well as flagellar assembly. These findings reveal that DsbB is required for the pathogenesis process of X. campestris pv. campestris.

A small non-coding RNA of the invasion gene island (SPI-1) represses outer membrane protein synthesis from the Salmonella core genome

The Salmonella pathogenicity island (SPI-1) encodes approximately 35 proteins involved in assembly of a type III secretion system (T3SS) which endows Salmonella with the ability to invade eukaryotic cells. We have discovered a novel SPI-1 gene, invR, which expresses an abundant small non-coding RNA (sRNA). The invR gene, which we identified in a global search for new Salmonella sRNA genes, is activated by the major SPI-1 transcription factor, HilD, under conditions that favour host cell invasion. The RNA chaperone, Hfq, is essential for the in vivo stability of the approximately 80 nt InvR RNA. Hfq binds InvR with high affinity in vitro, and InvR co-immunoprecipitates with FLAG epitope-tagged Hfq in Salmonella extracts. Surprisingly, deletion/overexpression of invR revealed no phenotype in SPI-1 regulation. In contrast, we find that InvR represses the synthesis of the abundant OmpD porin encoded by the Salmonella core genome. As invR is conserved in the early branching Salmonella bongori, we speculate that porin repression by InvR may have aided successful establishment of the SPI-1 T3SS after horizontal acquisition in the Salmonella lineage. This study identifies the first regulatory RNA of an enterobacterial pathogenicity island, and new roles for Hfq and HilD in SPI-1 gene expression.

The role of Dsb proteins of Gram-negative bacteria in the process of pathogenesis

Tertiary and quaternary structures of extracytoplasmic proteins containing more than one cysteine residue often require introduction of disulfide bonds. This process takes place in an oxidative environment, such as the periplasm of Gram-negative bacteria, and is catalyzed by Dsb (disulfide bond formation) proteins. Mutations in dsb genes influence the conformation and stability of many extracytoplasmic proteins. Thus, many pathogens become partially or fully attenuated due to improper folding of proteins that act as virulence factors. This review summarizes the current knowledge on Dsb proteins and their effect on the pathogenicity of Gram-negative bacteria. The potential application of Dsb proteins in biotechnology is also discussed.

Neutrophil influx during non-typhoidal salmonellosis: who is in the driver's seat?

A massive neutrophil influx in the intestine is the histopathological hallmark of Salmonella enterica serovar Typhimurium-induced enterocolitis in humans. Two major hypotheses on the mechanism leading to neutrophil infiltration in the intestinal mucosa have emerged. One hypothesis suggests that S. enterica serovar Typhimurium takes an active role in triggering this host response by injecting proteins, termed effectors, into the host cell cytosol which induce a proinflammatory gene expression profile in the intestinal epithelium. The second hypothesis suggests a more passive role for the pathogen by proposing that bacterial invasion stimulates the innate pathways of inflammation because the pathogen-associated molecular patterns of S. enterica serovar Typhimurium are recognized by pathogen recognition receptors on cells in the lamina propria. A review of the current literature reveals that, while pathogen recognition receptors are clearly involved in eliciting neutrophil influx during S. enterica serovar Typhimurium infection, a direct contribution of effectors in triggering proinflammatory host cell responses cannot currently be ruled out.

Two periplasmic disulfide oxidoreductases, DsbA and SrgA, target outer membrane protein SpiA, a component of the Salmonella pathogenicity island 2 type III secretion system

The formation of disulfide is essential for the folding, activity, and stability of many proteins secreted by Gram-negative bacteria. The disulfide oxidoreductase, DsbA, introduces disulfide bonds into proteins exported from the cytoplasm to periplasm. In pathogenic bacteria, DsbA is required to process virulence determinants for their folding and assembly. In this study, we examined the role of the Dsb enzymes in Salmonella pathogenesis, and we demonstrated that DsbA, but not DsbC, is required for the full expression of virulence in a mouse infection model of Salmonella enterica serovar Typhimurium. Salmonella strains carrying a dsbA mutation showed reduced function mediated by type III secretion systems (TTSSs) encoded on Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2). To obtain a more detailed understanding of the contribution of DsbA to both SPI-1 and SPI-2 TTSS function, we identified a protein component of the SPI-2 TTSS apparatus affected by DsbA. Although we found no substrate protein for DsbA in the SPI-1 TTSS apparatus, we identified SpiA (SsaC), an outer membrane protein of SPI-2 TTSS, as a DsbA substrate. Site-directed mutagenesis of the two cysteine residues present in the SpiA protein resulted in the loss of SPI-2 function in vitro and in vivo. Furthermore, we provided evidence that a second disulfide oxidoreductase, SrgA, also oxidizes SpiA. Analysis of in vivo mixed infections demonstrated that a Salmonella dsbA srgA double mutant strain was more attenuated than either single mutant, suggesting that DsbA acts in concert with SrgA in vivo.

Use of mixed infections to study cell invasion and intracellular proliferation of Salmonella enterica in eukaryotic cell cultures

Epithelial cell lines are widely used as an in vitro model to study cell invasion by Salmonella. In turn, phagocytic cell lines are used to study Salmonella intracellular survival and proliferation. We describe a novel method, derived from the classical mixed infection procedure, to quantify invasion and proliferation defects in Salmonella enterica serovar Typhimurium. A eukaryotic cell culture is infected with two strains (e.g., a mutant and the wild-type). After infection, bacterial cells that remain extracellular are eliminated with gentamicin. At the end of the trial, intracellular bacteria are recovered and plated. Colonies from each strain are then counted for the calculation of a competitive index. Strain discrimination can be achieved either with antibiotic resistance markers or using plasmids encoding color markers (e.g., fluorescent proteins). Because both strains are exposed to the same conditions throughout the process, the procedure decreases the variability between independent trials and allows a direct measurement of the impairment of the mutant in invasion or intracellular proliferation.

Bacterial type III secretion systems are ancient and evolved by multiple horizontal-transfer events

Type III secretion systems (TTSS) are unique bacterial mechanisms that mediate elaborate interactions with their hosts. The fact that several of the TTSS proteins are closely related to flagellar export proteins has led to the suggestion that TTSS had evolved from flagella. Here we reconstruct the evolutionary history of four conserved type III secretion proteins and their phylogenetic relationships with flagellar paralogs. Our analysis indicates that the TTSS and the flagellar export mechanism share a common ancestor, but have evolved independently from one another. The suggestion that TTSS genes have evolved from genes encoding flagellar proteins is effectively refuted. A comparison of the species tree, as deduced from 16S rDNA sequences, to the protein phylogenetic trees has led to the identification of several major lateral transfer events involving clusters of TTSS genes. It is hypothesized that horizontal gene transfer has occurred much earlier and more frequently than previously inferred for TTSS genes and is, consequently, a major force shaping the evolution of species that harbor type III secretion systems.

Protein-peptidoglycan interactions modulate the assembly of the needle complex in the Salmonella invasion-associated type III secretion system

The invasion-associated type III secretion system of Salmonella enterica assembles as a supra-molecular structure, termed needle complex, which spans the bacterial envelope. Here, we present evidence for protein-peptidoglycan interactions that modulate the assembly of this organelle. The presence of major membrane components of the needle complex (PrgH, PrgK and InvG) and InvH, required for efficient assembly of the organelle, was examined in peptidoglycan purified by extensive boiling of bacteria in 4% SDS. InvH, PrgH and PrgK, but not InvG, were detected in this purified material. InvH was present in the peptidoglycan in higher relative amounts than PrgH or PrgK, and was the only protein efficiently bound to peptidoglycan in cross-linking experiments. Analysis in mutants defective for needle complex proteins showed that the needle proteins PrgI and PrgJ and, to a lesser extent, InvH, sustain the association of PrgH and PrgK with peptidoglycan. In contrast, the association of InvH with peptidoglycan did not necessitate other needle complex proteins. Functional analysis showed that the association of InvH, PrgH and PrgK with peptidoglycan is abolished in live bacteria carrying structural modifications in the peptidoglycan. The loss of these interactions caused a marked reduction in the number of needle complexes and, concomitantly, in protein secretion and bacterial invasion of cultured eukaryotic cells. Altogether, these data provide the first evidence for an association between proteins of the Salmonella needle complex and the peptidoglycan. In addition, we demonstrate that these protein-peptidoglycan interactions are critical for an efficient and correct assembly of this specialized organelle.

Comparison of intestinal invasion and macrophage response of Salmonella Gallinarum and other host-adapted Salmonella enterica serovars in the avian host

The purpose of this investigation was to study the host specific infection of Salmonella Gallinarum in chickens and to determine the contribution of intestinal invasion and macrophage survival in relation to systemic infection in the host. This was carried out by comparing the kinetics of infection of S. Gallinarum to that of other Salmonella host-adapted (S. Cholerae-suis, S. Dublin and S. Typhimurium) and host-specific (S. Pullorum and S. Abortus-ovis) serovars. Establishment of the rate of colonisation in intestinal tissue, bursa and systemic sites was carried out by oral infection in day-old and week-old birds. Salmonella Gallinarum was the only serovar capable of causing systemic infection in chickens, however, general colonising ability in the intestine and bursa demonstrated no apparent selective advantage for S. Gallinarum. Further quantification of gastrointestinal invasion was carried out using ligated loops in the small intestine. Invasion in the jejunum of the chicken intestine over 3h demonstrated that Salmonella Typhimurium invasion was statistically higher (P<0.01) when compared with S. Gallinarum. Specific sites of high lymphoid tissue concentration in the chicken, including the bursa of Fabricius and caecal tonsils, were also targeted in invasion assays to investigate possible areas of tissue tropism. S. Typhimurium demonstrated significantly higher (P<0.01) invasion at these sites when compared with S. Gallinarum. Infection of chicken macrophages with S. Gallinarum did not demonstrate increased multiplication and survival intracellularly when compared with other Salmonella serotypes. The only difference seen was with S. Abortus-ovis, which demonstrated a significantly lower (P<0.05 to 0.001) intracellular survival. Together these data suggest that although S. Gallinarum host specificity in the chicken correlates with systemic infection, intestinal and lymphoid tissue invasion in the bursa and caeca, and macrophage survival does not influence this outcome.

HilE interacts with HilD and negatively regulates hilA transcription and expression of the Salmonella enterica serovar Typhimurium invasive phenotype

The ability of Salmonella enterica serovar Typhimurium to traverse the intestinal mucosa of a host is an important step in its ability to initiate gastrointestinal disease. The majority of the genes required for this invasive characteristic are encoded on Salmonella pathogenicity island 1 (SPI1), and their expression is controlled by the transcriptional activator HilA, a member of the OmpR/ToxR family of proteins. A variety of genes (hilC, hilD, fis, sirA/barA, csrAB, phoB, fadD, envZ/ompR, fliZ, hilE, ams, lon, pag, and hha) have been identified that exert positive or negative effects on hilA expression, although the mechanisms by which these gene products function remain relatively unclear. Recent work indicates that the small DNA-binding protein, Hha, has a significant role in repressing hilA transcription and the invasive phenotype, particularly in response to osmolarity signals. We have characterized the Salmonella-specific gene, hilE, and found that it plays an important regulatory role in hilA transcription and invasion gene expression. Mutation of hilE causes derepression of hilA transcription, and overexpression of hilE superrepresses hilA expression and the invasive phenotype. Bacterial two-hybrid experiments indicate that the HilE protein interacts with HilD, suggesting a possible mechanism for HilE negative regulation of hilA gene expression and the Salmonella invasive phenotype. Finally, we have found that the hilE gene resides on a region of the serovar Typhimurium chromosome that has many characteristics of a pathogenicity island.

AraC/XylS family members, HilD and HilC, directly activate virulence gene expression independently of HilA in Salmonella typhimurium

Salmonella typhimurium is a Gram-negative enteric pathogen that can infect intestinal epithelial cells and induce inflammation of the intestinal mucosa. These processes are mediated by a type III secretion system (TTSS), which is encoded on Salmonella pathogenicity island 1 (SPI1). Previous studies showed that four SPI1-encoded transcriptional regulators, HilD, HilC, HilA and InvF, act in an ordered fashion to co-ordinately activate expression of the SPI1 TTSS. HilD and HilC derepress hilA transcription. HilA activates invF as well as SPI1 genes that encode components of the TTS apparatus. InvF then activates genes that encode proteins secreted by the SPI1 TTS apparatus. In this scheme, HilD and HilC indirectly activate expression of the SPI1 TTS apparatus and its secreted substrates by affecting hilA expression. Here, we report that HilD and HilC can also activate expression of a subset of SPI1 genes independently of HilA. Our studies show that HilD and HilC activate transcription of invF from a promoter that is far upstream of its HilA-dependent promoter. This activation is most probably through direct binding of HilD and HilC to sequences upstream and downstream of this alternative HilA-independent promoter. We conclude that HilD and HilC have a second role in SPI1 gene regulation that is separate from their role in co-ordinating expression of the SPI1 TTSS through hilA.

The Salmonella enterica serotype typhimurium effector proteins SipA, SopA, SopB, SopD, and SopE2 act in concert to induce diarrhea in calves

Salmonella enterica serotype Typhimurium requires a functional type III secretion system encoded by Salmonella pathogenicity island 1 (SPI1) to cause diarrhea. We investigated the role of genes encoding secreted target proteins of the SPI1-associated type III secretion system for enteropathogenicity in calves. Salmonella serotype Typhimurium strains having mutations in sptP, avrA, sspH1, or slrP induced fluid secretion in the bovine ligated ileal loop model at levels similar to that of the wild type. In contrast, mutations in sipA, sopA, sopB, sopD, or sopE2 significantly reduced fluid accumulation in bovine ligated ileal loops at 8 h postinfection. A strain carrying mutations in sipA, sopA, sopB, sopD, and sopE2 (sipA sopABDE2 mutant) caused the same level of fluid accumulation in bovine ligated ileal loops as a strain carrying a mutation in sipB, a SPI1 gene required for the translocation of effector proteins into host cells. A positive correlation was observed between the severity of histopathological lesions detected in the ileal mucosa and the levels of fluid accumulation induced by the different mutants. After oral infection of calves, the Salmonella serotype Typhimurium sipAsopABDE2 mutant caused only mild diarrhea and was more strongly attenuated than strains having only single mutations. These data demonstrate that SipA, SopA, SopB, SopD, and SopE2 are major virulence factors responsible for diarrhea during Salmonella serotype Typhimurium infection of calves.

Characterization of the mutS-proximal region of the Salmonella typhimurium SPI-1 identifies a group of pathogenicity island-associated genes

The virulence properties of Salmonella enterica are largely encoded within a set of horizontally acquired gene blocks termed pathogenicity islands. One such pathogenicity island, SPI-1, located at centisome 63 of the Salmonella chromosome between the mutS and fhlA genes, encodes a type III protein secretion system and an iron uptake system. We have characterized the mutS-proximal border of this pathogenicity island and have identified two sets of genes, pigAB and pigCD. All four genes have homologs of unknown function in several bacteria that share the ability to establish an intimate association with higher eukaryotic hosts. The expression of at least two of these genes, pigA and pigB, is controlled by SprA, a transcription factor encoded within SPI-1 that controls the expression of genes associated with the type III secretion system of this island. In addition, we found that homologs of the pig genes are also found at different locations of the S. enterica chromosome in association with segments of DNA that exhibit features of pathogenicity islands. The presence of several apparently functional copies of these genes argues for an important role in the biology of this bacterial pathogen. Furthermore, they constitute a valuable tool to identify potential pathogenicity islands.

Blockade of Salmonella enteritidis passage across the basolateral barriers of human intestinal epithelial cells by specific antibody

Antibodies specific to Salmonella enteritidis (S.E.) were obtained from immunized egg yolk, and their protective effects against S.E. were studied by using monolayer-cultured human intestinal epithelial cells, Caco-2 and T84. The Salmonella adherence and entry to the cells were partially inhibited by the antibodies. The antibodies inhibited the decrease in transepithelial electrical resistance (TEER) of the intestinal epithelial monolayers and IL-8 secretion of the cells induced by S.E. invasion. Also, the antibodies blocked the penetration of bacteria through the cell layer although they did not inhibit the growth of bacteria in the cells. Confocal microscopic photographs revealed the bacteria in the infected monolayer cells were bound to antibodies. These results indicate that anti-S.E. antibodies may protect the cells from destruction induced by S.E. invasion in intestinal epithelial cells in addition to the partial inhibition of adhesion and invasion of S.E. at the cell surface. Passive antibodies against invasive bacteria would be useful to prevent the migration of S.E. to blood not only at the cell surface but also inside of intestinal epithelial cells.

Genetic analysis of assembly of the Salmonella enterica serovar Typhimurium type III secretion-associated needle complex

Several pathogenic bacteria have evolved a specialized protein secretion system termed type III to secrete and deliver effector proteins into eukaryotic host cells. Salmonella enterica serovar Typhimurium uses one such system to mediate entry into nonphagocytic cells. This system is composed of more than 20 proteins which are encoded within a pathogenicity island (SPI-1) located at centisome 63 of its chromosome. A subset of these components form a supramolecular structure, termed the needle complex, that resembles the flagellar hook-basal body complex. The needle complex is composed of a multiple-ring cylindrical base that spans the bacterial envelope and a needle-like extension that protrudes from the bacterial outer surface. Although the components of this structure have been identified, little is known about its assembly. In this study we examined the effect of loss-of-function mutations in each of the type III secretion-associated genes encoded within SPI-1 on the assembly of the needle complex. This analysis indicates that the assembly of this organelle occurs in discrete, genetically separable steps. A model for the assembly pathway of this important organelle is proposed that involves a sec-dependent step leading to the assembly of the base substructure followed by a sec-independent process resulting in the assembly of the needle portion.

Sequence analysis and distribution in Salmonella enterica serovars of IS3-like elements

The genome of Salmonella enterica serovar Enteritidis was shown to possess three IS3-like insertion elements, designated IS1230A, B and C, and each was cloned and their respective deoxynucleotide sequences determined. Mutations in elements IS1230A and B resulted in frameshifts in the open reading frames that encoded a putative transposase to be inactive. IS1230C was truncated at nucleotide 774 relative to IS1230B and therefore did not possess the 3' terminal inverted repeat. The three IS1230 derivatives were closely related to each other based on nucleotide sequence similarity. IS1230A was located adjacent to the sef operon encoding SEF14 fimbriae located at minute 97 of the genome of S. Enteritidis. IS1230B was located adjacent to the umuDC operon at minute 42.5 on the genome, itself located near to one terminus of an 815-kb genome inversion of S. Enteritidis relative to S. Typhimurium. IS1230C was located next to attB, the bacteriophage P22 attachment site, and proB, encoding gamma-glutamyl phosphate reductase. A truncated 3' remnant of IS1230, designated IS1230T, was identified in a clinical isolate of S. Typhimurium DT193 strain 2391. This element was located next to attB adjacent to which were bacteriophage P22-like sequences. Southern hybridisation of total genomic DNA from eighteen phage types of S. Enteritidis and eighteen definitive types of S. Typhimurium showed similar, if not identical, restriction fragment profiles in the respective serovars when probed with IS1230A.

Salmonella enterica serovars gallinarum and pullorum expressing Salmonella enterica serovar typhimurium type 1 fimbriae exhibit increased invasiveness for mammalian cells

Salmonella enterica serovars Gallinarum and Pullorum are S. enterica biotypes that exhibit host specificity for poultry and aquatic birds and are not normally capable of causing disease in mammalian hosts. During their evolution toward host restriction serovars Gallinarum and Pullorum lost their ability to mediate mannose-sensitive hemagglutination (MSHA), a phenotype correlated with adherence to certain cell types. Because adherence is an essential requirement for invasion of cells by bacterial pathogens, we examined whether MHSA type 1 fimbriae would increase the ability of serovars Pullorum and Gallinarum to invade normally restrictive cells. Serovars Gallinarum and Pullorum expressing S. enterica serovar Typhimurium strain LT2 type 1 fimbriae exhibited a 10- to 20-fold increased ability to adhere to and a 20- to 60-fold increased invasion efficiency of the human epithelial HEp-2 cell line. Invasion was accompanied by extensive ruffling of the membranes of the HEp-2 cells. In a murine ligated ileal loop model, a 32% increase in the number of M-cell ruffles was seen when serovar Gallinarum expressed serovar Typhimurium type 1 fimbriae.

Apoptosis of human keratinocytes after bacterial invasion

In this study, we examined the invasive capacity of Staphylococcus aureus and Salmonella typhi in human keratinocytes and monitored the number of viable intracellular bacteria at different post-infection times. The strains tested entered keratinocytes; both S. typhi and S. aureus were internalized within 30 min to 2 h after infection. No intracellular multiplication was observed, but S. typhi and S. aureus remained viable 72 h after infection. We also demonstrated that keratinocyte death following S. typhi and S. aureus invasion occurs by apoptosis as shown by DNA fragmentation. After 24 h of infection with S. typhi, the number of cells undergoing apoptosis were higher compared to infection with S. aureus. For prolonged infection times (48 h, 72 h) with both bacteria, there was no significant change in the number of cells undergoing apoptosis. The results demonstrated that viable intracellular S. typhi and S. aureus induced apoptosis in keratinocyte cells.

Short-chain fatty acids affect cell-association and invasion of HEp-2 cells by Salmonella typhimurium

This study demonstrates that the growth of S. typhimurium in Luria Bertani broth supplemented with acetate, propionate, butyrate, or a mixture of the three SCFA, affected cell-association and the ability to invade cultured HEp-2 cells. Cell-association and invasion was determined after growth for 4 h of growth in the presence of the SCFA at pH 6 and 7. The results suggest that the growth rate of the culture may have affected cell-association and invasion since accompanying the significant decrease in growth rate in the presence of SCFA at pH 6 was a decrease in cell-association and invasion. However, the results also suggest that the individual SCFA may play a role in modulating cell-association and the invasion phenotype and the regulation of cell-association and invasion by the SCFA was dependent on the concentration and the pH of the medium. Although the growth rates were similar for S. typhimurium in the SCFA mixture, butyrate (100 mM) and propionate (50 mM) at pH 6, differences in cell-association and invasion were observed among these cultures. Also, at pH 7, differences were observed among the SCFA treatments even though the growth rates were similar.

Genomic subtractive hybridization and selective capture of transcribed sequences identify a novel Salmonella typhimurium fimbrial operon and putative transcriptional regulator that are absent from the Salmonella typhi genome

Salmonella typhi, the etiologic agent of typhoid fever, is adapted to the human host and unable to infect nonprimate species. The genetic basis for host specificity in S. typhi is unknown. The avirulence of S. typhi in animal hosts may result from a lack of genes present in the broad-host-range pathogen Salmonella typhimurium. Genomic subtractive hybridization was successfully employed to isolate S. typhimurium genomic sequences which are absent from the S. typhi genome. These genomic subtracted sequences mapped to 17 regions distributed throughout the S. typhimurium chromosome. A positive cDNA selection method was then used to identify subtracted sequences which were transcribed by S. typhimurium following macrophage phagocytosis. A novel putative transcriptional regulator of the LysR family was identified as transcribed by intramacrophage S. typhimurium. This putative transcriptional regulator was absent from the genomes of the human-adapted serovars S. typhi and Salmonella paratyphi A. Mutations within this gene did not alter the level of S. typhimurium survival within macrophages or virulence within mice. A subtracted genomic fragment derived from the ferrichrome operon also hybridized to the intramacrophage cDNA. Nucleotide sequence analysis of S. typhimurium and S. typhi chromosomal sequences flanking the ferrichrome operon identified a novel S. typhimurium fimbrial operon with a high level of similarity to sequences encoding Proteus mirabilis mannose-resistant fimbriae. The novel fimbrial operon was absent from the S. typhi genome. The absence of specific genes may have allowed S. typhi to evolve as a highly invasive, systemic human pathogen.

InvF is required for expression of genes encoding proteins secreted by the SPI1 type III secretion apparatus in Salmonella typhimurium

The expression of genes encoding proteins secreted by the SPI1 (Salmonella pathogenicity island) type III secretion apparatus is known to require the transcriptional activators SirA and HilA. However, neither SirA nor HilA is believed to directly activate the promoters of these genes. invF, the first gene of the inv-spa gene cluster, is predicted to encode an AraC-type transcriptional activator and is required for invasion into cultured epithelial cells. However, the genes which are regulated by InvF have not been identified. In this work, an in-frame deletion in invF was constructed and tested for the expression of Phi(sigD-lacZYA), sipC::Tn5lacZY, and a plasmid-encoded Phi(sicA-lacZYA). SigD (Salmonella invasion gene) is a secreted protein required for the efficient invasion of Salmonella typhimurium into cultured eucaryotic cells. sicA (Salmonella invasion chaperone) is the first gene of a putative operon encoding the Sip/Ssp (Salmonella invasion/Salmonella secreted proteins) invasion proteins secreted by the SPI1 type III export apparatus. invF was required for the expression of the sigD, sicA, and sipC fusions. This is the first demonstration that there is a functional promoter in the intergenic sequence between spaS and sicA. In addition, several proteins were either absent from or found in reduced amounts in the culture supernatants of the invF mutant. Therefore, invF is required for the optimal expression of several genes encoding SPI1-secreted proteins. Genetic evidence is also presented suggesting there is HilA-dependent readthrough transcription from the invF promoter at least through sipC.

Molecular basis of the interaction of Salmonella with the intestinal mucosa

Salmonella is one of the most extensively characterized bacterial pathogens and is a leading cause of bacterial gastroenteritis. Despite this, we are only just beginning to understand at a molecular level how Salmonella interacts with its mammalian hosts to cause disease. Studies during the past decade on the genetic basis of virulence of Salmonella have significantly advanced our understanding of the molecular basis of the host-pathogen interaction, yet many questions remain. In this review, we focus on the interaction of enterocolitis-causing salmonellae with the intestinal mucosa, since this is the initiating step for most infections caused by Salmonella. Animal and in vitro cell culture models for the interaction of these bacteria with the intestinal epithelium are reviewed, along with the bacterial genes that are thought to affect this interaction. Lastly, recent studies on the response of epithelial cells to Salmonella infection and how this might promote diarrhea are discussed.

Mechanisms of Helicobacter pylori infection: bacterial factors

Since the discovery of H. pylori in 1982 (MARSHALL 1983; WARREN 1983), research on the mechanisms of virulence of H. pylori has advanced substantially. It is now well established that urease and flagella are virulence factors of H. pylori. Although known for some time to be toxic to epithelial cells in vitro, VacA has only recently been established as a virulence factor. The cag pathogenicity island has also emerged as another virulence contender, although the specific genes involved in virulence are still being determined. Other possible virulence factors, not yet confirmed by gene disruptions, are hapA, katA, sodA, cagA, and iron-regulated genes. As of yet, no adhesins have been confirmed as being important for in vivo survival of H. pylori. With the sequence of the H. pylori genome in hand, it should be possible to more easily determine the role of specific genes in virulence. Genes of immediate interest are the OMPs, which may under go phase and antigenic variation and may represent adhesins. Additionally, virulence-related orthologs and vacA-related genes may provide some interesting findings. Once we define the genes that contribute to H. pylori virulence, we may be able to more easily develop novel therapeutic drugs or vaccines to treat and prevent H. pylori infection.

Salmonella InvG forms a ring-like multimer that requires the InvH lipoprotein for outer membrane localization

Salmonella species translocate virulence effector proteins from the bacterial cytoplasm into mammalian host cells by means of a type III secretion apparatus, encoded by the pathogenicity island-1 (SPI-1). Little is known about the assembly and structure of this secretion apparatus, but the InvG protein is essential and could be an outer membrane secretion channel for the effector proteins. We observed that in recombinant Escherichia coli, the yield of InvG was enhanced by co-expression of InvH, and showed that mutation of invH decreased the level of InvG in wild-type Salmonella typhimurium. In E. coli, InvG alone was able to form an SDS-resistant multimer, but InvG localization to the outer membrane was dependent upon InvH, a lipoprotein itself located in the outer membrane, and no other SPI-1 specific protein. InvG targeted to the outer membrane by InvH became accessible to extracellular protease. InvG and InvH did not, however, appear to form a stable complex. Electron microscopy of InvG membrane protein purified from E. coli revealed that it forms an oligomeric ring-like structure with inner and outer diameters, 7 nm and 15 nm respectively.

The RcsB-RcsC regulatory system of Salmonella typhi differentially modulates the expression of invasion proteins, flagellin and Vi antigen in response to osmolarity

Entry into intestinal epithelial cells is an essential feature in the pathogenicity of Salmonella typhi, which causes typhoid fever in humans. This process requires intact motility and secretion of the invasion-promoting Sip proteins, which are targets of the type III secretion machinery encoded by the inv, spa and prg loci. During our investigations into the entry of S. typhi into cultured epithelial cells, we observed that the secretion of Sip proteins and flagellin was impaired in Vi-expressing strains. We report here that the production of Sip proteins, flagellin and Vi antigen is differentially modulated by the RcsB-RcsC regulatory system and osmolarity. This regulation occurs at both transcriptional and post-translational levels. Under low-osmolarity conditions, the transcription of iagA, invF and sipB genes is negatively controlled by the RcsB regulator, which probably acts in association with the viaB locus-encoded TviA protein. The cell surface-associated Vi polysaccharide, which was maximally produced under these growth conditions, prevented the secretion of Sip proteins and flagellin. As the NaCl concentration in the growth medium was increased, transcription of iagA, invF and sipB was found to be markedly increased, whereas transcription of genes involved in Vi antigen biosynthesis was greatly reduced. The expression of iagA, whose product is involved in invF and sipB transcription, occurred selectively during the exponential growth phase and was maximal in the presence of 300mM NaCl. At this osmolarity, large amounts of Sips and flagellin were secreted in culture supernatants. As expected from these results, and given the essential role of Sip proteins and motility in entry, RcsB and osmolarity modulated the invasive capacity of S. typhi. Together, these findings might reflect the adaptive response of S. typhi to the environments encountered during the different stages of pathogenesis.

The Salmonella typhimurium InvH protein is an outer membrane lipoprotein required for the proper localization of InvG

The secretion of pathogenicity factors by Salmonella typhimurium is mediated by a type III secretion system that includes an outer membrane protein of the secretin family. Related secretins are also required for f1 phage assembly and type II secretion. When the C-terminal 43 amino acids of the S. typhimurium secretin InvG are added to f1 pIV, the chimeric f1 pIV-'InvG43 protein becomes dependent on the co-expression of another gene, invH, for function in phage assembly. [3H]-palmitic acid labelling, globomycin sensitivity and density gradient flotation were used to demonstrate that InvH is an outer membrane lipoprotein that is processed by signal peptidase II. A complex between chimeric f1 pIV-'InvG43 and InvH was demonstrated in vivo. InvH was shown to be required for the proper localization of InvG in the outer membrane and for the secretion of the virulence factor SipC. These results suggest that InvH and InvG are part of the functional outer membrane translocation complex in type III secretion systems.

Type III protein secretion systems in bacterial pathogens of animals and plants

Various gram-negative animal and plant pathogens use a novel, sec-independent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion systems inject (translocate) proteins into the cytosol of eukaryotic cells, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and other cellular processes. Accordingly, some type III secretion systems are activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pathogenesis phenotypes. In contrast to the secreted virulence factors, most of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion apparatus show additional homologies to flagellar biosynthetic proteins, while a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones which specifically bind to individual secreted proteins play an important role in type III protein secretion, apparently by preventing premature interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizontal genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental stimuli by networks of transcription factors. This review comprises a comparison of the structure, function, regulation, and impact on host cells of the type III secretion systems in the animal pathogens Yersinia spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimurium, enteropathogenic Escherichia coli, and Chlamydia spp. and the plant pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearum, Xanthomonas campestris, and Rhizobium spp.

Cell-contact-stimulated formation of filamentous appendages by Salmonella typhimurium does not depend on the type III secretion system encoded by Salmonella pathogenicity island 1

The formation of filamentous appendages on Salmonella typhimurium has been implicated in the triggering of bacterial entry into host cells (C. C. Ginocchio, S. B. Olmsted, C. L. Wells, and J. E. Galán, Cell 76:717-724, 1994). We have examined the roles of cell contact and Salmonella pathogenicity island 1 (SPI1) in appendage formation by comparing the surface morphologies of a panel of S. typhimurium strains adherent to tissue culture inserts, to cultured epithelial cell lines, and to murine intestine. Scanning electron microscopy revealed short filamentous appendages 30 to 50 nm in diameter and up to 300 nm in length on many wild-type S. typhimurium bacteria adhering to both cultured epithelial cells and to murine Peyer's patch follicle-associated epithelia. Wild-type S. typhimurium adhering to cell-free culture inserts lacked these filamentous appendages but sometimes exhibited very short appendages which might represent a rudimentary form of the cell contact-stimulated filamentous appendages. Invasion-deficient S. typhimurium strains carrying mutations in components of SPI1 (invA, invG, sspC, and prgH) exhibited filamentous appendages similar to those on wild-type S. typhimurium when adhering to epithelial cells, demonstrating that formation of these appendages is not itself sufficient to trigger bacterial invasion. When adhering to cell-free culture inserts, an S. typhimurium invG mutant differed from its parent strain in that it lacked even the shorter surface appendages, suggesting that SPI1 may be involved in appendage formation in the absence of epithelia. Our data on S. typhimurium strains in the presence of cells provide compelling evidence that SPI1 is not an absolute requirement for the formation of the described filamentous appendages. However, appendage formation is controlled by PhoP/PhoQ since a PhoP-constitutive mutant very rarely possessed such appendages when adhering to any of the cell types examined.

Mutation of invH, but not stn, reduces Salmonella-induced enteritis in cattle

The induction of secretory and inflammatory responses in calves by Salmonella typhimurium and Salmonella dublin strains was compared, and the effects of mutations in the invH and stn genes were assessed. S. typhimurium induced greater secretory and inflammatory responses than S. dublin in bovine ileal loops, despite the fact that these serotypes were recovered from bovine ileal mucosa in comparable numbers (P. R. Watson, S. M. Paulin, A. P. Bland, P. W. Jones, and T. S. Wallis, Infect. Immun. 63:2743-2754, 1995). These results implicate serotype-specific factors other than, or in addition to, intestinal invasion in the induction of enteritis. The secretory and inflammatory responses induced by S. typhimurium and S. dublin in bovine ligated ileal loops were not significantly altered by mutation of stn, which suggests that stn does not have a major role in Salmonella-induced enteritis. The invH mutation significantly reduced the secretory and inflammatory responses induced in bovine ileal loops, and this correlated with a reduction in the severity of enteritis following oral inoculation of calves. The attenuation associated with the invH mutation did not appear to be due to an increased susceptibility to the innate host defense mechanisms, because the resistance of S. typhimurium to the bactericidal action of either bovine polymorphonuclear leukocytes or bovine serum was not significantly altered. However, lysis of macrophages following infection with S. typhimurium was significantly reduced by the invH mutation. The invH mutation prevented the normal secretion of several proteins, including SipC, by S. typhimurium, indicating that the function of the inv-spa-encoded type III protein secretion system was disrupted. Taken together, these observations implicate inv-spa-dependent effectors in mediation of Salmonella-induced enteritis in cattle. Clearly, however, other undefined serotype-specific virulence factors are also involved in Salmonella-induced enteritis.

A substrate of the centisome 63 type III protein secretion system of Salmonella typhimurium is encoded by a cryptic bacteriophage

Salmonella enterica has evolved a type III protein secretion system that allows these enteropathogens to translocate effector molecules directly into the host cell cytoplasm. These effectors mediate a variety of responses, including cytoskeletal rearrangements, cytokine production, and in certain cells, the induction of apoptosis. We report here the characterization of a substrate of this secretion system in S. enterica serovar typhimurium (Salmonella typhimurium) that is homologous to the SopE protein of Salmonella dublin implicated in bacterial entry into cultured epithelial cells. The sopE locus is located within a cluster of genes that encode tail and tail fiber proteins of a cryptic P2-like prophage, outside of the centisome 63 pathogenicity island that encodes the invasion-associated type III secretion system. Southern hybridization analysis revealed that sopE is present in only a subset of S. enterica serovars and that the flanking bacteriophage genes are also highly polymorphic. Encoding effector proteins that are delivered through type III secretion systems in highly mobile genetic elements may allow pathogens to adapt rapidly by facilitating the assembly of an appropriate set of effector proteins required for successful replication in a new environment.

Environmental regulation of Salmonella typhi invasion-defective mutants

Salmonella typhi is the etiologic agent of human typhoid. During infection, S. typhi adheres to and invades epithelial and M cells that line the distal ileum. To survive in the human host, S. typhi must overcome numerous complex extracellular and intracellular environments. Since relatively little is known about S. typhi pathogenesis, studies were initiated to identify S. typhi genes involved in the early steps of interaction with the host and to evaluate the environmental regulation of these genes. In the present study, TnphoA mutagenesis was used to study these early steps. We isolated 16 Salmonella typhi TnphoA mutants that were defective for both adherence and invasion of the human small intestinal epithelial cell line Int407. Twelve of sixteen mutations were identified in genes homologous to the S. typhimurium invG and prgH genes, which are known to be involved in the type III secretion pathway of virulence proteins. Two additional insertions were identified in genes sharing homology with the cpxA and damX genes from Escherichia coli K-12, and two uncharacterized invasion-deficient mutants were nonmotile. Gene expression of TnphoA fusions was examined in response to environmental stimuli. We found that the cpxA, invG, and prgH genes were induced when grown under conditions of high osmolarity (0.3 M NaCl). Expression of invG and prgH genes was optimal at pH 6.5 and strongly reduced at low pH (5.0). Transcription of both invG and prgH TnphoA gene fusions was initiated during the late logarithmic growth phase and was induced under anaerobic conditions. Finally, we show that both invG and prgH genes appear to be regulated by DNA supercoiling, a mechanism influenced by environmental factors. These results are the first to demonstrate that in S. typhi, (i) the prgH and cpxA genes are osmoregulated, (ii) the invG gene is induced under low oxygen conditions, (iii) the invG gene is pH regulated and growth phase dependent, and (iv) the prgH gene appears to be regulated by DNA supercoiling. Since our experimental conditions were designed to mimic the in vivo environmental milieu, our results suggest that specific environmental conditions act as signals to induce the expression of S. typhi invasion genes.

Flagellar flhA, flhB and flhE genes, organized in an operon, cluster upstream from the inv locus in Yersinia enterocolitica

The inv gene of Yersinia enterocolitica codes for invasin, a member of the invasin/intimin-like protein family, which mediates the internalization of the bacterium into cultured epithelial cells. The putative inclusion of inv into a pathogenicity island was tested by investigating its flanking sequences. Indeed, the enteropathogenic Escherichia coli (EPEC) intimin, a member of the same family of proteins, is encoded by eaeA, a gene which belongs to a pathogenicity island. An ORF located upstream from inv was of particular interest since it appeared homologous both to the flagellar flhA gene and to sepA, an EPEC gene lying inside the same pathogenicity island as eaeA. A mutant in this ORF was non-motile and non-flagellated while its invasion phenotype remained unaffected. These data indicated that the ORF corresponded to the flhA gene of Y. enterocolitica. Subsequently, the flhB and flhE genes, located respectively upstream and downstream from flhA, were identified. The three flh genes appear to be transcribed from a single operon called flhB, according to the nomenclature used for Salmonella typhimurium. Intergenic sequence between flhE and inv includes a grey hole, with no recognizable function. Downstream from inv, we have detected the flagellar flgM operon as already reported. Finally, the incongruous localization of inv amidst the flagellar cluster is discussed; while transposition could explain this phenomenon, no trace of such an event was detected.

Sequence analysis and distribution of an IS3-like insertion element isolated from Salmonella enteritidis

The nucleotide sequence of a 3 kb region immediately upstream of the sef operon of Salmonella enteritidis was determined. A 1230 base pair insertion sequence which shared sequence identity (> 75%) with members of the IS3 family was revealed. This element, designated IS1230, had almost identical (90% identity) terminal inverted repeats to Escherichia coli IS3 but unlike other IS3-like sequences lacked the two characteristic open reading frames which encode the putative transposase. S. enteritidis possessed only one copy of this insertion sequence although Southern hybridisation analysis of restriction digests of genomic DNA revealed another fragment located in a region different from the sef operon which hybridised weakly which suggested the presence of an IS1230 homologue. The distribution of IS1230 and IS1230-like elements was shown to be widespread amongst salmonellas and the patterns of restriction fragments which hybridised differed significantly between Salmonella serotypes and it is suggested that IS1230 has potential for development as a differential diagnostic tool.

The invasion-associated type-III protein secretion system in Salmonella--a review

The genetic determinants that confer upon Salmonella the ability to enter non-phagocytic cells are largely encoded in a pathogenicity island located at centisome 63 of the bacterial chromosome. Molecular genetic analysis has revealed that this region encodes a specialized protein secretion system that mediates the export and/or translocation of putative signaling proteins into the host cell. This protein secretion system, which has been termed type III or contact-dependent, has also been identified in other plant and animal pathogens that have, in common, the ability to interact with eukaryotic host cells in an intimate manner.

Molecular analysis of the genes mediating Salmonella invasion

To identify invasion determinants, a genomic library of Salmonella typhimurium was cloned into a cosmid vector, pLA2917. A clone, pSI623 which was invasive for HEp-2 and Henle-407 epithelial cells, was subcloned into a plasmid vector, pGEM-7Z to define the invasion genes. The subclone, pSV6235 containing a 4.5 kbp fragment of the Salmonella genomic region, was highly invasive for HEp-2 and Henle-407 cells, compared with other subclones whose Salmonella genomic regions are 7.4, 6.3, 5.5 and 1.4 kbp, respectively. This study reflects that the invasion efficiency of the host strain, Escherichia coli to HEp-2 and Henle-407 cell lines was significantly increased by the introduction of the genomic region of the virulent S. typhimurium. Restriction enzyme analysis showed that there was a single PstI site on the 27 kbp of Salmonella genomic region of pSI623, and no PstI site was found on the 4.5 kbp of genomic region of pSV6235. This finding suggests that the invasion related genes different from the inv and spa gene clusters which were identified in S. typhimurium, may exist in genomic DNA of S. typhimurium.

Naturally occurring deletions in the centisome 63 pathogenicity island of environmental isolates of Salmonella spp

We have identified several environmental isolates of Salmonella senftenberg and S. litchfield which carry a deletion encompassing a vast segment of the centisome 63 region of the Salmonella chromosome. The deletion includes the entire inv, spa, and hil loci, which are required for entry of Salmonella spp. into mammalian cells. Consequently, these isolates were found to be markedly deficient in the ability to enter cultured epithelial cells. In contrast, no deletions were found in the corresponding regions of the chromosomes of clinical isolates of these serovars; consequently, these isolates were found to be highly invasive for cultured epithelial cells. These data confirm the importance of the centisome 63 region of the Salmonella chromosome in mediating the entry of these organisms into cultured mammalian cells and indicate that additional entry pathways are presumably not utilized by these environmental isolates. These results are also consistent with the notion that this region constitutes a pathogenicity island which remains unstable in certain Salmonella serotypes.

Comparative genetics of the inv-spa invasion gene complex of Salmonella enterica

The chromosomal region containing the Salmonella enterica pathogenic island inv-spa was present in the last common ancestor of all the contemporary lineages of salmonellae. For multiple strains of S. enterica, representing all eight subspecies, nucleotide sequences were obtained for five genes of the inv-spa invasion complex, invH, invE, invA, spaM, and spaN, al of which encode proteins that are required for entry of the bacteria into cultured epithelial cells. The invE, invA, spaM, and spaN genes were present in all eight subspecies of S. enterica, and for invE and invA and their products, levels of sequence variation among strains were within the ranges reported for housekeeping genes. In contrast, the InvH, SpaM, and SpaN proteins were unusually variable in amino acid sequence. Furthermore, invH was absent from the subspecies V isolates examined. The SpaM and SpaN proteins provide further evidence of a relationship (first detected by Li et al. [J. Li, H. Ochman, E. A. Groisman, E. F. Boyd, F. Solomon, K. Nelson, and R. K. Selander, Proc. Natl. Acad. Sci. USA 92:7252-7256, 1995]) between the cellular location of the products of the inv-spa genes and evolutionary rate, as reflected in the level of polymorphism within S. enterica. Invasion proteins that are membrane bound or membrane associated are relatively conserved in amino acid sequence, whereas those that are exported to the extracellular environment are hypervariable, possibly reflecting the action of diversifying selection.