Growth-phase regulation of plasmid virulence genes in Salmonella

Analysis of Virulence Factors and Antimicrobial Resistance in Salmonella Using Molecular Techniques and Identification of Clonal Relationships Among the Strains

A total of 50 Salmonella enterica strains were isolated from clinical samples from 2009 to 2012 and analyzed for the presence of virulence genes found in SPI-1, SPI-2, and plasmids. The distribution and frequency of the antimicrobial resistance genes and plasmids were revealed, and pulsed-field gel electrophoresis (PFGE) patterns were investigated. Five genes were identified from the seven strains with resistance or intermediate resistance to ampicillin: blaSHV-1 (present in six strains), qnrS1 (present in five strains), blaTEM-1 (present in three strains), blaCTX-M-1 (present in one strain), and qnrB1 (present in one strain). One trimethoprim-sulfamethoxazole-resistant strain was positive for sulI but negative for sulII. In addition, we detected TEM-1 and qnrS1 in one strain; SHV-1 and qnrS1 in two strains; TEM-1, SHV-1, CTX-M-1, and qnrS1 in one strain; TEM-1, SHV-1, and qnrB1 in one strain; and SHV-1 and sulI genes in one strain together. Plasmid-based replicon typing assay revealed that all 50 strains carried FIIS, 13 carried I1, 1 carried I2, 4 carried P, 1 carried A/C, and 4 carried X1 replicon. PFGE was used to type 46 of the 50 strains and classify them into 22 major groups, 33 pulsotypes, and 8 major clusters. All strains carried all the virulence genes of interest on both Salmonella Pathogenicity Islands 1 and 2 and plasmids suggested high potential for pathogenicity. All antimicrobial-resistant strains contained at least one of the resistance genes of interest, confirming a phenotype-genotype association in antimicrobial resistance.

Global transcriptional analysis of dehydrated Salmonella enterica serovar Typhimurium

Despite the scientific and industrial importance of desiccation tolerance in Salmonella, knowledge regarding its genetic basis is still scarce. In the present study, we performed a transcriptomic analysis of dehydrated and water-suspended Salmonella enterica serovar Typhimurium using microarrays. Dehydration induced expression of 90 genes and downregulated that of 7 genes. Ribosomal structural genes represented the most abundant functional group with a relatively higher transcription during dehydration. Other main induced functional groups included genes involved in amino acid metabolism, energy production, ion transport, transcription, and stress response. The highest induction was observed in the kdpFABC operon, encoding a potassium transport channel. Knockout mutations were generated in nine upregulated genes. Five mutants displayed lower tolerance to desiccation, implying the involvement of the corresponding genes in the adaptation of Salmonella to desiccation. These included genes encoding the isocitrate-lyase AceA, the lipid A biosynthesis palmitoleoyl-acyltransferase Ddg, the modular iron-sulfur cluster scaffolding protein NifU, the global regulator Fnr, and the alternative sigma factor RpoE. Notably, these proteins were previously implicated in the response of Salmonella to oxidative stress, heat shock, and cold shock. A strain with a mutation in the structural gene kdpA had a tolerance to dehydration comparable to that of the parent strain, implying that potassium transport through this system is dispensable for early adaptation to the dry environment. Nevertheless, this mutant was significantly impaired in long-term persistence during cold storage. Our findings indicate the involvement of a relatively small fraction of the Salmonella genome in transcriptional adjustment from water to dehydration, with a high prevalence of genes belonging to the protein biosynthesis machinery.

Adaptations of Candida albicans for growth in the mammalian intestinal tract

Although the fungus Candida albicans is a commensal colonizer of humans, the organism is also an important opportunistic pathogen. Most infections caused by C. albicans arise from organisms that were previously colonizing the host as commensals, and therefore successful establishment of colonization is a prerequisite for pathogenicity. To elucidate fungal activities that promote colonization, an analysis of the transcription profile of C. albicans cells recovered from the intestinal tracts of mice was performed. The results showed that within the C. albicans colonizing population, cells expressed genes characteristic of the laboratory-grown exponential phase and genes characteristic of post-exponential-phase cells. Thus, gene expression both promoted the ability to grow rapidly (a characteristic of exponential-phase cells) and enhanced the ability to resist stresses (a characteristic of post-exponential-phase cells). Similarities in gene expression in commensal colonizing cells and cells invading host tissue during disease were found, showing that C. albicans cells adopt a particular cell surface when growing within a host in both situations. In addition, transcription factors Cph2p and Tec1p were shown to regulate C. albicans gene expression during intestinal colonization.

Phage WO of Wolbachia: lambda of the endosymbiont world

The discovery of an extraordinarily high level of mobile elements in the genome of Wolbachia, a widespread arthropod and nematode endosymbiont, suggests that this bacterium could be an excellent model for assessing the evolution and function of mobile DNA in specialized bacteria. In this paper, we discuss how studies on the temperate bacteriophage WO of Wolbachia have revealed unexpected levels of genomic flux and are challenging previously held views about the clonality of obligate intracellular bacteria. We also discuss the roles this phage might play in the Wolbachia-arthropod symbiosis and infer how this research can be translated to combating human diseases vectored by arthropods. We expect that this temperate phage will be a preeminent model system to understand phage genetics, evolution and ecology in obligate intracellular bacteria. In this sense, phage WO might be likened to phage lambda of the endosymbiont world.

Leucine-responsive regulatory protein (Lrp) acts as a virulence repressor in Salmonella enterica serovar Typhimurium

Leucine-responsive regulatory protein (Lrp) is a global gene regulator that influences expression of a large number of genes including virulence-related genes in Escherichia coli and Salmonella. No systematic studies examining the regulation of virulence genes by Lrp have been reported in Salmonella. We report here that constitutive expression of Lrp [lrp(Con)] dramatically attenuates Salmonella virulence while an lrp deletion (Deltalrp) mutation enhances virulence. The lrp(Con) mutant caused pleiotropic effects that include defects in invasion, cytotoxicity, and colonization, whereas the Deltalrp mutant was more proficient at these activities than the wild-type strain. We present evidence that Lrp represses transcription of key virulence regulator genes--hilA, invF, and ssrA--in Salmonella pathogenicity island 1 (SPI-1) and 2 (SPI-2), by binding directly to their promoter regions, P(hilA), P(invF), and P(ssrA). In addition, Western blot analysis showed that the expression of the SPI-1 effector SipA was reduced in the lrp(Con) mutant and enhanced in the Deltalrp mutant. Computational analysis revealed putative Lrp-binding consensus DNA motifs located in P(hilA), P(invF), and P(ssrA). These results suggest that Lrp binds to the consensus motifs and modulates expression of the linked genes. The presence of leucine enhanced Lrp binding to P(invF) in vitro and the addition of leucine to growth medium decreased the level of invF transcription. However, leucine had no effect on expression of hilA and ssrA or on cellular levels of Lrp. In addition, Lrp appears to be an antivirulence gene, since the deletion mutant showed enhanced cell invasion, cytotoxicity, and hypervirulence in BALB/c mice.

Self-regulation of Candida albicans population size during GI colonization

Interactions between colonizing commensal microorganisms and their hosts play important roles in health and disease. The opportunistic fungal pathogen Candida albicans is a common component of human intestinal flora. To gain insight into C. albicans colonization, genes expressed by fungi grown within a host were studied. The EFH1 gene, encoding a putative transcription factor, was highly expressed during growth of C. albicans in the intestinal tract. Counterintuitively, an efh1 null mutant exhibited increased colonization of the murine intestinal tract, a model of commensal colonization, whereas an EFH1 overexpressing strain exhibited reduced colonization of the intestinal tract and of the oral cavity of athymic mice, the latter situation modeling human mucosal candidiasis. When inoculated into the bloodstream of mice, both efh1 null and EFH1 overexpressing strains caused lethal infections. In contrast, other mutants are attenuated in virulence following intravenous inoculation but exhibited normal levels of intestinal colonization. Finally, although expression of several genes is dependent on transcription factor Efg1p during laboratory growth, Efg1p-independent expression of these genes was observed during growth within the murine intestinal tract. These results show that expression of EFH1 regulated the level of colonizing fungi, favoring commensalism as opposed to candidiasis. Also, different genes are required in different host niches and the pathway(s) that regulates gene expression during host colonization can differ from well-characterized pathways used during laboratory growth.

Although the fungus Candida albicans commonly colonizes the human gastrointestinal tract as a commensal, the organism is also an opportunistic pathogen, responsible for a wide range of infections in immunocompromised persons. While numerous studies of infection have been conducted, few studies have analyzed the commensal state. The studies described here analyze C. albicans cells colonizing the intestinal tract of immunocompetent mice in the absence of disease, a model for commensalism. Results showed that expression of the putative transcription factor Efh1p by cells colonizing the intestinal tract was relatively high, but paradoxically, expression of Efh1p was associated with lower colonization. Efh1p had no detectable effect on the ability of C. albicans to cause lethal disseminated infection in mice. In contrast, Rbt1p and Rbt4p, two proteins of poorly defined function required for normal disseminated infection, were not required for intestinal colonization. These results argue that the commensal state is distinct from the pathogenic state and that different factors are important in different states. Also, the regulation of expression of genes RBT1, RBT4, and ECE1 during intestinal colonization differed from their well-characterized regulation during laboratory growth. Further studies of commensal colonization are needed to understand this important stage of C. albicans biology.

Comparison of growth phase on Salmonella enterica serovar Typhimurium invasion in an epithelial cell line (IPEC J2) and mucosal explants from porcine small intestine

Salmonella Typhimurium DT104 is a zoonotic enteropathogen of increasing concern for human health. In this study, the influence of growth phase on invasiveness of a S. Typhimurium DT104 field isolate and two reference strains (SL1344 and ATCC 14028) was compared in IPEC J2 cells and mucosal explants from porcine ileum. Internalized bacteria were quantified by a gentamicin resistance assay. After 90 min of exposure to the apical aspect of epithelial monolayers or luminal surface of explants, internalization of all S. Typhimurium strains in mid-logarithmic phase of bacterial growth was comparable. Internalization of stationary phase bacteria was reduced relative to log phase bacteria, with DT104 exhibiting the greatest decrease. Growth phase-related differences in S. Typhimurium invasion are similar in porcine intestinal epithelial cells and mucosal explants, but may be greater in multidrug-resistant strains.

A cell-permeable fusion toxin as a tool to study the consequences of actin-ADP-ribosylation caused by the salmonella enterica virulence factor SpvB in intact cells

The virulence factor SpvB is a crucial component for the intracellular growth and infection process of Salmonella enterica. The SpvB protein mediates the ADP-ribosylation of actin in infected cells and is assumed to be delivered directly from the engulfed bacteria into the host cell cytosol. Here we used the binary Clostridium botulinum C2 toxin as a transport system for the catalytic domain of SpvB (C/SpvB) into the host cell cytosol. A recombinant fusion toxin composed of the enzymatically inactive N-terminal domain of C. botulinum C2 toxin (C2IN) and C/SpvB was cloned, expressed, and characterized in vitro and in intact cells. When added together with C2II, the C2IN-C/SpvB fusion toxin was efficiently delivered into the host cell cytosol and ADP-ribosylated actin in various cell lines. The cellular uptake of the fusion toxin requires translocation from acidic endosomes into the cytosol and is facilitated by Hsp90. The N- and C-terminal domains of SpvB are linked by 7 proline residues. To elucidate the function of this proline region, fusion toxins containing none, 5, 7, and 9 proline residues were constructed and analyzed. The existence of the proline residues was essential for the translocation of the fusion toxins into host cell cytosol and thereby determined their cytopathic efficiency. No differences concerning the mode of action of the C2IN-C/SpvB fusion toxin and the C2 toxin were obvious as both toxins induced depolymerization of actin filaments, resulting in cell rounding. The acute cellular responses following ADP-ribosylation of actin did not immediately induce cell death of J774.A1 macrophage-like cells.

Towards more virulent and antibiotic-resistantSalmonella?

Salmonella are well-known pathogens. Virulence determinants can be present on the chromosome, usually encoded on pathogenicity islands, or on plasmids and bacteriophages. Antibiotic resistance determinants usually are encoded on plasmids, but can also be present on the multidrug resistance region of Salmonella Genomic Island 1 (SGI1). Virulence plasmids show a remarkable diversity in the combination of virulence factors they encode, which appears to adapt them to specific hosts and the ability to cause gastroenteritidis or systemic disease. The appearance of plasmids with two replicons may help to extend the host range of these plasmids and thereby increase the virulence of previously non- or low pathogenic serovars. Antibiotic resistance among Salmonella is also increasing. This increase is not only in the percentage isolates resistant to a particular antibiotic, but also the development of resistance against newer antibiotics. The increased occurrence of integrons is particularly worrying. Integrons can harbour a varying set of antibiotic resistance encoding gene cassettes. Gene cassettes can be exchanged between integrons. Although the gene cassettes currently present in Salmonella integrons encode for older antibiotics (however, some still frequently used) gene cassettes encoding resistance against the newest antibiotics has been documented in Enterobacteriaceae. Furthermore, beta-lactamases with activity against broad-spectrum cephalosporins, which are often used in empiric therapy, have been found associated with integrons. So, empiric treatment of Salmonella infections becomes increasingly more difficult. The most worrisome finding is that virulence and resistance plasmids form cointegrates. These newly formed plasmids can be selected by antibiotic pressure and thereby for virulence factors. Taken together these trends may lead to more virulent and antibiotic-resistant Salmonella.

Salmonella enterica serotype Choleraesuis: epidemiology, pathogenesis, clinical disease, and treatment

Nontyphoid Salmonella strains are important causes of reportable food-borne infection. Among more than 2,000 serotypes, Salmonella enterica serotype Choleraesuis shows the highest predilection to cause systemic infections in humans. The most feared complication of serotype Cholearesuis bacteremia in adults is the development of mycotic aneurysm, which previously was almost uniformally fatal. The advances in diagnostic techniques, surgical care, and antimicrobial therapy have greatly improved the survival of these patients. However, the recent emergence of serotype Choleraesuis that is resistant to ampicillin, chloramphenicol, trimethoprim-sulfamethoxazole, and, notably, fluoroquinolone antibiotics has aroused concern about the use of these agents for the empirical treatment of systemic infection caused by this organism. In view of the serious implications of the situation, the chain of transmission and mechanism of resistance should be carefully studied to reduce the spread of infection and threat to human health. To date, there are no vaccines available to prevent serotype Choleraesuis infections in humans. The availability, in the near future, of the genome sequence of serotype Cholearesuis will facilitate the development of effective vaccines as well as the discovery of new targets for novel antimicrobial agents.

Neuromodulation of enteropathogen internalization in Peyer's patches from porcine jejunum

Jejunal Peyer's patches (JPP) are innervated sites of immune induction and enteropathogen infection. We investigated the role of enteric nerves in modulating pathogen entry into porcine JPP. Presumptive norepinephrine (NE)-containing nerve fibers were localized in JPP domes and follicle-associated villi by secondary immunofluorescence histochemistry. NE or the neuronal conduction blocker saxitoxin increased intracellular internalization of pathogenic Salmonella choleraesuis and Escherichia coli O157:H7, but not nonpathogenic E. coli, into isolated JPP mucosa. NE action was prevented by the alpha-adrenergic antagonist phentolamine. Withdrawal of enteric neural activity or NE administration appears to modulate JPP interactions with pathogenic bacteria.

Extracellular secretion of the virulence plasmid-encoded ADP-ribosyltransferase SpvB in Salmonella

Nontyphoid Salmonella enterica requires the plasmid-encoded spv genes to establish successful systemic infection in experimental animals. The SpvB virulence-associated protein has recently been shown to contain the ADP-ribosyltransferase domain. SpvB ADP-ribosilates actin and depolymerizes actin filaments when expressed in cultured epithelial cells. However, spontaneous secretion or release of SpvB has not been observed under in vitro growth conditions. In the present study we investigated the secretion of SpvB from Salmonella using in vitro and in vivo assay systems. We showed that SpvB is secreted into supernatant from Salmonella strains that contain the cloned spvB gene on a plasmid when they grew in intracellular salts medium (ISM), a minimal medium mimicing the intracellular iron concentrations of eukaryotic cells. A series of mutant SpvB proteins revealed that an N-terminal region of SpvB located at amino acids 1-229 was sufficient to promote secretion into extracellular milieu. Confocal immunofluorescence microscopy also demonstrated efficient localization of the N-terminal domain of SpvB(1-360) tagged with biotinylated peptide within infected host cell cytosol but not truncated SpvB(1-179) fusion protein. In addition, mutations that inactivate genes within Salmonella pathogenicity island 1 or Salmonella pathogenicity island 2 that encode type III secretion systems (TTSS) could secrete the SpvB protein into the culture medium. These results indicate that SpvB protein is transported from the bacteria and into the host cytoplasm independent of TTSS.

The alternative sigma factor sigma(E) plays an important role in intestinal survival and virulence in Vibrio cholerae

The alternative sigma factor sigma(E) (RpoE) is involved in the response to extracytoplasmic stress and plays a role in the virulence of a variety of different bacteria. To assess the role of sigma(E) in Vibrio cholerae pathogenesis, a DeltarpoE mutant was constructed and analyzed using the infant mouse model. The results here show that sigma(E) contributes significantly to the virulence of V. cholerae. The DeltarpoE mutant was highly attenuated with a 50% lethal dose more than 3 logs higher than that for the parental strain, and its ability to colonize the intestine was reduced approximately 30-fold. A time course of infection revealed that the number of CFU of the DeltarpoE mutant was approximately 1 log lower than that of the parental strain by 12 h postinoculation and decreased further by 24 h. The defect in virulence in the DeltarpoE mutant thus appears to be a diminished ability to survive within the intestinal environment. The results here also show that sigma(E) is not required for growth and survival of V. cholerae in vitro at high temperatures but is required under other stressful conditions, such as in the presence of 3% ethanol. As in Escherichia coli, the expression of rpoE in V. cholerae is dependent upon two promoters located upstream of the gene, P1 and P2. P1 appears to be sigma(70) dependent, whereas the downstream promoter, P2, is positively autoregulated by sigma(E).

A two-component regulator induces the transmission phenotype of stationary-phase Legionella pneumophila

Pathogenic Legionella pneumophila evolved as a parasite of aquatic amoebae. To persist in the environment, the microbe must be proficient at both replication and transmission. In laboratory cultures, as nutrients become scarce a stringent response-like pathway coordinates exit from the exponential growth phase with induction of traits correlated with virulence, including motility. A screen for mutants that express the flagellin gene poorly identified five activators of virulence: LetA/LetS, a two-component regulator homologous to GacA/GacS of Pseudomonas and SirA/BarA of Salmonella; the stationary-phase sigma factor RpoS; the flagellar sigma factor FliA; and a new locus, letE. Unlike wild type, post-exponential-phase letA and letS mutants were not motile, cytotoxic, sodium sensitive or proficient at infecting macrophages. L. pneumophila also required fliA to become motile, cytotoxic and to infect macrophages efficiently and letE to express sodium sensitivity and maximal motility and cytotoxicity. When induced to express RelA, all of the strains exited the exponential phase, but only wild type converted to the fully virulent form. In contrast, intracellular replication was independent of letA, letS, letE or fliA. Together, the data indicate that, as the nutrient supply wanes, ppGpp triggers a regulatory cascade mediated by LetA/ LetS, RpoS, FliA and letE that coordinates differentiation of replicating L. pneumophila to a transmissible form.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

Virulence gene regulation in Salmonella enterica

In order to infect a host, a microbe must be equipped with special properties known as virulence factors. Bacterial virulence factors are required to facilitate colonization, to survive under host defenses, and to permit multiplication inside the host. However, the possession of genes encoding virulence factors does not guarantee effective infection. There is considerable evidence that tight regulation of a given virulence factor is as important as the possession of the virulence factors themselves. Thus, an understanding of the regulation of virulence expression is fundamental to our comprehension of any infection process and can identify potential targets for disease prevention and therapy. We have summarized the lessons learned from experimental salmonellosis in terms of virulence regulation and hope to illustrate the differing requirements for gene and virulence expression.

Complete DNA sequence and comparative analysis of the 50-kilobase virulence plasmid of Salmonella enterica serovar Choleraesuis

The complete nucleotide sequence of pKDSC50, a large virulence plasmid from Salmonella enterica serovar Choleraesuis strain RF-1, has been determined. We identified 48 of the open reading frames (ORFs) encoded by the 49,503-bp molecule. pKDSC50 encodes a known virulence-associated operon, the spv operon, which is composed of genes essential for systemic infection by nontyphoidal Salmonella. Analysis of the genetic organization of pKDSC50 suggests that the plasmid is composed of several virulence-associated genes, which include the spvRABCD genes, plasmid replication and maintenance genes, and one insertion sequence element. A second virulence-associated region including the pef (plasmid-encoded fimbria) operon and rck (resistance to complement killing) gene, which has been identified on the virulence plasmid of S. enterica serovar Typhimurium, was absent. Two different replicon regions, similar to the RepFIIA and RepFIB replicons, were found. Both showed high similarity to those of the pO157 plasmid of enterohemorrhagic Escherichia coli O157:H7 and the enteropathogenic E. coli (EPEC) adherence factor plasmid harbored by EPEC strain B171 (O111:NM), as well as the virulence plasmids of Salmonella serovars Typhimurium and Enteritidis. Comparative analysis of the nucleotide sequences of the 50-kb virulence plasmid of serovar Choleraesuis and the 94-kb virulence plasmid of serovar Typhimurium revealed that 47 out of 48 ORFs of the virulence plasmid of serovar Choleraesuis are highly homologous to the corresponding ORFs of the virulence plasmid of serovar Typhimurium, suggesting a common ancestry.

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.

Invasiveness in chickens, stress resistance and RpoS status of wild-type Salmonella enterica subsp. enterica serovar typhimurium definitive type 104 and serovar enteritidis phage type 4 strains

The heat and acid resistance and the ability to survive airdrying on commonly used kitchen surfaces were assessed for clinical and environmental strains of Salmonella enterica subsp. enterica serovar Typhimurium, definitive type (DT) 104. Three out of thirty-eight strains of DT 104 were found to be more sensitive in stationary phase to the stresses examined than the other strains. This compares to a previous study by the authors which showed that seven out of forty serovar Enteritidis phage type (PT) 4 strains were more sensitive. RpoS activity was examined indirectly in selected strains of DT 104 and PT 4. In those with normal stress resistance a 100-fold induction of an RpoS-dependent spvR/A:'::luxCDABE fusion was observed upon entry into stationary phase. The sensitive strains examined showed either no induction or a reduced level of spvR/A:'::luxCDABE expression. The rpoS gene was sequenced from these strains and three were found to harbour mutations including one deletion, one base-pair substitution resulting in a nonsense codon, and one insertion causing a frameshift resulting in an early stop codon. Strains with negligible or reduced spvR/A:'::luxCDABE expression had low stress resistance. All strains of DT 104 could be recovered from liver and spleen tissues of infected hens 14 d post-infection, but one with no induction of spvR/A:'::luxCDABE expression was significantly less likely to be recovered from chicken reproductive tissues, liver or spleen than the majority of other strains, including one with reduced spvR/A:'::luxCDABE expression. This work has demonstrated that clinical and environmental strains of DT 104 and PT 4 not infrequently harbour mutations in the rpoS allele. It is possible that the rpoS mutations may have occurred during the initial isolation of the strains. The ability of a strain to cause infection, however, also depends on factors such as host susceptibility and dose.

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.

The virulence plasmid of Salmonella typhimurium contains an autoregulated gene, rlgA, that codes for a resolvase-like DNA binding protein

The virulence plasmid of Salmonella typhimurium contains a gene, rlgA, that shows strong homology to several reported resolvase-like proteins. This gene maps 5 kb upstream of spv locus, the major virulence determinant on the plasmid. Regulation of rlgA was studied using a lacZ transcriptional reporter fusion. The rlgA gene was found to be repressed at the level of transcription by its own product and to be expressed maximally in the late exponential phase of growth. The transcription start site of the rlgA gene was determined and the RlgA binding site was mapped and found to overlap with the transcription initiation signals. A derivative of the virulence plasmid was constructed with a knockout mutation in rlgA. This mutation did not alter the stability of the virulence plasmid nor did it affect the ability of S. typhimurium to cause systemic disease in mice.

Unravelling the mysteries of virulence gene regulation in Salmonella typhimurium

Salmonella typhimurium, which causes gastroenteritis in calves and humans as well as a typhoid-like disease in mice, uses numerous virulence factors to infect its hosts. Genes encoding these factors are regulated by many environmental conditions and regulatory pathways in vitro. Many virulence genes are specifically induced at particular sites during infection or in cultured host cells. The complex regulation of virulence genes observed in vitro may be necessary to restrict their expression to specific locations within the host. In vitro and in vivo studies provide clues about how virulence genes might be regulated in vivo. Future studies must assess the actual environmental signals and regulators that modulate each virulence gene in vivo and determine how multiple regulatory pathways are integrated to co-ordinate the appropriate expression of virulence factors at specific sites in vivo.

DNA topology and adaptation of Salmonella typhimurium to an intracellular environment

The expression of genes coding for determinants of DNA topology in the facultative intracellular pathogen Salmonella typhimurium was studied during adaptation by the bacteria to the intracellular environment of J774A.1 macrophage-like cells. A reporter plasmid was used to monitor changes in DNA supercoiling during intracellular growth. Induction of the dps and spv genes, previously shown to be induced in the macrophage, was detected, as was expression of genes coding for DNA gyrase, integration host factor and the nucleoid-associated protein H-NS. The topA gene, coding for the DNA relaxing enzyme topoisomerase I, was not induced. Reporter plasmid data showed that bacterial DNA became relaxed following uptake of S. typhimurium cells by the macrophage. These data indicate that DNA topology in S. typhimurium undergoes significant changes during adaptation to the intracellular environment. A model describing how this process may operate is discussed.

Granulocyte selected live Salmonella enteritidis vaccine is species specific

Selection of Salmonellae from granulocytes may result in safe and effective vaccine strains. We demonstrated that the reduced virulence of such strains is limited to the species in which the selection was made. Repeated (sequential) selection of Salmonella enteritidis from chicken granulocytes yielded an avirulent strain for chickens. Repeated (sequential) selection of Salmonella enteritidis from pig granulocytes (neutrophils) yielded a strain that was comparable to the original wildtype strain.

Legionella pneumophila pathogesesis: a fateful journey from amoebae to macrophages

Legionella pneumophila first commanded attention in 1976, when investigators from the Centers for Disease Control and Prevention identified it as the culprit in a massive outbreak of pneumonia that struck individuals attending an American Legion convention (). It is now clear that this gram-negative bacterium flourishes naturally in fresh water as a parasite of amoebae, but it can also replicate within alveolar macrophages. L. pneumophila pathogenesis is discussed using the following model as a framework. When ingested by phagocytes, stationary-phase L. pneumophila bacteria establish phagosomes which are completely isolated from the endosomal pathway but are surrounded by endoplasmic reticulum. Within this protected vacuole, L. pneumophila converts to a replicative form that is acid tolerant but no longer expresses several virulence traits, including factors that block membrane fusion. As a consequence, the pathogen vacuoles merge with lysosomes, which provide a nutrient-rich replication niche. Once the amino acid supply is depleted, progeny accumulate the second messenger guanosine 3',5'-bispyrophosphate (ppGpp), which coordinates entry into the stationary phase with expression of traits that promote transmission to a new phagocyte. A number of factors contribute to L. pneumophila virulence, including type II and type IV secretion systems, a pore-forming toxin, type IV pili, flagella, and numerous other factors currently under investigation. Because of its resemblance to certain aspects of Mycobacterium, Toxoplasma, Leishmania, and Coxiella pathogenesis, a detailed description of the mechanism used by L. pneumophila to manipulate and exploit phagocyte membrane traffic may suggest novel strategies for treating a variety of infectious diseases. Knowledge of L. pneumophila ecology may also inform efforts to combat the emergence of new opportunistic macrophage pathogens.

A role for the leucine-responsive regulatory protein and integration host factor in the regulation of the Salmonella plasmid virulence (spv ) locus in Salmonella typhimurium

The Salmonella plasmid virulence (spv ) genes of Salmonella typhimurium are activated at the level of transcription as the bacteria enter stationary phase in vitro or in response to signals received during intracellular growth. Activation requires the LysR-like transcription factor SpvR and the alternative sigma factor RpoS. In this report, we show by biochemical and genetic analyses that two chromosomally encoded DNA-binding proteins contribute to the control of spv expression. These are the integration host factor (IHF), which binds to DNA sequences upstream of the spvR regulatory gene, and the leucine-responsive regulatory protein (Lrp), which binds to sequences upstream of the spvABCD operon. Under all conditions tested, inactivation of IHF expression reduces the level of spvR transcription by twofold. It also alters the response of the spv regulon to loss of DNA gyrase activity, consistent with a role for IHF in organizing DNA structure in the vicinity of the spvR promoter. Lrp represses spvA gene expression by up to fivefold and Lrp-mediated repression is antagonized by leucine. The Lrp binding site upstream of the spvA gene overlaps one of the binding sites for the positive regulator SpvR, suggesting a mechanism by which Lrp repression is exerted. This is a first demonstration of a role for Lrp in controlling genes that are also subject to intracellular regulation. These data show that the spv virulence genes belong simultaneously to several regulons in the cell, raising the possibility that spv expression can be fine-tuned in response to multiple environmental inputs.

Co-ordination of legionella pneumophila virulence with entry into stationary phase by ppGpp

Legionella pneumophila survives in aquatic environments, but replicates within amoebae or the alveolar macrophages of immunocompromised individuals. Here, the signal transduction pathway that co-ordinates L. pneumophila virulence expression in response to amino acid depletion was investigated. To facilitate kinetic and genetic studies, a phenotypic reporter of virulence was engineered by fusing flaA promoter sequences to a gene encoding green fluorescent protein. When subjected to amino acid depletion, L. pneumophila accumulated ppGpp and converted from a replicative to a virulent state, as judged by motility and sodium sensitivity. ppGpp appeared to initiate this response, as L. pneumophila induced to express the Escherichia coli RelA ppGpp synthetase independently of nutrient depletion accumulated ppGpp, exited the exponential growth phase and expressed flaAgfp, motility, sodium sensitivity, cytotoxicity and infectivity, five traits correlated with virulence. Although coincident with the stationary phase, L. pneumophila virulence expression appeared to require an additional factor: mutant Lp120 accumulated ppGpp and acquired two stationary phase traits but none of six virulence phenotypes analysed. We propose that, when nutrients are limiting, ppGpp acts as an alarmone, triggering the expression of multiple traits that enable L. pneumophila to escape its spent host, to survive and disperse in the environment and to re-establish a protected intracellular replication niche.

Role of the 85-kilobase plasmid and plasmid-encoded virulence-associated protein A in intracellular survival and virulence of Rhodococcus equi

Rhodococcus equi is a facultative intracellular pathogen of macrophages and a cause of pneumonia in young horses (foals) and immunocompromised people. Isolates of R. equi from pneumonic foals typically contain large, 85- or 90-kb plasmids encoding a highly immunogenic virulence-associated protein (VapA). The objective of this study was to determine the role of the 85-kb plasmid and VapA in the intracellular survival and virulence of R. equi. Clinical isolates containing the plasmid and expressing VapA efficiently replicated within mouse macrophages in vitro, while plasmid-cured derivatives of these organisms did not multiply intracellularly. An isolate harboring the large plasmid also replicated in the tissues of experimentally infected mice, whereas its plasmid-cured derivative was rapidly cleared. All foals experimentally infected with a plasmid-containing clinical isolate developed severe bronchopneumonia, whereas the foals infected with its plasmid-cured derivative remained asymptomatic and free of visible lung lesions. By day 14 postinfection, lung bacterial burdens had increased considerably in foals challenged with the plasmid-containing clinical isolate. In contrast, bacteria could no longer be cultured from the lungs of foals challenged with the isogenic plasmid-cured derivative. A recombinant, plasmid-cured derivative expressing wild-type levels of VapA failed to replicate in macrophages and remained avirulent for both mice and foals. These results show that the 85-kb plasmid of R. equi is essential for intracellular replication within macrophages and for development of disease in the native host, the foal. However, expression of VapA alone is not sufficient to restore the virulence phenotype.

An homologue of the human 100-kDa protein (p100) is differentially expressed by Histoplasma capsulatum during infection of murine macrophages

Using differential display reverse transcription-PCR (DDRT-PCR) we have identified several sequences that are specifically expressed by Histoplasma capsulatum during infection of murine macrophages (MPhi). Here, we report the characterization of a clone, pHc12, identified as a differentially expressed gene 1 hour after infection of MPhi. Screening of a cDNA library of H. capsulatum allowed us to isolate a clone, pHc12-E, that contains the complete coding sequence. We show that after infection the level of transcription of this gene increases about 5 fold. Analysis of its sequence revealed the presence of an open reading frame of 890 aa (ORF890) that shares respectively 30 and 33% identity with human and Caenorhabditis elegans p100 kD and rat p105 kD co-activator proteins. Using the two-dimensional Hydrophobic Cluster Analysis (HCA) method, we showed that H. capsulatum ORF890 and p100 kD co-activator proteins are clearly related. The H. capsulatum protein consists of a four-fold repeated module (domains I to IV) like the p100 kD co-activator proteins, whose three-dimensional (3D) structure is related to staphylococcal thermonuclease, followed by a modified fifth "hybrid" domain which partially resembles the structure of the tudor domain found in multiple copies in the Drosophila melanogaster tudor protein. These data strongly suggest that ORF890 is homologous to human p100 kD and that this protein, named Hcp100, may play an essential role during infection by co-activating the expression of specific genes.

Environmental growth conditions influence the ability of Escherichia coli K1 to invade brain microvascular endothelial cells and confer serum resistance

A major limitation to advances in prevention and therapy of neonatal meningitis is our incomplete understanding of the pathogenesis of this disease. In an effort to understand the pathogenesis of meningitis due to Escherichia coli K1, we examined whether environmental growth conditions similar to those that the bacteria might be exposed to in the blood could influence the ability of E. coli K1 to invade brain microvascular endothelial cells (BMEC) in vitro and to cross the blood-brain barrier in vivo. We found that the following bacterial growth conditions enhanced E. coli K1 invasion of BMEC 3- to 10-fold: microaerophilic growth, media buffered at pH 6.5, and media supplemented with 50% newborn bovine serum (NBS), magnesium, or iron. Growth conditions that significantly repressed invasion (i.e., 2- to 250-fold) included iron chelation, a pH of 8.5, and high osmolarity. More importantly, E. coli K1 traversal of the blood-brain barrier was significantly greater for the growth condition enhancing BMEC invasion (50% NBS) than for the condition repressing invasion (osmolarity) in newborn rats with experimental hematogenous meningitis. Of interest, bacterial growth conditions that enhanced or repressed invasion also elicited similar serum resistance phenotype patterns. This is the first demonstration that bacterial ability to enter the central nervous system can be affected by environmental growth conditions.

In vivo analysis of the interactions of the LysR-like regulator SpvR with the operator sequences of the spvA and spvR virulence genes of Salmonella typhimurium

The interaction of the Salmonella typhimurium virulence gene regulator, SpvR, with its operator sites upstream of the spvA and spvR genes was analysed in vivo by dimethyl sulphate (DMS) footprinting and site-directed mutagenesis. DMS methylation protection assays showed that, in vivo, SpvR forms direct protein-DNA contacts with nucleotides clustered in two regions (+1 to -27 and -51 to -71) of the spvA regulatory region. These regions were subjected to site-directed mutagenesis and the effects on SpvR binding and gene activation assessed. Mutations that prevented occupancy of the promoter distal site (-51 to -71) in vivo also prevented occupancy of the promoter proximal site (+1 to -27), whereas mutations in the proximal site affected binding only at the proximal site and not the distal site. SpvR binding at the promoter proximal site was an essential prerequisite for transcription activation. These findings demonstrated a hierarchy of SpvR binding in which the promoter distal site is dominant to the proximal. The spvR gene was found to possess an operator site that resembled closely the distal SpvR binding site of the spvA operator. Nonetheless, SpvR interaction with the spvR operator was difficult to detect in vivo. When the nucleotide sequence of the spvR operator was altered at two nucleotides so that it corresponded more precisely to that of the distal site of the spvA operator, strong SpvR-DNA interactions were detected, with nucleotides in the region -31 to -67 being protected from DMS methylation in vivo. However, despite the improved interaction with the transcriptional activator, the altered regulatory region was poorer at promoting spvR gene transcription than the wild type. We describe a two-step model for activation of the spvA promoter and discuss the possibility that a specific cofactor in addition to sigma factor RpoS is required for SpvR action at this promoter in vivo.

MglA and MglB are required for the intramacrophage growth of Francisella novicida

Francisella novicida is a facultative intracellular pathogen capable of growing in macrophages. A spontaneous mutant of F. novicida defective for growth in macrophages was isolated on LB media containing the chromogenic phosphatase substrate 5-bromo-4-chloro-3-indolyl phosphate (X-p) and designated GB2. Using an in cis complementation strategy, four strains were isolated that are restored for growth in macrophages. A locus isolated from one of these strains complements GB2 for both the intracellular growth defect and the colony morphology on LB (X-p) media. The locus consists of an apparent operon of two genes, designated mgIAB, for Macrophage Growth Locus. Both mglA and mglB transposon insertion mutants are defective for intracellular growth and have a phenotype similar to GB2 or LB (X-p) media. Sequencing on mglA cloned from GB2 identified a missense mutation, providing evidence that both mglA and mglB are required for the intramacrophage growth of F. novicida. mglB expression in GB2 was confirmed using antiserum against recombinant MglB. Cell fractionation studies revealed several differences in the protein profiles of mgI mutants compared with wild-type F. novicida. The deduced amino acid sequences of mglA and mglB show similarity to the SspA and SspB proteins of Escherichia coli and Haemophilus spp. In E. coli, SspA and/or SspB influence the levels of multiple proteins under conditions of nutritional stress, and SspA can associate with the RNA polymerase holoenzyme. Taken together, these observations suggest that in Francisella MglA and MglB may affect the expression of genes whose products contribute to survival and growth within macrophages.

Expression of Legionella pneumophila virulence traits in response to growth conditions

In nature, Legionella pneumophila replicates exclusively as an intracellular parasite of amoebae, but it also persists in the environment as a free-living microbe. Studies of how this opportunistic pathogen recognizes and responds to distinct extracellular and intracellular environments identified a link between the growth phase and expression of traits previously correlated with virulence. When cultured in broth, only post-exponential-phase L. pneumophila was sodium sensitive, cytotoxic, osmotically resistant, competent to evade macrophage lysosomes, infectious, and motile. Likewise, the L. pneumophila phenotype changed during growth in macrophages. During the intracellular replication period, this bacterium was sodium resistant and lacked flagella; concomitant with macrophage lysis, L. pneumophila became sodium sensitive and flagellated. Expression of the virulent phenotype was a response to starvation, since exponential-phase L. pneumophila became cytotoxic, sodium sensitive, and motile after incubation in broth from stationary-phase cultures, except when it was supplemented with amino acids. Together, these data indicate that while nutrients are plentiful, intracellular L. pneumophila organisms are dedicated to replication; when amino acids become limiting, the progeny express virulence factors to escape the spent host, to disperse and survive in the aquatic environment, and to reestablish a protected intracellular niche favorable for growth.

sigA is an essential gene in Mycobacterium smegmatis

sigA encodes a sigma factor of the sigma70 family, sigmaA, that is found in all mycobacterial species. As sigmaA shows high similarity to the primary sigma factor in Streptomyces coelicolor, it was postulated that sigmaA has the same role in mycobacteria. However, a point mutation in sigA, resulting in the replacement of arginine 522 by histidine, was found responsible for the attenuated virulence of the Mycobacterium bovis strain ATCC 35721. This raised the possibility that sigmaA was an alternative sigma factor specifically required for virulence gene expression. In this work, we show that sigA can not be disrupted in Mycobacterium smegmatis unless an extra copy of the gene is provided at another chromosomal site, which demonstrates that sigA is essential. To characterize the pattern of sigA expression during exponential and stationary phase in M. smegmatis, we measured the beta-galactosidase activity in a strain carrying a sigA-lacZ transcriptional fusion and monitored sigmaA levels using Western blotting. Our results indicate that sigA is expressed throughout the growth of the culture. The essential character of sigA and its pattern of expression corroborate the hypothesis that sigA codes for the primary sigma factor in M. smegmatis and, most likely, in all mycobacteria.

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.

Drug targeting by polyalkylcyanoacrylate nanoparticles is not efficient against persistent Salmonella

PURPOSE

We have investigated the efficacy of colistin and ciprofloxacin, free or bound to polyalkylcyanoacrylate nanoparticles, for the targeting and eradication of Salmonella persisting in the organs of the mononuclear phagocyte system.

METHODS

A model of persistent S. typhimurium infection was developed in C57BL/6 mice using i.v. inoculation of the plasmid-cured strain C53.

RESULTS

In vivo and ex vivo experiments showed that the persisting bacteria seem to evolve to a nongrowing state during experimental salmonellosis. In vivo treatment with free or nanoparticle-bound colistin did not significantly reduce the number of viable Salmonella C53, either in the liver or the spleen of infected mice. In contrast, in vivo treatment with ciprofloxacin led to a significant decrease of bacterial counts in the liver whatever the stage of infection and the form used. However, none of the treatments were able to sterilize the spleen or the liver. In ex vivo experiments, colistin was only active against bacteria recovered during the early phase of infection, whereas ciprofloxacin exerted its activity at all times postinfection.

CONCLUSIONS

We suggest that the micro-environment in which the bacterial cells persist in vivo probably causes dramatic changes in their susceptibility to antimicrobial agents.

Salmonella typhimurium encodes an SdiA homolog, a putative quorum sensor of the LuxR family, that regulates genes on the virulence plasmid

Quorum sensing is a phenomenon in which bacteria sense and respond to their own population density by releasing and sensing pheromones. In gram-negative bacteria, quorum sensing is often performed by the LuxR family of transcriptional regulators, which affect phenotypes as diverse as conjugation, bioluminescence, and virulence gene expression. The gene encoding one LuxR family member, named sdiA (suppressor of cell division inhibition), is present in the Escherichia coli genome. In this report, we have cloned the Salmonella typhimurium homolog of SdiA and performed a systematic screen for sdiA-regulated genes. A 4.4-kb fragment encoding the S. typhimurium sdiA gene was sequenced and found to encode the 3' end of YecC (homologous to amino acid transporters of the ABC family), all of SdiA and SirA (Salmonella invasion regulator), and the 5' end of UvrC. This gene organization is conserved between E. coli and S. typhimurium. We determined that the S. typhimurium sdiA gene was able to weakly complement the E. coli sdiA gene for activation of ftsQAZ at promoter 2 and for suppression of filamentation caused by an ftsZ(Ts) allele. To better understand the function of sdiA in S. typhimurium, we screened 10,000 random lacZY transcriptional fusions (MudJ transposon mutations) for regulation by sdiA. Ten positively regulated fusions were isolated. Seven of the fusions were within an apparent operon containing ORF8, ORF9, rck (resistance to complement killing), and ORF11 of the S. typhimurium virulence plasmid. The three ORFs have now been named srgA, srgB, and srgC (for sdiA-regulated gene), respectively. The DNA sequence adjacent to the remaining three fusions shared no similarity with previously described genes.

Evidence for expressional crosstalk between the central virulence regulator PrfA and the stress response mediator ClpC in Listeria monocytogenes

Virulence is a multifactorial trait which depends on the coordinated expression of many bacterial products, hence it is to be expected that the regulatory circuits that control the relevant genetic determinants are somehow interconnected. Two pleiotropic regulatory elements acting at different levels, the transcription factor PrfA which controls virulence gene expression and the potential chaperone ClpC which is involved in tolerance to environmental stress, are required for Listeria monocytogenes survival within the host. We analyzed the influence of PrfA on clpC expression in L. monocytogenes. clpC transcription is maximal under heat-shock conditions, i.e. at 42 degrees C, and is very weak or undetectable at 37 degrees C. In a prfA* mutant which constitutively overexpresses PrfA and PrfA-dependent virulence genes, clpC transcription dropped to basal levels during exponential growth at 42 degrees C. This repression was not observed during stationary phase, indicating growth phase-dependent regulation of clpC. Culture in charcoal-treated medium, which triggers in wild-type strains the transcriptional activation of the PrfA regulon, also caused a strong downregulation of clpC. Moreover, in a prfA deletion mutant, clpC transcription during exponential growth at 37 degrees C was clearly enhanced, reaching the same high levels of the wild-type at 42 degrees C. Overall, our results indicate that clpC expression is negatively controlled at the transcriptional level, directly or indirectly, by the central virulence regulator PrfA.

Systemic infection of mice by wild-type but not Spv- Salmonella typhimurium is enhanced by neutralization of gamma interferon and tumor necrosis factor alpha

The spv genes of the virulence plasmid of Salmonella typhimurium and other nontyphoidal serovars of S. enterica are involved in systemic infection by increasing the replication rate of the bacteria in host tissues beyond the intestines. We considered the possibility that the Spv virulence function is to evade suppression by the host response to infection. To examine this possibility, gamma interferon (IFN-gamma) and/or tumor necrosis factor alpha (TNF-alpha) were neutralized in BALB/c mice by intraperitoneal administration of monoclonal antibodies. Neutralization of IFN-gamma and/or TNF-alpha resulted in increased splenic infection with wild-type salmonellae after oral inoculation; however, Spv- salmonellae were defective at increasing splenic infection in cytokine-depleted mice. The use of a temperature-sensitive marker plasmid, pHSG422, indicated that neutralization of IFN-gamma caused less killing of wild-type S. typhimurium, while neutralization of TNF-alpha resulted in an increased in vivo replication rate for wild-type salmonellae. These results demonstrate that the Spv virulence function is not to evade suppression of bacterial infection normally mediated by IFN-gamma or TNF-alpha.

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.

SlyA, a transcriptional regulator of Salmonella typhimurium, is required for resistance to oxidative stress and is expressed in the intracellular environment of macrophages

Appropriate regulation of genes enables Salmonella typhimurium to adapt to the intracellular environment of the host. The Salmonella slyA gene is in a family of transcriptional regulators that may play an important role in this adaptation. We have previously shown that slyA mutant Salmonella strains are profoundly attenuated for virulence and do not survive in macrophages. In this study, we demonstrate that the expression of multiple Salmonella proteins is regulated by SlyA during stationary phase and during infection of macrophages. Both of these conditions also induced the expression of a slyA::lacZ transcriptional fusion. Expression of the slyA::lacZ transcriptional fusion increased 15-fold in stationary phase and was not dependent on the stationary-phase sigma factor, RpoS. slyA mutant Salmonella strains were sensitive to oxidative products of the respiratory burst, including hydrogen peroxide and the products of the redox cycling compound paraquat, but not to nitric oxide donors. These results suggest that the SlyA regulon is activated during infection of the host and is required for resistance to toxic oxidative products of the reticuloendothelial system.

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.

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.

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.

Avirulence of LT2 strains of Salmonella typhimurium results from a defective rpoS gene

In order to identify the genetic basis for the attenuation of Salmonella typhimurium LT2 strains, experiments were performed to identify a gene(s) which restores virulence to an avirulent LT2 strain. These and further experiments confirmed that an rpoS mutation is the sole determinant of the attenuation of S. typhimurium LT2.

Expression of Salmonella typhimurium rpoS and rpoS-dependent genes in the intracellular environment of eukaryotic cells

Adaptation to the intracellular environment of host cells is crucial for the pathogenesis of Salmonella infections. The alternative sigma factor RpoS is a global regulator of gene expression during starvation and stress conditions and is required for virulence in Salmonella spp. We have used lacZ reporter fusions to rpoS and rpoS-dependent genes to study rpoS regulation after entry of Salmonella typhimurium into macrophages and epithelial cells. The results demonstrate that expression of an rpoS::lacZ translational fusion increases rapidly in S. typhimurium after phagocytosis. Activity of RpoS also increases after bacterial entry into both macrophages and epithelial cells, as demonstrated by the induction of the rpoS-regulated genes katE and spvB. A control rpoS-independent promoter for neomycin resistance does not show significant induction after cell entry. These results demonstrate that the regulatory system mediated by RpoS in S. typhimurium is activated by the intracellular environment of eukaryotic cells.