Identification of a Salmonella virulence gene required for formation of filamentous structures containing lysosomal membrane glycoproteins within epithelial cells

Salmonella enterica serovar Typhimurium response involved in attenuation of pathogen intracellular proliferation

Salmonella enterica serovar Typhimurium proliferates within cultured epithelial and macrophage cells. Intracellular bacterial proliferation is, however, restricted within normal fibroblast cells. To characterize this phenomenon in detail, we investigated the possibility that the pathogen itself might contribute to attenuating the intracellular growth rate. S. enterica serovar Typhimurium mutants were selected in normal rat kidney fibroblasts displaying an increased intracellular proliferation rate. These mutants harbored loss-of-function mutations in the virulence-related regulatory genes phoQ, rpoS, slyA, and spvR. Lack of a functional PhoP-PhoQ system caused the most dramatic change in the intracellular growth rate. phoP- and phoQ-null mutants exhibited an intracellular growth rate 20- to 30-fold higher than that of the wild-type strain. This result showed that the PhoP-PhoQ system exerts a master regulatory function for preventing bacterial overgrowth within fibroblasts. In addition, an overgrowing clone was isolated harboring a mutation in a previously unknown serovar Typhimurium open reading frame, named igaA for intracellular growth attenuator. Mutations in other serovar Typhimurium virulence genes, such as ompR, dam, crp, cya, mviA, spiR (ssrA), spiA, and rpoE, did not result in pathogen intracellular overgrowth. Nonetheless, lack of either SpiA or the alternate sigma factor RpoE led to a substantial decrease in intracellular bacterial viability. These results prove for the first time that specific serovar Typhimurium virulence regulators are involved in a response designed to attenuate the intracellular growth rate within a nonphagocytic host cell. This growth-attenuating response is accompanied by functions that ensure the viability of intracellular bacteria.

Transfer of the Salmonella type III effector sopE between unrelated phage families

Salmonella spp. are pathogenic enterobacteria that employ type III secretion systems to translocate effector proteins and modulate responses of host cells. The repertoire of translocated effector proteins is thought to define host specificity and epidemic virulence, and varies even between closely related Salmonella strains. Therefore, horizontal transfer of effector protein genes between Salmonella strains plays a key role in shaping the Salmonella-host interaction. Several effector protein genes are located in temperate phages. The P2-like phage SopE Phi encodes SopE and the lambda-like GIFSY phages encode several effector proteins of the YopM/IpaH-family. Lysogenic conversion with these phages is responsible for much of the diversity of the effector protein repertoires observed among Salmonella spp. However, free exchange of effector proteins by lysogenic conversion can be restricted by superinfection immunity. To identify genetic mechanisms that may further enhance horizontal transfer of effector genes, we have analyzed sopE loci from Salmonella spp. that do not harbor P2-like sequences of SopE Phi. In two novel sopE loci that were identified, the 723 nt sopE gene is located in a conserved 1.2 kb cassette present also in SopE Phi. Most strikingly, in Salmonella enterica subspecies I serovars Gallinarum, Enteritidis, Hadar and Dublin, the sopE-cassette is located in a cryptic lambda-like prophage with similarity to the GIFSY phages. This provides the first evidence for transfer of virulence genes between different phage families. We show that such a mechanism can circumvent restrictions to phage-mediated gene transfer and thereby enhances reassortment of the effector protein repertoires in Salmonella spp.

Characterization of Salmonella-induced filaments (Sifs) reveals a delayed interaction between Salmonella-containing vacuoles and late endocytic compartments

Salmonella typhimurium is a facultative intracellular pathogen that colonizes host cells throughout the course of infection. A unique feature of this pathogen is its ability to enter into (invade) epithelial cells and elongate the vacuole within which it resides into tubular structures called Salmonella-induced filaments (Sifs). In this study we sought to characterize the mechanism of Sif formation by immunofluorescence analysis using subcellular markers. The late endosomal lipid lysobisphosphatidic acid associated in a punctate pattern with the Salmonella-containing vacuole, starting 90 min after infection and increasing thereafter. Lysobisphosphatidic acid-rich vesicles were also found to interact with Sifs, at numerous sites along the tubules. Similarly, cholesterol-rich vesicles were also found in association with intracellular bacteria and Sifs. The lysosomal hydrolase cathepsin D was present in Sifs, both in a punctate pattern and, at later times, predominantly in an uninterrupted linear pattern. Rab7 associated with Sifs and expression of the N125I dominant negative mutant of this GTPase inhibited Sif formation. Transfection of HeLa cells with a vector encoding SifA fused to the green fluorescent protein caused swelling and aggregation of lysobisphosphatidic acid-containing compartments, suggesting that this virulence factor directs membrane fusion events involving late endosomes. Our findings demonstrate that Sif formation involves fusion of late endocytic compartments with the Salmonella-containing vacuole, and suggest that SifA modulates this event.

Pathogenic trickery: deception of host cell processes

Microbial pathogens cause a spectrum of diseases in humans. Although the disease mechanisms vary considerably, most pathogens have developed virulence factors that interact with host molecules, often usurping normal cellular processes, including cytoskeletal dynamics and vesicle targeting. These virulence factors often mimic host molecules, and mediate events as diverse as bacterial invasion, antiphagocytosis, and intracellular parastism.

Pathogenicity islands and the evolution of microbes

Virulence factors of pathogenic bacteria (adhesins, toxins, invasins, protein secretion systems, iron uptake systems, and others) may be encoded by particular regions of the prokaryotic genome termed pathogenicity islands. Pathogenicity islands were first described in human pathogens of the species Escherichia coli, but have recently been found in the genomes of various pathogens of humans, animals, and plants. Pathogenicity islands comprise large genomic regions [10-200 kilobases (kb) in size] that are present on the genomes of pathogenic strains but absent from the genomes of nonpathogenic members of the same or related species. The finding that the G+C content of pathogenicity islands often differs from that of the rest of the genome, the presence of direct repeats at their ends, the association of pathogenicity islands with transfer RNA genes, the presence of integrase determinants and other mobility loci, and their genetic instability argue for the generation of pathogenicity islands by horizontal gene transfer, a process that is well known to contribute to microbial evolution. In this article we review these and other aspects of pathogenicity islands and discuss the concept that they represent a subclass of genomic islands. Genomic islands are present in the majority of genomes of pathogenic as well as nonpathogenic bacteria and may encode accessory functions which have been previously spread among bacterial populations.

SifA permits survival and replication of Salmonella typhimurium in murine macrophages

SifA was originally identified as a virulence factor required for formation of Salmonella-induced filaments (Sifs), elongated tubules rich in lysosomal glycoproteins that extend from the Salmonella-containing vacuole in infected epithelial cells. Here, we demonstrate that deletion mutants of ssaR, a component of the SPI-2 type III secretion system, do not form Sifs in HeLa epithelial cells. This suggests that SifA is a translocated effector of this system, acting within host cells to form Sifs. In support of this hypothesis, transfection of HeLa cells with a vector encoding SifA fused to the green fluorescent protein caused extensive vacuolation of LAMP-1-positive compartments. Filamentous tubules that closely resembled Sifs were also observed in transfected cells, demonstrating that SifA is sufficient to initiate alteration of host cell endosomal structures. deltasifA mutants were impaired in their ability to survive/replicate in RAW 264.7 murine macrophages, a phenotype similar to ssaR mutants. Our findings suggest that SifA is an effector of the SPI-2 type III secretion system and allows colonization of murine macrophages, the host niche exploited during systemic phases of disease in these animals. A family of SifA-related proteins and their importance to Salmonella pathogenesis is also discussed.

A fluorescence microscopy based genetic screen to identify mutants altered for interactions with host cells

The study of microbial intracellular pathogenesis has benefited from the application of immunofluorescence microscopy to characterize interactions of the pathogen with host cells. Unfortunately, immunofluorescence microscopy is impractical for screening the large number of bacterial mutants necessary to represent the entire genome of the pathogen. Screening has been limited due to the lack of materials suitable for high-throughput processing (e.g. 96-well plates) that also possess the optical features needed for high resolution fluorescence microscopy. Recently marketed 96-well Special Optics (SO) plates provide both the 96-well template ideal for high-throughput analysis and optical features suitable for fluorescence microscopy. Until this work, mutants needed for the study of a fluorescence-based virulence phenotype could not be obtained by direct screening approaches. In this study, SO plates were used to examine 11520 individual Salmonella typhimurium MudJ mutants for the loss of the ability to disrupt host cell endocytic compartments. The direct application of the fluorescence phenotype for screening allowed us to obtain a set of mutants to characterize the formation of lysosomal membrane glycoprotein (lgp) containing tubules upon Salmonella infection of HeLa epithelial cells. This approach will facilitate the characterization of a wide range of microbial phenotypes detectable by fluorescence microscopy.

Aggregation of host endosomes by Salmonella requires SPI2 translocation of SseFG and involves SpvR and the fms-aroE intragenic region

Salmonella-induced aggregation of host endosomal compartments into tubules, termed lgp-tubules, requires sifA and ompR. Lgp-tubules result from Salmonella-directed alteration of the endocytic system and typify the unique intracellular locale where Salmonella replicate. A high-throughput method devised to screen 11 520 MudJ mutants for loss of lgp-tubule formation identified one auxotrophic and nine prototrophic mutants. Molecular characterization identified four new loci required to alter epithelial endocytic structure. Salmonella pathogenicity island 2 (SPI2) is the locus central to the phenotype. A subset of SPI2 effectors is essential: SpiC and SseFG are required, but not SseE. A subset of apparatus proteins is also implicated: SsaJ, L, M, V and P are required. SPI2 was implicated further, as SifA shows similarity with known SPI2 translocation targets, and OmpR regulates SPI2. Another locus lies within the smf-aroE intragenic region. Lgp-tubule formation also involves a locus on the virulence plasmid pSLT. The pSLT-encoded SpvR negatively regulates an unknown repressor of the phenotype located on pSLT. Finally, disruption of carB leads to multiple auxotrophy that prevents lgp-tubule formation. This study demonstrates that lgp-tubule formation is a virulence mechanism that underlies the selective disruption of host endocytic trafficking and is associated with the formation of a replication-permissive locale.

Salmonella maintains the integrity of its intracellular vacuole through the action of SifA

A method based on the Competitive Index was used to identify Salmonella typhimurium virulence gene interactions during systemic infections of mice. Analysis of mixed infections involving single and double mutant strains showed that OmpR, the type III secretion system of Salmonella pathogenicity island 2 (SPI-2) and SifA [required for the formation in epithelial cells of lysosomal glycoprotein (lgp)-containing structures, termed Sifs] are all involved in the same virulence function. sifA gene expression was induced after Salmonella entry into host cells and was dependent on the SPI-2 regulator ssrA. A sifA(-) mutant strain had a replication defect in macrophages, similar to that of SPI-2 and ompR(-) mutant strains. Whereas wild-type and SPI-2 mutant strains reside in vacuoles that progressively acquire lgps and the vacuolar ATPase, the majority of sifA(-) bacteria lost their vacuolar membrane and were released into the host cell cytosol. We propose that the wild-type strain, through the action of SPI-2 effectors (including SpiC), diverts the Salmonella-containing vacuole from the endocytic pathway, and subsequent recruitment and maintenance of vacuolar ATPase/lgp-containing membranes that enclose replicating bacteria is mediated by translocation of SifA.

A conserved amino acid sequence directing intracellular type III secretion by Salmonella typhimurium

Type III secretion systems (TTSS) are important virulence factors that Gram-negative bacteria use to translocate proteins into the cytoplasm of eukaryotic host cells. Salmonellae encode two virulence-associated TTSS. The Salmonella pathogenicity island 1 (SPI1)-encoded TTSS is active on contact with host cells, whereas the Salmonella pathogenicity island 2 (SPI2)-encoded TTSS is expressed after phagocytosis of bacteria by host cells. Previously, no consensus signal sequence for translocation has been identified among TTSS effector proteins. In this work, seven proteins, termed Salmonella-translocated effectors (STE), are described that contain conserved amino acid sequences that direct translocation by TTSS in Salmonella typhimurium. STE that are coordinately regulated with SPI2 gene expression contain translocation signals that are recognized by the SPI2 but not by the SPI1 TTSS. STE that are constitutively expressed contain signals that direct translocation through both SPI1 and SPI2 TTSS. Of the seven STE examined, SspH1 and SspH2 have been previously shown to be translocated and involved in virulence; SlrP and SifA were identified as virulence factors, but were not previously known to be associated with TTSS; and SseI, SseJ, and SifB were previously unidentified. Three STE genes (sspH1, sspH2, and sseI) are located within temperate bacteriophages, suggesting a common mechanism for the dissemination of more recently evolved STE.

The NRAMP proteins of Salmonella typhimurium and Escherichia coli are selective manganese transporters involved in the response to reactive oxygen

NRAMPs (natural resistance-associated macrophage proteins) have been characterized in mammals as divalent transition metal transporters involved in iron metabolism and host resistance to certain pathogens. The mechanism of pathogen resistance is proposed to involve sequestration of Fe2+ and Mn2+, cofactors of both prokaryotic and eukaryotic catalases and superoxide dismutases, not only to protect the macrophage against its own generation of reactive oxygen species, but to deny the cations to the pathogen for synthesis of its protective enzymes. NRAMP homologues are also present in bacteria. We report the cloning and characterization of the single NRAMP genes in Escherichia coli and Salmonella enterica ssp. typhimurium, and the cloning of two distinct NRAMP genes from Pseudomonas aeruginosa and an internal fragment of an NRAMP gene in Burkholderia cepacia. The genes are designated mntH because the two enterobacterial NRAMPs encode H+-stimulated, highly selective manganese(II) transport systems, accounting for all Mn2+ uptake in each species under the conditions tested. For S. typhimurium MntH, the Km for 54Mn2+ ( approximately 0.1 microM) was pH independent, but maximal uptake increased as pH decreased. Monovalent cations, osmotic strength, Mg2+ and Ca2+ did not inhibit 54Mn2+ uptake. Ni2+, Cu2+ and Zn2+ inhibited uptake with Kis greater than 100 microM, Co2+ with a Ki of 20 microM and Fe2+ with a Ki that decreased from 100 microM at pH 7. 6 to 10 microM at pH 5.5. Fe3+ and Pb2+ inhibited weakly, exhibiting Kis of 50 microM, while Cd2+ was a potent inhibitor with a Ki of about 1 microM. E. coli MntH had a similar inhibition profile, except that Kis were three- to 10-fold higher. Both S. typhimurium and E. coli MntH also transport 55Fe2+ however, the Kms are equivalent to the Kis for Fe2+ inhibition of Mn2+ uptake, and are thus too high to be physiologically relevant. In both S. typhimurium and E. coli, mntH:lacZ constructs were strongly induced by hydrogen peroxide, weakly induced by EDTA and unresponsive to paraquat, consistent with the presence of Fur and OxyR binding sites in the promoters. Strains overexpressing mntH were more susceptible to growth inhibition by Mn2+ and Cd2+ than wild type, and strains lacking a functional mntH gene were more susceptible to killing by hydrogen peroxide. In S. typhimurium strain SL1344, mntH mutants showed no defect in invasion of or survival in cultured HeLa or RAW264.7 macrophage cells; however, expression of mntH:lacZ was induced severalfold by 3 h after invasion of the macrophages. S. typhimurium mntH mutants showed only a slight attenuation of virulence in BALB/c mice. Thus, the NRAMP Mn2+ transporter MntH and Mn2+ play a role in bacterial response to reactive oxygen species and possibly have a role in pathogenesis.

OmpR regulates the two-component system SsrA-ssrB in Salmonella pathogenicity island 2

Salmonella pathogenicity island 2 (SPI-2) encodes a putative, two-component regulatory system, SsrA-SsrB, which regulates a type III secretion system needed for replication inside macrophages and systemic infection in mice. The sensor and regulator homologs, ssrAB (spiR), and genes within the secretion system, including the structural gene ssaH, are transcribed after Salmonella enters host cells. We have studied the transcriptional regulation of ssrAB and the secretion system by using gfp fusions to the ssrA and ssaH promoters. We found that early transcription of ssrA, after entry into macrophages, is most efficient in the presence of OmpR. An ompR mutant strain does not exhibit replication within cultured macrophages. Furthermore, footprint analysis shows that purified OmpR protein binds directly to the ssrA promoter region. We also show that minimal medium, pH 4.5, induces SPI-2 gene expression in wild-type but not ompR mutant strains. We conclude that the type III secretion system of SPI-2 is regulated by OmpR, which activates expression of ssrA soon after Salmonella enters the macrophage.

Salmonella pathogenicity islands: big virulence in small packages

Reflecting a complex set of interactions with its host, Salmonella spp. require multiple genes for full virulence. Many of these genes are found in 'pathogenicity islands' in the chromosome. Salmonella typhimurium possesses at least five such pathogenicity islands (SPI), which confer specific virulence traits and may have been acquired by horizontal transfer from other organisms. We highlight recent progress in characterizing these SPIs and the function of some of their genes. The role of virulence genes found on a highly conserved plasmid is also discussed. Collectively, these packages of virulence cassettes are essential for Salmonella pathogenesis.

A novel chromosomal locus of enteropathogenic Escherichia coli (EPEC), which encodes a bfpT-regulated chaperone-like protein, TrcA, involved in microcolony formation by EPEC

The bfpTVW operon, also known as the per operon, of enteropathogenic Escherichia coli (EPEC) is required for the transcriptional activation of the bfp operon, which encodes the major subunit and assembly machinery of bundle-forming pili (BFP). An immobilized T7-tagged BfpT fusion protein that binds specifically to upstream promoter sequences of bfpA and eae was used to 'fish out' from a promoter library other EPEC chromosomal fragments that are bound by the BfpT protein. After screening for promoters exhibiting bfpTVW-dependent expression, one was identified that was positively regulated by bfpTVW and that is not present in the chromosomes of two non-virulent E. coli laboratory strains, DH5alpha and HB101. Further analysis of this positively regulated promoter in EPEC showed that it resided within a 4.9 kb sequence that is not present in E. coli K12. This locus, located downstream of the potB gene, was found to contain four open reading frames (ORFs): bfpTVW-activated promoter was localized upstream of ORF1. An ORF1 knockout mutant produced less of the BFP structural subunit (BfpA) and formed smaller than normal adherent microcolonies on cultured epithelial cells; however, this mutation did not affect bfp transcription. An ORF1-His6 fusion protein specifically bound the preprocessed and mature forms of the BfpA protein and thus appears to stabilize the former within the cytoplasmic compartment. ORF1 therefore is a newly isolated EPEC chromosomal gene that encodes a chaperone-like protein involved in the production of BFP. Hence, ORF1 was designated trcA (bfpT-regulated chaperone-like protein gene). The TrcA protein also specifically bound 39 kDa and 90 kDa proteins that are expressed by EPEC but not by E. coli K12. The 90 kDa protein was revealed to be intimin, a protein product of the eae gene, which is required for the EPEC attaching/effacing phenotype, suggesting a direct interaction of TrcA with intimin in the cytoplasmic compartment.

Biogenesis of Salmonella typhimurium-containing vacuoles in epithelial cells involves interactions with the early endocytic pathway

In epithelial cells, the intracellular pathogen Salmonella typhimurium resides and replicates within a unique cytoplasmic organelle, the Salmonella-containing vacuole (SCV). In vitro studies have shown that the SCV is a dynamic organelle that selectively acquires lysosomal glycoproteins (Igps) without fusing directly with lyosomes. Here, we have investigated early events in SCV biogenesis using immunofluorescence microscopy and epitope-specific flow cytometry. We show that proteins specific to the early endocytic pathway, EEA1 and transferrin receptor (TR), are present on early SCVs. The association of these proteins with SCVs is transient, and both proteins are undetectable at later time points when Igp and vATPase are acquired. Analysis of the fraction of SCVs containing both TR and lamp-1 showed that TR is lost from SCVs as the Igp is acquired, and that these processes occur progressively and not as the result of a single fusion/fission event. These experiments reveal a novel mechanism of SCV biogenesis, involving previously undetected initial interactions with the early endocytic pathway followed by the sequential delivery of Igp. The pathway does not involve interactions with the late endosome/prelysosome and is distinct from traditional phagocytic and endocytic pathways. Our study indicates that intracellular S. typhimurium occupies a unique niche, branching away from the traditional endocytic pathway between the early and late endosomal compartments.

Inhibition of Salmonella intracellular proliferation by non-phagocytic eucaryotic cells

Salmonella typhimurium is an intracellular pathogen capable of proliferating within vacuolar compartments of non-phagocytic eucaryotic cells. This process has been shown to be essential for virulence in the mouse typhoid model (Leung and Finlay, Proc. Natl. Acad. Sci. USA, 88, 11470-11474, 1990). Here we present evidence that certain non-phagocytic eucaryotic cell lines, such as 3T3 (mouse fibroblasts) and NRK (rat fibroblasts) cells, are not permissive for S. typhimurium intracellular proliferation. Moreover, viability of intracellular bacteria residing within both cell types notably decreases at late postinfection times (72 h). These results clearly demonstrate that non-phagocytic eucaryotic cells are capable of destroying intracellular S. typhimurium. Experimentation with 3T3 and NRK cell lines might provide an appropriate in vitro model for identifying new bacterial and/or eucaryotic factors regulating Salmonella intracellular proliferation within vacuoles of the host eucaryotic cell.

Trafficking of porin-deficient Salmonella typhimurium mutants inside HeLa cells: ompR and envZ mutants are defective for the formation of Salmonella-induced filaments

Outer membrane porin genes of Salmonella typhimurium, including ompC, ompF, and tppB, are regulated by the products of ompB, a two-component regulatory locus encoding OmpR and EnvZ. S. typhimurium ompR mutants are attenuated in mice, but to date no one has studied the intracellular trafficking of S. typhimurium porin-deficient mutants. In this study, isogenic transposon mutants of S. typhimurium with insertions in ompR, envZ, ompF, ompC, ompD, osmZ, and tppB were compared with wild-type SL1344 for trafficking in the human epithelial cell line HeLa. We found that ompR and envZ mutants were reduced or completely inhibited for the formation of Salmonella-induced filaments (Sifs). This result was confirmed with an ompB deletion mutant. Sifs are tubular structures containing lysosomal glycoprotein which are induced specifically by intracellular Salmonella. Genetic analysis showed that the ompR mutation could be complemented in trans by cloned ompR to restore its ability to induce Sifs. In contrast, mutations in the known ompR-regulated genes ompF, ompC, and tppB (as well as the ompR-independent porin gene, ompD) had no effect on Sif formation relative to that of wild-type SL1344, thus indicating that OmpR does not exert its role on these genes to induce Sif formation. The omp mutants studied were able to invade and replicate in HeLa cells at levels comparable to those in wild-type SL1344. We conclude that OmpR and EnvZ appear to regulate Sif formation triggered by intracellular S. typhimurium.

Models of invasion of enteric and periodontal pathogens into epithelial cells: a comparative analysis

Bacterial invasion of epithelial cells is associated with the initiation of infection by many bacteria. To carry out this action, bacteria have developed remarkable processes and mechanisms that co-opt host cell function and stimulate their own uptake and adaptation to the environment of the host cell. Two general types of invasion processes have been observed. In one type, the pathogens (e.g., Salmonella and Yersinia spp.) remain in the vacuole in which they are internalized and replicate within the vacuole. In the other type, the organism (e.g., Actinobacillus actinomycetemcomitans, Shigella flexneri, and Listeria monocytogenes) is able to escape from the vacuole, replicate in the host cell cytoplasm, and spread to adjacent host cells. The much-studied enteropathogenic bacteria usurp primarily host cell microfilaments for entry. Those organisms which can escape from the vacuole do so by means of hemolytic factors and C type phospholipases. The cell-to-cell spread of these organisms is mediated by microfilaments. The investigation of invasion by periodontopathogens is in its infancy in comparison with that of the enteric pathogens. However, studies to date on two invasive periodontopathogens. A actinomycetemcomitans and Porphyromonas (Bacteroides) gingivalis, reveal that these bacteria have developed invasion strategies and mechanisms similar to those of the enteropathogens. Entry of A. actinomycetemcomitans is mediated by microfilaments, whereas entry of P. gingivalis is mediated by both microfilaments and microtubules. A. actinomycetemcomitans, like Shigella and Listeria, can escape from the vacuole and spread to adjacent cells. However, the spread of A. actinomycetemcomitans is linked to host cell microtubules, not microfilaments. The paradigms presented establish that bacteria which cause chronic infections, such as periodontitis, and bacteria which cause acute diseases, such as dysentery, have developed similar invasion strategies.

Use of a novel approach, termed island probing, identifies the Shigella flexneri she pathogenicity island which encodes a homolog of the immunoglobulin A protease-like family of proteins

The she gene of Shigella flexneri 2a, which also harbors the internal enterotoxin genes set1A and set1B (F. R. Noriega, GenBank accession no. U35656, 1995) encodes a homolog of the virulence-related immunoglobulin A (IgA) protease-like family of secreted proteins, Tsh, EspC, SepA, and Hap, from an avian pathogenic Escherichia coli, an enteropathogenic E. coli, S. flexneri 5, and Haemophilus influenzae, respectively. To investigate the possibility that this locus was carried on a larger deletable element, the S. flexneri 2a YSH6000T she gene was insertionally disrupted by allelic exchange using a Tn10-derived tetAR(B) cassette. Then, to detect loss of the she locus, the tetracycline-resistant derivative was plated onto fusaric acid medium to select for tetracycline-sensitive revertants, which were observed to arise at a frequency of 10(-5) to 10(-6). PCR and pulsed-field gel electrophoresis analysis confirmed loss of the she::tetAR(B) locus in six independent tetracycline-sensitive isolates. Sample sequencing over a 25-kb region flanking she identified four insertion sequence-like elements, the group II intron-like sequence Sf.IntA, and the 3' end of a second IgA protease-like homolog, sigA, lying 3.6 kb downstream and in an orientation inverted with respect to she. The deletion was mapped to chromosomal NotI fragment A and determined to have a size of 51 kb. Hybridization with flanking probes confirmed that at least 17.7 kb of the 51-kb deletable element was unique to the seven she+ strains investigated, supporting the conclusion that she lay within a large pathogenicity island. The method described in this study, termed island probing, provides a useful tool to further the study of pathogenicity islands in general. Importantly, this approach could also be of value in constructing safer live attenuated bacterial vaccines.

How Salmonella became a pathogen

In many pathogens, virulence can be conferred by a single region of the genome. In contrast, the facultative intracellular lifestyle of Salmonella demands a large number of genes distributed around the chromosome. The evolution of Salmonella has been marked by the acquisition of several 'pathogenicity islands', each contributing to the unique virulence properties of this microorganism.

The record of horizontal gene transfer in Salmonella

The evolution of virulence in Salmonella is driven by horizontal gene transfer. This has given rise to highly flexible pathogens that are able to colonize new niches and extend their host range. Tracing the record of horizontal gene transfer can provide clues to the virulence factors that contribute to the formation of new pathovars.

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.

Molecular mechanisms of bacterial virulence: type III secretion and pathogenicity islands

Recently, two novel but widespread themes have emerged in the field of bacterial virulence: type III secretion systems and pathogenicity islands. Type III secretion systems, which are found in various gram-negative organisms, are specialized for the export of virulence factors delivered directly to host cells. These factors subvert normal host cell functions in ways that seem beneficial to invading bacteria. The genes encoding several type III secretion systems reside on pathogenicity islands, which are inserted DNA segments within the chromosome that confer upon the host bacterium a variety of virulence traits, such as the ability to acquire iron and to adhere to or enter host cells. Many of these segments of DNA appear to have been acquired in a single step from a foreign source. The ability to obtain complex virulence traits in one genetic event, rather than by undergoing natural selection for many generations, provides a mechanism for sudden radical changes in bacterial-host interactions. Type III secretion systems and pathogenicity islands must have played critical roles in the evolution of known pathogens and are likely to lead to the emergence of novel infectious diseases in the future.