The Salmonella typhimurium virulence plasmid encodes a positive regulator of a plasmid-encoded virulence gene

Bidirectional Comparisons Revealed Functional Patterns in Interaction between Salmonella enterica and Plants

Plants may harbor the human pathogen Salmonella enterica. Interactions between S. enterica and different plant species have been studied in individual reports. However, disparities arising from the distinct experimental conditions may render a meaningful comparison very difficult. This study explored interaction patterns between different S. enterica strains including serovars Typhimurium 14028s and LT2 and serovar Senftenberg, and different plants (Arabidopsis, lettuce, and tomato) in one approach. Better persistence of S. enterica serovar Typhimurium strains was observed in all tested plants, whereas the resulting symptoms varied depending on plant species. Genes encoding pathogenesis-related proteins were upregulated in plants inoculated with Salmonella. Furthermore, transcriptome of tomato indicated dynamic responses to Salmonella, with strong and specific responses already 24 h after inoculation. By comparing with publicly accessible Arabidopsis and lettuce transcriptome results generated in a similar manner, constants and variables were displayed. Plants responded to Salmonella with metabolic and physiological adjustments, albeit with variability in reprogrammed orthologues. At the same time, Salmonella adapted to plant leaf-mimicking media with changes in biosynthesis of cellular components and adjusted metabolism. This study provides insights into the Salmonella-plant interaction, allowing for a direct comparison of responses and adaptations in both organisms.

Speaking the host language: how Salmonella effector proteins manipulate the host

Salmonella injects over 40 virulence factors, termed effectors, into host cells to subvert diverse host cellular processes. Of these 40 Salmonella effectors, at least 25 have been described as mediating eukaryotic-like, biochemical post-translational modifications (PTMs) of host proteins, altering the outcome of infection. The downstream changes mediated by an effector's enzymatic activity range from highly specific to multifunctional, and altogether their combined action impacts the function of an impressive array of host cellular processes, including signal transduction, membrane trafficking, and both innate and adaptive immune responses. Salmonella and related Gram-negative pathogens have been a rich resource for the discovery of unique enzymatic activities, expanding our understanding of host signalling networks, bacterial pathogenesis as well as basic biochemistry. In this review, we provide an up-to-date assessment of host manipulation mediated by the Salmonella type III secretion system injectosome, exploring the cellular effects of diverse effector activities with a particular focus on PTMs and the implications for infection outcomes. We also highlight activities and functions of numerous effectors that remain poorly characterized.

A LysR Transcriptional Regulator Manipulates Macrophage Autophagy Flux During Brucella Infection

Brucella, the intracellular bacteria, have evolved subtle strategies to efficiently survive and replicate in macrophages. However, the virulence effector proteins involved are still unclear. LysR-type transcriptional regulators (lttrs) are the largest regulator family with diverse function in prokaryotes. However, very little is known about the role of LysR regulators in the Brucella spp. Here, a BSS2_II0858 gene, encoded as one of the LysR-type regulators, was studied. We successfully constructed a BSS2_II0858 deletion mutant, Δ0858, and complementation strain CΔ0858 in Brucella suis S2. The cell apoptosis induced by B. suis S2 and its derivatives were detected by flow cytometry. The autophagy was then assessed by immunoblot analysis using the IL3I/II and p62 makers. In addition, the autophagy flux was evaluated by double fluorescent labeling method for autophagy marker protein LC3. Our studies demonstrated that B. suis S2 and its derivatives inhibited the programmed cell death in early stage and promoted apoptosis in the later stage during infection in RAW264.7 cells. The BSS2_II0858 gene was found to play no role during apoptosis according to these results. Compared with the wild-type strain, Δ0858 mutant can stimulate the conversion of LC3-I to LC3-II and markedly inhibited the autophagy flux at early stage leading to obvious autophagosome accumulation. This study explored the function of BSS2_II0858 gene and may provide new insights for understanding the mechanisms involved in the survival of Brucella in macrophages.

Detection of Virulence Plasmid-Encoded Genes in Salmonella Typhimurium and Salmonella Kentucky Isolates Recovered from Commercially Processed Chicken Carcasses

Salmonella enterica serovar Typhimurium is one of the leading causes of nontyphoidal gastroenteritis of humans in the United States. Commercially processed poultry carcasses are frequently contaminated with Salmonella serovar Kentucky in the United States. The aim of the study was to detect the Salmonella virulence plasmid containing the spv genes from Salmonella isolates recovered from commercially processed chicken carcasses. A total of 144 Salmonella isolates (Salmonella Typhimurium, n = 72 and Salmonella Kentucky, n = 72) were used for isolation of plasmids and detection of corresponding virulence genes (spvA, spvB, and spvC). Only four (5.5%) Salmonella Typhimurium isolates tested positive for all three virulence genes and hence were classified as possessing the virulence plasmid. All isolates of Salmonella Kentucky were negative for the virulence plasmid and genes. These results indicate that the virulence plasmid, which is very common among clinical isolates of Typhimurium and other Salmonella serovars (e.g., Enteritidis, Dublin, Choleraesuis, Gallinarum, Pullorum, and Abortusovis), may not be present in a significant portion of commercially processed chicken carcass isolates.

Salmonella, E. coli, and Citrobacter Type III Secretion System Effector Proteins that Alter Host Innate Immunity

Bacteria deliver virulence proteins termed 'effectors' to counteract host innate immunity. Protein-protein interactions within the host cell ultimately subvert the generation of an inflammatory response to the infecting pathogen. Here we briefly describe a subset of T3SS effectors produced by enterohemorrhagic Escherichia coli (EHEC), enteropathogenic E. coli (EPEC), Citrobacter rodentium, and Salmonella enterica that inhibit innate immune pathways. These effectors are interesting for structural and mechanistic reasons, as well as for their potential utility in being engineered to treat human autoimmune disorders associated with perturbations in NF-κB signaling.

ADP-Ribosylation, a Multifaceted Posttranslational Modification Involved in the Control of Cell Physiology in Health and Disease

Posttranslational modifications (PTMs) regulate protein functions and interactions. ADP-ribosylation is a PTM, in which ADP-ribosyltransferases use nicotinamide adenine dinucleotide (NAD⁺) to modify target proteins with ADP-ribose. This modification can occur as mono- or poly-ADP-ribosylation. The latter involves the synthesis of long ADP-ribose chains that have specific properties due to the nature of the polymer. ADP-Ribosylation is reversed by hydrolases that cleave the glycosidic bonds either between ADP-ribose units or between the protein proximal ADP-ribose and a given amino acid side chain. Here we discuss the properties of the different enzymes associated with ADP-ribosylation and the consequences of this PTM on substrates. Furthermore, the different domains that interpret either mono- or poly-ADP-ribosylation and the implications for cellular processes are described.

Salmonella enterica serovar Typhimurium skills to succeed in the host: virulence and regulation

Salmonella enterica serovar Typhimurium is a primary enteric pathogen infecting both humans and animals. Infection begins with the ingestion of contaminated food or water so that salmonellae reach the intestinal epithelium and trigger gastrointestinal disease. In some patients the infection spreads upon invasion of the intestinal epithelium, internalization within phagocytes, and subsequent dissemination. In that case, antimicrobial therapy, based on fluoroquinolones and expanded-spectrum cephalosporins as the current drugs of choice, is indicated. To accomplish the pathogenic process, the Salmonella chromosome comprises several virulence mechanisms. The most important virulence genes are those located within the so-called Salmonella pathogenicity islands (SPIs). Thus far, five SPIs have been reported to have a major contribution to pathogenesis. Nonetheless, further virulence traits, such as the pSLT virulence plasmid, adhesins, flagella, and biofilm-related proteins, also contribute to success within the host. Several regulatory mechanisms which synchronize all these elements in order to guarantee bacterial survival have been described. These mechanisms govern the transitions from the different pathogenic stages and drive the pathogen to achieve maximal efficiency inside the host. This review focuses primarily on the virulence armamentarium of this pathogen and the extremely complicated regulatory network controlling its success.

Structure and function of the LysR-type transcriptional regulator (LTTR) family proteins

The LysR family of transcriptional regulators represents the most abundant type of transcriptional regulator in the prokaryotic kingdom. Members of this family have a conserved structure with an N-terminal DNA-binding helix-turn-helix motif and a C-terminal co-inducer-binding domain. Despite considerable conservation both structurally and functionally, LysR-type transcriptional regulators (LTTRs) regulate a diverse set of genes, including those involved in virulence, metabolism, quorum sensing and motility. Numerous structural and transcriptional studies of members of the LTTR family are helping to unravel a compelling paradigm that has evolved from the original observations and conclusions that were made about this family of transcriptional regulators.

A steric antagonism of actin polymerization by a salmonella virulence protein

Salmonella spp. require the ADP-ribosyltransferase activity of the SpvB protein for intracellular growth and systemic virulence. SpvB covalently modifies actin, causing cytoskeletal disruption and apoptosis. We report here the crystal structure of the catalytic domain of SpvB, and we show by mass spectrometric analysis that SpvB modifies actin at Arg177, inhibiting its ATPase activity. We also describe two crystal structures of SpvB-modified, polymerization-deficient actin. These structures reveal that ADP-ribosylation does not lead to dramatic conformational changes in actin, suggesting a model in which this large family of toxins inhibits actin polymerization primarily through steric disruption of intrafilament contacts.

Polynucleotide phosphorylase negatively controls spv virulence gene expression in Salmonella enterica

Mutational inactivation of the cold-shock-associated exoribonuclease polynucleotide phosphorylase (PNPase; encoded by the pnp gene) in Salmonella enterica serovar Typhimurium was previously shown to enable the bacteria to cause chronic infection and to affect the bacterial replication in BALB/c mice (M. O. Clements et al., Proc. Natl. Acad. Sci. USA 99:8784-8789, 2002). Here, we report that PNPase deficiency results in increased expression of Salmonella plasmid virulence (spv) genes under in vitro growth conditions that allow induction of spv expression. Furthermore, whole-genome microarray-based transcriptome analyses of bacteria growing inside murine macrophage-like J774.A.1 cells revealed six genes as being significantly up-regulated in the PNPase-deficient background, which included spvABC, rtcB, entC, and STM2236. Mutational inactivation of the spvR regulator diminished the increased expression of spv observed in the pnp mutant background, implying that PNPase acts upstream of or at the level of SpvR. Finally, competition experiments revealed that the growth advantage of the pnp mutant in BALB/c mice was dependent on spvR as well. Combined, our results support the idea that in S. enterica PNPase, apart from being a regulator of the cold shock response, also functions in tuning the expression of virulence genes and bacterial fitness during infection.

Serological characterization and prevalence of spvR genes in Salmonella isolated from foods involved in outbreaks in Brazil

Salmonella strains (n = 75) isolated from foods involved in foodborne outbreaks occurred in Rio Grande do Sul State, Brazil, during 1999 and 2000 were studied. Strains were serotyped and submitted to PCR analysis to verify the prevalence of Salmonella plasmid virulence (spvR) regulatory gene. Among the 75 isolates, 73 (97%) were classified as Salmonella enterica serovar Enteritidis. All of the Salmonella strains isolated in 1999 were classified as serotype Enteritidis, whereas in 2000 two isolates were serotyped as Salmonella Derby and Salmonella Typhimurium. Regarding the prevalence of spvR gene, 62 strains (82.7%) were PCR positive, and a positive correlation (P < 0.05) between the strains of Salmonella Enteritidis and the presence of spvR gene was demonstrated, which suggests that this gene is a characteristic of the Salmonella Enteritidis analyzed.

Detection of pathogenic bacteria in shellfish using multiplex PCR followed by CovaLink NH microwell plate sandwich hybridization

Outbreak of diseases associated with consumption of raw shellfish especially oysters is a major concern to the seafood industry and public health agencies. A multiplex PCR amplification of targeted gene segments followed by DNA-DNA sandwich hybridization was optimized to detect the etiologic agents. First, a multiplex PCR amplification of hns, spvB, vvh, ctx and tl was developed enabling simultaneous detection of total Salmonella enterica serotype Typhimurium, Vibrio vulnificus, Vibrio cholerae and Vibrio parahaemolyticus from both pure cultures and seeded oysters. Amplicons were then subjected to a colorimetric CovaLink NH microwell plate sandwich hybridization using phosphorylated and biotinlylated oligonucleotide probes, the nucleotide sequences of which were located internal to the amplified DNA. The results from the hybridization with the multiplexed PCR amplified DNA exhibited a high signal/noise ratio ranging between 14.1 and 43.2 measured at 405 nm wavelength. The sensitivity of detection for each pathogen was 10(2) cells/g of oyster tissue homogenate. The results from this study showed that the combination of the multiplex PCR with a colorimetric microwell plate sandwich hybridization assay permits a specific, sensitive, and reproducible system for the detection of the microbial pathogens in shellfish, thereby improving the microbiological safety of shellfish to consumers.

Genes in the Salmonella pathogenicity island 2 and the Salmonella virulence plasmid are essential for Salmonella-induced apoptosis in intestinal epithelial cells

Intestinal epithelial cells are an important site of the host's interaction with enteroinvasive bacteria. Genes in the chromosomally encoded Salmonella pathogenicity island 2 (SPI 2) that encodes a type III secretion system and genes on the virulence plasmid pSDL2 of Salmonella enteritica serovar Dublin (spv genes) are thought to be important for Salmonella dublin survival in host cells. We hypothesized that genes in those loci may be important also for prolonged Salmonella growth and the induction of apoptosis induced by Salmonella in human intestinal epithelial cells. HT-29 human intestinal epithelial cells were infected with wild-type S. dublin or isogenic mutants deficient in the expression of spv genes or with SPI 2 locus mutations. Neither the spv nor the SPI 2 mutations affected bacterial entry into epithelial cells or intracellular proliferation of Salmonella during the initial 8 h after infection. However, at later periods, bacteria with mutations in the SPI 2 locus or in the spv locus compared to wild-type bacteria, manifested a marked decrease in intracellular proliferation and a different distribution pattern of bacteria within infected cells. Epithelial cell apoptosis was markedly increased in response to infection with wild-type, but not the mutant Salmonella. However, apoptosis of epithelial cells infected with wild-type S. dublin was delayed for approximately 28 h after bacterial entry. Apoptosis was preceded by caspase 3 activation, which was also delayed for approximately 24 h after infection. Despite its late onset, the cellular commitment to apoptosis was determined in the early period after infection as inhibition of bacterial protein synthesis during the first 6 h after epithelial cell infection with wild-type S. dublin, but not at later times, inhibited the induction of apoptosis. These studies indicate that genes in the SPI 2 and the spv loci are crucial for prolonged bacterial growth in intestinal epithelial cells. In addition to their influence on intracellular proliferation of Salmonella, genes in those loci determine the ultimate fate of infected epithelial cells with respect to caspase 3 activation and undergoing death by apoptosis.

Virulence plasmid-borne spvB and spvC genes can replace the 90-kilobase plasmid in conferring virulence to Salmonella enterica serovar Typhimurium in subcutaneously inoculated mice

In a mouse model of systemic infection, the spv genes carried on the Salmonella enterica serovar Typhimurium virulence plasmid increase the replication rate of salmonellae in host cells of the reticuloendothelial system, most likely within macrophages. A nonpolar deletion in the spvB gene greatly decreased virulence but could not be complemented by spvB alone. However, a low-copy-number plasmid expressing spvBC from a constitutive lacUV5 promoter did complement the spvB deletion. By examining a series of spv mutations and cloned spv sequences, we deduced that spvB and spvC could be sufficient to confer plasmid-mediated virulence to S. enterica serovar Typhimurium. The spvBC-bearing plasmid was capable of replacing all of the spv genes, as well as the entire virulence plasmid, of serovar Typhimurium for causing systemic infection in BALB/c mice after subcutaneous, but not oral, inoculation. A point mutation in the spvBC plasmid preventing translation but not transcription of spvC eliminated the ability of the plasmid to confer virulence. Therefore, it appears that both spvB and spvC encode the principal effector factors for Spv- and plasmid-mediated virulence of serovar Typhimurium.

Overlapping binding sites for the virulence gene regulators AphA, AphB and cAMP-CRP at the Vibrio cholerae tcpPH promoter

The expression of the Vibrio cholerae virulence factors, toxin-co-regulated pilus (TCP) and cholera toxin (CT), are dependent on the ability of the LysR regulator AphB to co-operate with a second protein, AphA, to activate the expression of the membrane-bound transcription factors TcpP and TcpH. To gain insights into the mechanism by which AphA and AphB co-operate to activate the expression of tcpPH, we have purified these two proteins to near homogeneity and show that they are each capable of interacting with the classical tcpPH promoter at distinct binding sites. As shown by tcpP-lacZ promoter deletion experiments, gel shift and DNase I footprinting, AphA binds to and activates from a region of the promoter between -101 and -71 from the start of transcription. AphB binds to and activates from a partially overlapping downstream site between -78 and -43, and these functions are dependent upon a region of partial dyad symmetry that resembles the well-characterized LysR-binding motif. A single basepair difference in this region of dyad symmetry has been shown previously to play a critical role in the expression of virulence genes between the two disease-causing biotypes of V. cholerae, classical and El Tor. We also show here that the tcpPH promoter is negatively influenced by the global regulator cAMP-CRP. Purified CRP binds to a near-consensus sequence in the tcpPH promoter in a cAMP-dependent manner and protects from DNase I digestion a region that is completely within the region protected by AphA and AphB. These findings raise the possibility that the negative effect of cAMP-CRP on virulence gene expression is the result of its ability to influence AphA- and AphB-dependent transcriptional activation of tcpPH under various conditions.

Induction of vap genes encoded by the virulence plasmid of Rhodococcus equi during acid tolerance response

The response of the intracellular pathogen Rhodococcus equi to acid shock, a stress potentially encountered after phagocytosis by macrophages, was analyzed. The wild-type and its avirulent plasmid-cured strain acquired increased acid tolerance during the exponential growth phase upon exposure to sublethal acid stress, a response referred to as the acid tolerance response. Maximal adaptation was observed when cells were pretreated for 90 min at pH 5.0 before exposure to the pH challenge. Search for plasmid-encoded proteins regulated by an acidic pH was performed using two-dimensional gel electrophoresis, and enabled us to detect several membrane and cytoplasmic proteins with altered expression during the adaptation phase, but none of them were plasmid-encoded. However, using a strategy based on plasmid-encoded gene expression, we showed that two operons located on the virulence plasmid of strain 85F were upregulated by acid pHs with a maximal induction at pH 5.0. One operon, containing vapA, was monocistronic whereas the other was polycistronic composed of vapD and an unknown open reading frame. Our combined results suggest that these genes may play an important role in the pathogenicity of R. equi.

Actin is ADP-ribosylated by the Salmonella enterica virulence-associated protein SpvB

The Salmonella enterica virulence-associated protein SpvB was recently shown to contain a carboxy-terminal mono(ADP-ribosyl)transferase domain. We demonstrate here that the catalytic domain of SpvB as well bacterial extracts containing full-length SpvB modifies a 43 kDa protein from macrophage-like J774-A.1 and epithelial MDCK cells as shown by label transfer from [32P]-nicotinamide adenine dinucleotide (NAD) to the 43 kDa protein. When analysed by two-dimensional gel electrophoresis, the same protein was modified in cells infected with S. enterica serovariant Dublin strain SH9325, whereas infection with an isogenic spvB mutant strain did not result in modification. Immunoprecipitation and immunoblotting experiments using SH9325-infected cells identified the modified protein as actin. The isolated catalytic domain of SpvB mediated transfer of 32P from [32P]-NAD to actins from various sources in vitro, whereas isolated eukaryotic control proteins or bacterial proteins were not modified. In an in vitro actin polymerization assay, the isolated catalytic SpvB domain prevented the conversion of G actin into F actin. Microscopic examination of MDCK cells infected with SH9325 revealed morphological changes and loss of filamentous actin content, whereas cells infected with the spvB mutant remained virtually unaffected. We conclude that actin is a target for an SpvB-mediated modification, most probably ADP-ribosylation, and that the modification of G actin interferes with actin polymerization.

Characterization of Salmonella isolates from beef cattle, broiler chickens and human sources on Prince Edward Island

Non-typhoid Salmonella serovars remain a potential threat to human health, and beef cattle and broiler chickens are possible sources of these organisms on Prince Edward Island (PEI). In this study, the ceca of beef cattle belonging to fasted and non-fasted groups, and broiler chickens were examined for Salmonella at the time of slaughter. The characteristics of the isolates, including antimicrobial resistance patterns and virulence genes, were studied along with the isolates obtained from cases of human salmonellosis on PEI during the study period (1996-97). The prevalence of Salmonella in beef cattle was 4.6% (11/240). The rate was significantly higher in fasted cattle (7.46%), than in non-fasted cattle (0.94%). The prevalence rate in chickens was 32.5% (39/120). In beef cattle, Salmonella typhimurium phage type (PT) or definitive type (DT) 104 which was resistant to ampicillin, chloramphenicol, streptomycin, sulfisoxazole and tetracycline, was the most predominant type (64%). In chickens, S. heidelberg, with resistance to gentamicin, streptomycin and sulfisoxazole, predominated. Of 26 isolates from humans, the most common serovar was S. typhimurium, including a multidrug-resistant strain of DT104. Examination by PCR revealed presence of the virulence gene invA in all serovars, and the spvC gene in all S. typhimurium isolates, of both beef cattle and human origin. Among the other serovars the latter gene was found in 7 human isolates, but in none of the chicken or beef isolates. All but 3 of the spvC-positive isolates possessed a 90 kilobasepair (kbp) plasmid suggesting that the 3 isolates had the spvC gene on their chromosome. These findings were confirmed by plasmid DNA isolation using 3 different protocols and by sequence analysis of the spvC-PCR product.

Role of the Salmonella abortusovis virulence plasmid in the infection of BALB/c mice

Following oral inoculation of BALB/c mice, Salmonella abortusovis strain SS44 was recovered in lower numbers from the Peyer's patches and mesenteric lymph nodes compared with S. typhimurium strain SL1344, whereas splenic infections were equivalent between the two serovars. SS44 was cured of its virulence plasmid or subjected to mutagenesis of the spv genes, and the Spv(-) derivatives were tested for virulence in mice. Plasmid-cured S. abortusovis SU40 retained virulence in BALB/c mice when inoculated intraperitoneally. On the other hand, mice infected orally with SU40 had greatly reduced splenic infection compared to those infected with wild-type SS44. Similar results were obtained after Tn5 insertion mutagenesis of the spvR gene or deletion of the spvABCD locus. These results suggest that in the gut-associated lymphoid tissues S. abortusovis may replicate less than S. typhimurium and that the S. abortusovis virulence plasmid primarily affects systemic infection after oral inoculation but not after intraperitoneal administration in the mouse model.

Intimate adhesion of Neisseria meningitidis to human epithelial cells is under the control of the crgA gene, a novel LysR-type transcriptional regulator

PilC1, a pilus-associated protein in Neisseria menin- gitidis, is a key element in initial meningococcal adhesion to target cells. A promoter element (CREN, contact regulatory element of Neisseria) is responsible for the transient induction of this gene upon cell contact. crgA (contact-regulated gene A) encodes a transcriptional regulator whose expression is also induced upon cell contact from a promoter region similar to the CREN of pilC1. CrgA shows significant sequence homologies to LysR-type transcriptional regulators. Its inactivation in meningococci provokes a dramatic reduction in bacterial adhesion to epithelial cells. Moreover, this mutant is unable to undergo intimate adhesion to epithelial cells or to provoke effacing of microvilli on infected cells. Purified CrgA is able to bind to pilC1 and crgA promoters, and CrgA seems to repress the expression of pilC1 and crgA. Our results support a dynamic model of bacteria-cell interaction involving a network of regulators acting in cascade. CrgA could be an intermediate regulator in such a network.

Identification of Brucella suis genes affecting intracellular survival in an in vitro human macrophage infection model by signature-tagged transposon mutagenesis

Bacteria of the genus Brucella are facultative intracellular pathogens which have developed the capacity to survive and multiply in professional and nonprofessional phagocytes. The genetic basis of this aspect of Brucella virulence is still poorly understood. To identify new virulence factors, we have adapted signature-tagged transposon mutagenesis, which has been used essentially in animal models, to an in vitro human macrophage infection model. A library of 1,152 Brucella suis 1330 tagged mini-Tn5 Km2 mutants, in 12 pools, was screened for intracellular survival and multiplication in vitamin D(3)-differentiated THP1 cells. Eighteen mutants were identified, and their attenuation was confirmed in THP1 macrophages and HeLa cells. For each avirulent mutant, a genomic fragment containing the transposon was cloned. The genomic DNA sequence flanking the transposon allowed us to assign functions to all of the inactivated genes. Transposon integration had occurred in 14 different genes, some of which were known virulence genes involved in intracellular survival or biosynthesis of smooth lipopolysaccharide (the virB operon and manB), thus validating the model. Other genes identified encoded factors involved in the regulation of gene expression and enzymes involved in biosynthetic or metabolic pathways. Possible roles in the virulence of Brucella for the different factors identified are discussed.

Contribution of Salmonella typhimurium virulence factors to diarrheal disease in calves

Limited knowledge is available about the virulence mechanisms responsible for diarrheal disease caused by Salmonella typhimurium. To assess the contribution to diarrheal disease of virulence determinants identified in models of infection, we tested a collection of S. typhimurium mutants for their ability to cause enteritis in calves. S. typhimurium strains carrying mutations in the virulence plasmid (spvR), Salmonella pathogenicity island 2 (SPI-2) (spiB), or SPI-5 (sopB) caused mortality and acute diarrhea in calves. An S. typhimurium rfaJ mutant, which is defective for lipopolysaccharide outer core biosynthesis, was of intermediate virulence. Mutations in SPI-1 (hilA and prgH) or aroA markedly reduced virulence and the severity of diarrhea. Furthermore, histopathological examination of calves infected with SPI-1 or aroA mutants revealed a marked reduction or absence of intestinal lesions. These data suggest that virulence factors, such as SPI-1, which are required during intestinal colonization are more important for pathogenicity in calves than are genes required during the systemic phase of S. typhimurium infection, including SPI-2 or the spv operon. This is in contrast to the degree of attenuation caused by these mutations in the mouse.

Regulation of the spvR gene of the Salmonella typhimurium virulence plasmid during exponential-phase growth in intracellular salts medium and at stationary phase in L broth

The authors previously showed that the SpvR-regulated spvABCD operon of the Salmonella typhimurium virulence plasmid is highly induced during exponential-phase growth by salmonellae intracellularly in mammalian cells and in a medium designed to mimic the intracellular environment of mammalian cells, intracellular salts medium (ISM), as well as at stationary phase in L broth (LB). The most relevant signal(s) for spv gene expression in vivo is not known. To elucidate the means by which salmonellae regulate the spv genes in response to the environment during the disease process, expression of the spvR gene, encoding the positive regulatory protein SpvR, was examined under these same growth conditions by using RNAse-protection analysis. spvR was expressed at a low, basal level during exponential growth in LB but was induced during exponential growth in ISM and during stationary phase in LB, the same conditions that increased expression of the spvABCD operon. Basal expression of spvR during exponential growth in LB was independent of both SpvR and the alternative sigma factor RpoS, whereas maximal induction of spvR was dependent on both SpvR and RpoS. In an RpoS- background, spvR message was decreased in stationary phase, whereas spvR exhibited residual RpoS-independent induction during exponential growth in ISM. Deletion of spvA from the virulence plasmid of S. typhimurium increased expression of spvR during stationary phase in LB, but not during exponential growth in ISM. These results suggest that expression of spvR is controlled by different regulatory factors, depending on the growth conditions encountered by the salmonellae.

Analysis of host cells associated with the Spv-mediated increased intracellular growth rate of Salmonella typhimurium in mice

The 90-kb virulence plasmid of Salmonella typhimurium encodes five spv genes which increase the growth rate of the bacteria within host cells within the first week of systemic infection of mice (P. A. Gulig and T. J. Doyle, Infect. Immun. 61:504-511, 1993). The presently described study was aimed at identifying the host cells associated with Spv-mediated virulence by manipulating the mouse host and the salmonellae. To test the effects of T cells and B cells on the Spv phenotype, salmonellae were orally inoculated into nude and SCID BALB/c mice. Relative to normal BALB/c mice, nude and SCID BALB/c mice were unaffected for splenic infection with either the Spv+ or Spv- S. typhimurium strains at 5 days postinoculation. When mice were pretreated with cyclophosphamide to induce granulocytopenia, there was a variable increase in total salmonella infection, but the relative splenic CFU of Spv+ versus Spv- S. typhimurium was not changed after oral inoculation. In contrast, depletion of macrophages from mice by treatment with cyclophosphamide plus liposomes containing dichloromethylene diphosphate resulted in equivalent virulence of Spv+ and Spv- salmonellae. To examine if the spv genes affected the growth of salmonellae in nonphagocytic cells, an invA::aphT mutation was transduced into Spv+ and Spv- S. typhimurium strains. InvA- Spv+ salmonellae were not significantly affected for splenic infection after subcutaneous inoculation compared with the wild-type strain, and InvA- Spv- salmonellae were only slightly attenuated relative to InvA+ Spv- salmonellae. Invasion-defective salmonellae still exhibited the Spv phenotype. Therefore, infection of nonphagocytes is not involved with the Spv virulence function. Taken together, these data demonstrate that macrophages are essential for suppressing the infection by Spv- S. typhimurium, by serving as the primary host cell for Spv-mediated intracellular replication and possibly by inhibiting the replication of salmonellae within other macrophages.

Homology and functional similarity of an hrp-linked pathogenicity locus, dspEF, of Erwinia amylovora and the avirulence locus avrE of Pseudomonas syringae pathovar tomato

The "disease-specific" (dsp) region next to the hrp gene cluster of Erwinia amylovora is required for pathogenicity but not for elicitation of the hypersensitive reaction. A 6.6-kb apparent operon, dspEF, was found responsible for this phenotype. The operon contains genes dspE and dspF and is positively regulated by hrpL. A BLAST search revealed similarity in the dspE gene to a partial sequence of the avrE locus of Pseudomonas syringae pathovar tomato. The entire avrE locus was sequenced. Homologs of dspE and dspF were found in juxtaposed operons and were designated avrE and avrF. Introduced on a plasmid, the dspEF locus rendered P. syringae pv. glycinea race 4 avirulent on soybean. An E. amylovora dspE mutant, however, elicited a hypersensitive reaction in soybean. The avrE locus in trans restored pathogenicity to dspE strains of E. amylovora, although restored strains were low in virulence. DspE and AvrE are large (198 kDa and 195 kDa) and hydrophilic. DspF and AvrF are small (16 kDa and 14 kDa) and acidic with predicted amphipathic alpha helices in their C termini; they resemble chaperones for virulence factors secreted by type III secretion systems of animal pathogens.

Exponential-phase expression of spvA of the Salmonella typhimurium virulence plasmid: induction in intracellular salts medium and intracellularly in mice and cultured mammalian cells

The spv genes of Salmonella typhimurium and other non-typhoidal Salmonella serovars are essential for efficient systemic infection beyond the intestines in orally inoculated mice as a model for enteric fever. These virulence genes are not significantly expressed by salmonellae during exponential growth in L broth but are induced when the bacteria enter the stationary phase of growth. Using RNase protection analysis to directly measure spvA mRNA from the virulence plasmid of S. typhimurium, we found that spvA was maximally induced in an SpvR- and RpoS-dependent manner during exponential growth in intracellular Salts Medium, which mimics the intracellular environment of mammalian cells. A cloned spvA-lacZ operon fusion in S. typhimurium was induced intracellularly in periotoneal cells of mice, correlating in vivo intracellular gene expression with intracellular function of the spv genes in infected mice. spvA was also induced intracellularly in vitro within both Henle-407 intestinal epithelial cells and J774.A1 macrophage-like cells when the bacteria were replicating with exponential kinetics. Prevention of invasion of salmonellae with cytochalasin D inhibited spvA induction within tissue culture cells, indicating that salmonellae must be internalized for spvA to be induced. The spvA-lacZ fusion was not induced by salmonellae in extracellular fluid of the peritoneal cavity or in serum. Since induction of the spv genes occurs intracellularly during exponential growth of salmonellae, cessation of growth may not be the most relevant inducing signal for spv gene expression.

Importance of the serovar-specific plasmid for virulence of salmonella strains in calves

To evaluate the influence of serovar-specific plasmids on salmonella virulence in calves, experiments were performed involving infection, by the oral route, with mixtures of strains containing equal counts of a plasmid-carrying and a plasmid-free strain of the same serovar. The concentration ratio between the plasmid-carrying and the plasmid-free strain which had developed in the organs of the infected animals was used for a comparative evaluation of virulence and pathogenetic behaviour of the strains. While in the S. typhimurium strains studied, the presence of the plasmid was accompanied by a significantly increased colonization and multiplication of the agent in the host's body, examination of S. enteritidis and S. dublin revealed that the plasmid-free strains exhibited identical or even significantly higher bacterial counts than the plasmid-carrying strains in organs. The fact that plasmid-free salmonella strains with a high virulence for calves have been found demonstrates that the presence of a serovar-specific plasmid is not an indispensable requirement for the development of salmonellosis in calves.

Mutational characterization of promoter regions recognized by the Salmonella dublin virulence plasmid regulatory protein SpvR

The virulence plasmid-encoded spv regulon is essential for virulence of Salmonella dublin in mice. The spvR gene product belongs to the LysR family of transcriptional regulator proteins. SpvR induces the expression of the spvABCD operon and positively regulates its own expression. DNase I protection analysis with purified SpvR fusion proteins identified SpvR binding sites within the spvA and spvR promoters (P. Grob and D. G. Guiney, J. Bacteriol. 178:1813-1820, 1996). We have used PCR mutagenesis, combined with functional selection for reduced SpvR affinity, to define the DNA elements essential for SpvR binding. For the spvR promoter fragment, a screen for reduced expression was also applied. Sequence analysis of the resulting mutant fragments reveals that the base pair changes are clustered in distinct regions. Determination of the apparent dissociation constants of SpvR for the mutant promoters showed that the spvA LysR-type motif and the upstream palindromic sequences of both promoters play an important role in SpvR recognition.

Expression profile and subcellular location of the plasmid-encoded virulence (Spv) proteins in wild-type Salmonella dublin

The plasmid-encoded virulence genes (spvABCD) in nontyphoid Salmonella strains mediate lethal infections in a variety of animals. Previous studies have shown that these genes are transcriptionally regulated by stationary-phase growth. We studied the expression profile and the subcellular locations of the SpvABCD proteins in wild-type S. dublin by using polyclonal antibodies against SpvA, SpvB, SpvC, and SpvD. The cellular levels of the individual proteins were determined during growth by quantitative immunoblotting. As expected, SpvA, SpvB, SpvC, and SpvD were not detectable before the late logarithmic growth phase and appeared in the sequence SpvA, SpvB, SpvC, and SpvD. In contrast to the transcriptional regulation, however, SpvA and SpvB reached their maximal expression shortly after induction and declined during further growth whereas SpvC and SpvD expression remained high throughout the stationary phase, indicating that the Spv proteins are individually regulated at a posttranscriptional level. To localize SpvABCD within the bacteria, the cells were fractionated into the periplasmic, cytoplasmic, inner membrane, and outer membrane components. The cell fractions and the culture supernatant were analyzed by immunoblotting. SpvA was present in the outer membrane, SpvB was present in the cytoplasm and the inner membrane, and SpvC was present in the cytoplasm. SpvD was secreted into the supernatant; however, a substantial portion of this protein was also detected in the cytoplasm and membranes. The molecular weights of SpvD in the supernatant and in the cytoplasm appeared to be equal, suggesting that SpvD is not cleaved upon secretion.

Common themes in microbial pathogenicity revisited

Bacterial pathogens employ a number of genetic strategies to cause infection and, occasionally, disease in their hosts. Many of these virulence factors and their regulatory elements can be divided into a smaller number of groups based on the conservation of similar mechanisms. These common themes are found throughout bacterial virulence factors. For example, there are only a few general types of toxins, despite a large number of host targets. Similarly, there are only a few conserved ways to build the bacterial pilus and nonpilus adhesins used by pathogens to adhere to host substrates. Bacterial entry into host cells (invasion) is a complex mechanism. However, several common invasion themes exist in diverse microorganisms. Similarly, once inside a host cell, pathogens have a limited number of ways to ensure their survival, whether remaining within a host vacuole or by escaping into the cytoplasm. Avoidance of the host immune defenses is key to the success of a pathogen. Several common themes again are employed, including antigenic variation, camouflage by binding host molecules, and enzymatic degradation of host immune components. Most virulence factors are found on the bacterial surface or secreted into their immediate environment, yet virulence factors operate through a relatively small number of microbial secretion systems. The expression of bacterial pathogenicity is dependent upon complex regulatory circuits. However, pathogens use only a small number of biochemical families to express distinct functional factors at the appropriate time that causes infection. Finally, virulence factors maintained on mobile genetic elements and pathogenicity islands ensure that new strains of pathogens evolve constantly. Comprehension of these common themes in microbial pathogenicity is critical to the understanding and study of bacterial virulence mechanisms and to the development of new "anti-virulence" agents, which are so desperately needed to replace antibiotics.

Role of sigma factor RpoS in initial stages of Salmonella typhimurium infection

The sigma factor RpoS mediates the stationary-phase expression of a large group of genes, including those involved in resistance to a variety of environmental stresses, such as starvation, oxidation, and low pH. In addition, RpoS has been shown to regulate Salmonella virulence. In Salmonella typhimurium, RpoS controls the expression of the Salmonella plasmid virulence (spv) genes, which are required for systemic infection. However, the mechanism by which RpoS affects the pathogenicity of Salmonella remains incompletely defined. In this study, we focused on the ability of rpoS to affect the early stages of the infection process of S. typhimurium. An rpoS mutant of S. typhimurium exhibited wild-type abilities to attach to and invade Int-407 cells and J774 macrophage-like cells. In addition, rpoS did not affect the intracellular survival of S. typhimurium in either J774 macrophage-like cells or rat bone marrow-derived macrophages. However, the rpoS mutant demonstrated a decreased ability to colonize murine Peyer's patches after oral inoculation than its wild-type virulent parent strain showed. In addition, virulence plasmid-cured derivatives of the rpoS mutant were recovered in lower numbers from murine Peyer's patches than were plasmid-cured derivatives of the isogenic wild-type S. typhimurium. This indicates that RpoS regulation of chromosomally encoded genes is important for colonization of the gut-associated lymphoid tissue (GALT) by S. typhimurium. Microscopic analysis of histological sections taken from Peyer's patches after peroral infection of mice showed that, unlike its wild-type virulent parent strain, the isogenic rpoS mutant did not destroy the follicle-associated epithelium of the GALT. Furthermore, the rpoS mutant demonstrated a decreased ability to adhere to histological sections of murine Peyer's patches than its wild-type parent showed. Our data provide evidence for a role of RpoS in the interaction of Salmonella with cells of the GALT, specifically the Peyer's patches. This implicates the involvement of rpoS in the initial stages of systemic infection by Salmonella as opposed to infection leading to gastroenteritis.

Plasmid virulence gene expression induced by short-chain fatty acids in Salmonella dublin: identification of rpoS-dependent and rpo-S-independent mechanisms

The Salmonella plasmid virulence spvABCD genes are growth phase regulated and require RpoS for maximal expression in stationary phase. We identified a growth phase-independent expression of spv which is mediated by short-chain fatty acids. During this fatty acid-mediated expression of spv, RpoS is required for induction only during exponential phase. In stationary phase, an rpoS-independent mechanism is responsible for expression of spv.

An altered rpoS allele contributes to the avirulence of Salmonella typhimurium LT2

Virulent Salmonella typhimurium strains differ from the attenuated laboratory strain LT2 at the rpoS locus. It was previously shown that the rpoS gene in strain LT2 contains a rare UUG start codon (I. S. Lee, J. Lin, H. K. Hall, B. Bearson, and J. W. Foster, Mol. Microbiol. 17:155-167, 1995). This difference is responsible for the inability of LT2 to display a sustained log-phase acid tolerance response. We show that the altered rpoS allele (rpoS(LT2)) also affects the stationary-phase acid tolerance response in Salmonella. By transducing the rpoS(LT2) allele into virulent strain backgrounds and crossing wild-type rpoS allele into strain LT2, we demonstrate that the rpoS(LT2) allele contributes to the attenuation of strain LT2. We examined the effect of the rpoS allele on invasion and found that the rpoS status of the cell had no effect on the ability of the strains to invade intestinal epithelial cells in tissue culture. Enumeration of bacteria from tissues of infected mice indicated that the presence of the rpoS(LT2) allele affected the ability of S. typhimurium to reach the liver and spleen and to persist in several tissues at 6 days postinfection. This is likely due, at least in part, to a decrease in spv gene expression in these mutants. We demonstrate that strains containing the rpoS(LT2) allele are not only sensitive to pH 3.0 (acid stress) but are also sensitive to the DNA-damaging agent methyl methanesulfonate. However, these strains appear to survive stationary-phase and oxidative stresses as well as strains containing a wild-type rpoS allele. Despite an increased sensitivity to acid stress and DNA damage, strains containing either an rpoS-null mutation or the rpoS(LT2) allele survived in J774 cells and bone marrow-derived macrophages as well as did otherwise isogenic strains with a wild-type rpoS allele.

Salmonella typhimurium cob mutants are not hyper-virulent

It was previously reported that Salmonella typhimurium LT2 cob mutants defective in the biosynthesis of vitamin B12 (cobalamin) are more virulent than the wild type in mice. Here we show that the strains used previously are non-isogenic and that the proposed increase in virulence of the cob mutant strain results from an uncharacterized mutation in the "wild type" which attenuates virulence, most likely by decreasing expression of the spv genes on the virulence plasmid. As a result the cob mutant will appear as hyper-virulent. Examination of the virulence of reconstructed wild-type and cob mutant strains showed that their growth rates were similar in mice, and we conclude that vitamin B12 does not affect the virulence of S. typhimurium LT2.

Salmonellosis: host immune responses and bacterial virulence determinants

The lifestyle of bacterial pathogens requires them to establish infection in the face of host immunity. Upon entering a potential host, a variety of interactions are initiated, the outcome of which depends upon a myriad of attributes of each of the participants. In this review we discuss the interactions that occur between pathogenic Salmonella species and the host immune systems, but when appropriate to broaden perspective, we have provided a general overview of the interactions between bacterial pathogens and animal hosts. Pathogenic Salmonella species possess an array of invasion genes that produce proteins secreted by a specialized type III secretion apparatus. These proteins are used by the bacteria to penetrate the intestinal mucosa by invading and destroying specialized epithelial M cells of the Peyer's patches. This maneuver deposits the bacteria directly within the confines of the reticuloendothelial system. The host responds to these actions with nonspecific phagocytic cells and an inflammatory response as well as by activating specific cellular and humoral immune responses. Salmonella responds to this show of force directly. It appears that the bacteria invade and establish a niche within the very cells that have been sent to destroy them. Efforts are underway to characterize the factors that allow these intracellular bacteria to customize intracellular vacuoles for their own purposes. It is the constant play between these interactions that determines the outcome of the host infection, and clearly they will also shape the evolution of new survival strategies for both the bacterium and the host.

In vitro binding of the Salmonella dublin virulence plasmid regulatory protein SpvR to the promoter regions of spvA and spvR

The spv regulon of Salmonella dublin is essential for virulence in mice. SpvR, a LysR-type regulator, induces the expression of the spvABCD operon and its own expression in the stationary phase of bacterial growth and in macrophages. We constructed fusion proteins to the maltose-binding protein (MBP) and a His tag peptide (His) to overcome the insolubility and to facilitate purification of SpvR. We demonstrated that both fusion proteins, MBP-SpvR and His-SpvR, were able to induce spvA expression in vivo. MBP-SpvR was produced as soluble protein, whereas His-SpvR was only marginally present in the soluble cell fraction. Affinity chromatography resulted in at least 95% pure MBP-SpvR protein and in an enrichment of His-SpvR. Gel mobility shift assay revealed that the SpvR fusion proteins were able to bind to 125-and 147-bp DNA fragments of the spvA and spvR promoter regions, respectively. DNase I footprint experiments showed that the fusion proteins protected DNA regions of 54 and 50 bp within the spvA and spvR promoter regions, respectively.

Central regulatory role for the RpoS sigma factor in expression of Salmonella dublin plasmid virulence genes

The plasmid virulence genes spvABCD of Salmonella spp. are regulated by SpvR and the stationary-phase sigma factor RpoS. The transcription of spv genes is induced during the post-exponential phase of bacterial growth in vitro. We sought to investigate the relationship between growth phase and RpoS in spv regulation. rpoS insertion mutations were constructed in S. dublin Lane and plasmid-cured LD842 strains, and the mutants were found to be attenuated for virulence and deficient in spv gene expression. We utilized the plasmid pBAD::rpoS to express rpoS independent of the growth phase under the control of the arabinose-inducible araBAD promoter. SpvA expression was induced within 2 h after the addition of 0.1% arabinose, even when bacteria were actively growing. This suggested that the level of RpoS, instead of the growth phase itself, controls induction of the spv genes. However, RpoS did not activate transcription of spvA in the absence of SpvR protein. Using a constitutive tet promoter to express spvR, we found that the spvA gene can be partially expressed in the rpoS mutant, suggesting that RpoS is required for SpvR synthesis. We confirmed that spvR is poorly expressed in the absence of RpoS. With an intact rpoS gene, spvR expression is not dependent on an intact spvR gene but is enhanced by spvR supplied in trans. We propose a model for Salmonella spv gene regulation in which both RpoS and SpvR are required for maximal expression at the spvR and spvA promoters.

Current perspectives in salmonellosis

Salmonellosis remains an important human and animal problem worldwide and, despite extensive research effort, many of the details of its pathogenesis are not known. While there have been recent advances in some aspects of pathogenesis, other areas are not understood. The host adaptation shown by several serotypes and the recent dramatic changes in the predominance of particular serotypes are examples. Molecular techniques using in vitro model systems have identified several genes involved in adhesion and invasion, though their function and even their relevance to disease remain poorly defined. Similarly, several potential toxins have been identified and the genes cloned, although their significance is far from clear. Some of the essential genes on the large virulence plasmids have been defined, and these are known to be necessary for the establishment of systemic infection. Two of these genes are regulatory, but the function of the other genes is unknown. A general theme has been the identification of gene systems involved in regulation of virulence. New vaccines, based on 'rational attenuation' are being designed, and these have also been used to carry heterologous antigens; such vaccines are currently undergoing trials. The improved understanding of the pathogenesis of salmonellosis may also provide a model of wide applicability to a more general understanding of bacterial pathogenesis. New techniques, including the polymerase chain reaction, are being applied to diagnose salmonellosis.

Genetic map of Salmonella typhimurium, edition VIII

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

The distribution of invA, pagC and spvC genes among Salmonella isolates from animals

New molecular diagnostic techniques often rely on hybridization or amplification of specific DNA regions to detect pathogenic bacteria. The choice of genes to be used as probes or as the targets of amplification techniques is critical to the success of these procedures. The genes so used might best be those associated with virulent isolates and having a wide distribution among such isolates. In this study three genes, invA, pagC and spvC, thought to be associated with the virulence of salmonellae, were labelled and used to probe the total DNA from 103 Salmonella isolates from animals in an attempt to determine whether these genes might be useful in diagnostic procedures. pagC was detected in 99% of the Salmonella tested, and invA was detected in 94.2% of the isolates. Both pagC and invA were detected with a significantly higher frequency than spvC in isolates from chickens and swine, but no significant difference in detection of these three genes occurred when bovine isolates were examined. Failure to detect any of these genes occurred in only one isolate. Isolates from apparently healthy or from clinically ill chickens and swine could not be distinguished by detecting these three genes. The genes were not detected in the non-Salmonella strains tested. These results suggest that, of these three genes, pagC may be the best choice for use as a probe or polymerase chain reaction target in future detection protocols.

Evidence for functional polymorphism of the spvR gene regulating virulence gene expression in Salmonella

The expression of Salmonella enterica spv virulence genes was studied in serovariants Dublin and Typhimurium using Western blotting (immunoblotting), spv-lacZ operon fusions and Northern blotting. The SpvA protein was detected in immunoblots from stationary phase cultures of Dublin but not from the corresponding cultures of Typhimurium. Transcriptional measurements, using a spvA-lacZ operon fusion, indicated 8-10 times higher spvA transcription in Dublin. In an isogenic Escherichia coli chromosomal background, virulence plasmids from various Dublin strains systematically had a significantly higher induction level of the spvA-lacZ operon fusion than virulence plasmids from Typhimurium strains. The cloned spvR transcriptional activator gene of Dublin strain 2229 was found to activate both spvR-lacZ and spvA-lacZ operon fusions, as well as to raise spv mRNA levels in E. coli TG1. In contrast, the corresponding cloned gene of Typhimurium strain SL2965 possessed a lower induction potential and required higher spvR gene dosage for activation. A comparison of the nucleotide sequences of spvR genes from two Dublin and four Typhimurium strains revealed conserved, serovariant-associated basepair substitutions. Our results indicate that the spv virulence gene cluster possesses different functional alleles of the regulator gene spvR. This finding has important consequences for comparative studies of regulation and virulence in different serovariants of Salmonella.

Role of rpoS in the regulation of Salmonella plasmid virulence (spv) genes

Salmonella plasmid virulence (spv) genes are organized into two transcriptional units: one formed by the spvR gene and the other by the spvA, spvB, spvC and spvD genes. Transcription of both units is activated by SpvR, a regulatory protein of the LysR family. The effect of RpoS, a stationary phase-associated sigma factor, on the expression of spv genes was studied using lacZ transcriptional fusions to spvR and spvA in wild-type and rpoS Escherichia coli backgrounds. Mutant and wild-type SpvR proteins were expressed in trans from a multicopy plasmid. The results show that the combined action of rpoS and spvR is necessary for transcription of spvA and that this combination also enhances transcription of spvR. Interestingly, spvR can also be transcribed in an alternative manner, i.e. in the absence of rpoS or spvR or both. The possible role for SpvR as a repressor of its own transcription is discussed.

Regulation of spvR gene expression of Salmonella virulence plasmid pKDSC50 in Salmonella choleraesuis serovar Choleraesuis

The expression regulation of spvR, a regulatory gene on the virulence plasmid (pKDSC50) of Salmonella choleraesuis serovar Choleraesuis, was investigated by spvR-lacZ translational fusion. The spvR gene was found to be positively regulated by its own product, the SpvR protein, and this unusual positive autoregulation was repressed by the products of spvA and spvB, virulence-associated genes present downstream from the spvR gene. Amino acid sequence analysis revealed that the amino-terminal region of SpvB had homology with the CatM repressor protein of Acinetobacter calcoaceticus, which belongs to the MetR/LysR protein family. On the other hand, the sigma factor RpoS was required for expression of the spvR gene in the stationary phase of bacterial growth. The SpvR protein was also necessary for self-activation, suggesting that an RNA polymerase holoenzyme containing RpoS requires SpvR protein in order to recognize the spvR promoter.

The spv virulence operon of Salmonella typhimurium LT2 is regulated negatively by the cyclic AMP (cAMP)-cAMP receptor protein system

The cyclic AMP (cAMP) receptor protein (CRP) was found to play a role in the growth phase regulation of the spv operon on the high-molecular-weight virulence plasmid of Salmonella typhimurium LT2. By using a lacZ reporter transcriptional fusion to the spvB structural gene on the single-copy virulence plasmid, it was found that while spvB transcription was induced in stationary-phase cultures, the induced level of expression was lower than that reported for the spv system in other serovars of Salmonella. Surprisingly, inactivation of the gene encoding the positive activator SpvR resulted in only a threefold reduction in spvB transcription. In contrast, spvB transcription in stationary-phase cultures was enhanced by 10-fold in mutants deficient in crp-encoded CRP or cya-encoded adenylate cyclase. Wild-type (i.e., 10-fold-lower) levels of spvB expression were restored by providing active copies of crp or cya on recombinant plasmids. Enhanced spvB transcription was not seen in crp or cya mutants in the absence of a functional spvR positive regulatory gene, showing that the cAMP-CRP system acted on spvB expression either in conjunction with or via SpvR. A lacZ transcriptional fusion to spvR could not be induced in stationary-phase cultures in the absence of functional SpvR, regardless of the cAMP-CRP status of the cells. When SpvR was provided in trans, transcription of the spvR-lacZ fusion was induced to similar levels in stationary-phase cultures with and without cAMP-CRP. These data are consistent with spvR being poorly transcribed from the single-copy virulence plasmid in S. typhimurium LT2 and with a suppression of this defect via inactivation of the cAMP-CRP system. The physiological significance of cAMP-CRP involvement in spv expression is discussed.

Pathogenicity of Salmonella enteritidis in poultry

Salmonella enteritidis is a common pathogen of all species of mammals and fowls. The recent increase in the number of outbreaks of food poisoning due to S. enteritidis in man was epidemiologically analysed, and it was considered that contaminated eggs or egg products were the major source of this infection. To assist in prevention and eradication of human food poisoning many investigators have studied the pathogenicity of S. enteritidis in poultry. Gross pathological observations after natural and experimental infections with S. enteritidis in poultry revealed that this organism may cause systemic infection in chicks and laying hens accompanied by prolonged faecal shedding. Some variations in the mortality rates, clinical symptoms, faecal shedding and frequency of production of contaminated eggs were observed in the chicks and hens experimentally infected with S. enteritidis isolates. Choice of bacterial strain, phage type, age of bird and inoculum size may affect the outcome of an infection. Moreover, isolation of the organisms from the ovaries, oviducts and egg contents indicates the possibility of transovarian infection of S. enteritidis in chickens. Some virulence factors associated with S. enteritidis are also reviewed in the present paper.

Phenotype conversion in Pseudomonas solanacearum due to spontaneous inactivation of PhcA, a putative LysR transcriptional regulator

Phenotype conversion (PC) in Pseudomonas solanacearum is the coordinated change in production of extracellular polysaccharide and a variety of extracellular proteins, some of which contribute to virulence. Although PC is normally spontaneous, it is mimicked by transposon inactivation of the phcA locus (S. M. Brumbley and T. P. Denny, J. Bacteriol. 172:5677-5685, 1990). The DNA sequence of a 1.8-kb region from strain AW1 that contains phcA revealed one open reading frame that should encode a polypeptide of 38.6 kDa. The PhcA protein produced in Escherichia coli by using a T7 RNA polymerase expression system was of the predicted size. The deduced amino acid sequence of PhcA is similar to that of some members of the LysR transcriptional activator gene family, especially in the amino terminus, where a putative helix-turn-helix DNA-binding motif was identified. An analogous allele (phcA1) was cloned from the spontaneous PC mutant strain AW1-PC and found to be nonfunctional in complementation studies. When phcA1 was expressed in E. coli, the PhcA1 protein was 35.5 kDa, 3 kDa smaller than PhcA. Sequence analysis of phcA1 and chimeric constructs of phcA and phcA1 confirmed that PhcA1 is truncated by a 2-bp insertion 147 nucleotides upstream of the carboxyl terminus of PhcA. Southern blot analysis of 10 additional independently isolated PC mutants of strain AW1 revealed that two strains have larger insertions (0.2 and 1.0 kb) within phcA. These results suggest that phcA encodes a DNA-binding protein that regulates the transcription of one or more of the genes involved in P. solanacearum virulence and that spontaneous PC can be attributed to one of several different insertions within this locus.

Signal transduction and virulence regulation in human and animal pathogens

Pathogens have developed many strategies for survival in animals and humans which possess very effective defense mechanisms. Although there are many different ways, in which pathogenic bacteria solved the problem to overcome the host defense, some common features of virulence mechanisms can be detected even in phylogenetically very distant bacteria (Finlay and Falkow (1989) Microb. Rev. 6, 1375-1383). One important feature is that the regulation of expression of virulence factors and the exact timing of their expression is very important for many of the pathogenic bacteria, as most of them have to encounter different growth situations during an infection cycle, which require a fast adaptation to the new situation by the expression of different factors. This review gives an overview about the mechanisms used by pathogenic bacteria to accomplish the difficult task of regulation of their virulence potential in response to environmental changes. In addition, the relationship of these virulence regulatory systems with other signal transduction mechanisms not involved in pathogenicity is discussed.

Molecular analysis of spv virulence genes of the Salmonella virulence plasmids

Genes on an 8 kb region common to the virulence plasmids of several serovars of Salmonella are sufficient to replace the entire plasmid in enabling systemic infection in animal models. This virulence region encompasses five genes which previously have been designated with different names from each investigating laboratory. A common nomenclature has been devised for the five genes, i.e. spv for salmonella plasmid virulence. The first gene, spvR, encodes a positive activator for the following four genes, spvABCD. DNA sequence analysis of the spv genes from Salmonella typhimurium, Salmonella dublin, and Salmonella choleraesuis demonstrated extremely high conservation of the DNA and amino acid sequences. The spv genes are induced at stationary phase and in carbon-poor media, and optimal expression is dependent on the katF locus. The virulence functions of the spv genes are not known, but these genes may increase the growth rate of salmonellae in host cells and affect the interaction of salmonellae with the host immune system.