The virulence plasmid does not contribute to growth of Salmonella in cultured murine macrophages

Nitric oxide produced by murine dendritic cells is cytotoxic for intracellular Salmonella enterica sv. Typhimurium

The pathogenicity of Salmonella enterica serovar Typhimurium has traditionally been correlated with its ability to survive and grow in macrophages. Macrophage-derived production of nitric oxide (NO) has been implicated as a major innate defence, restricting bacterial proliferation both in macrophage cultures and in mice. In the present study, we show that the ability of primary murine dendritic cells (DCs) to ingest Salmonella is low, but greatly enhanced by serum complement. Ingestion of bacteria was followed by the expression of inducible nitric oxide synthase (iNOS), as well as by NO production. iNOS mRNA was detected as early as 6 h post infection and production of NO 12 h post infection, rising further at 16 h post infection. Inhibition of the iNOS activity with the inhibitor N-monomethyl-l-arginine or using DCs from iNOS-/- mice resulted in increased intracellular bacterial yields. To further define the potential defensive role of DC-derived NO, the actual intracellular replication rate of S. Typhimurium in DCs was measured. DC-derived NO was shown to exert a bactericidal effect, whereas the effect of NO in macrophage-like J774-A.1 cells was found to be bacteriostatic. These results identified an important role for NO in restricting S. Typhimurium survival in DCs, indicating that DCs may actively participate in the innate defence against intracellular pathogens.

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.

Breaking through the acid barrier: an orchestrated response to proton stress by enteric bacteria

The ability of enteropathogens such as Salmonella and Escherichia coli to adapt and survive acid stress is fundamental to their pathogenesis. Once inside the host, these organisms encounter life-threatening levels of inorganic acid (H+) in the stomach and a combination of inorganic and organic acids (volatile fatty acids) in the small intestine. To combat these stresses, enteric bacteria have evolved elegant, overlapping strategies that involve both constitutive and inducible defense systems. This article reviews the recent progress made in understanding the pH 3 acid tolerance systems of Salmonella and the even more effective pH 2 acid resistance systems of E. coli. Focus is placed on how Salmonella orchestrates acid tolerance by modulating the activities or levels of diverse regulatory proteins in response to pH stress. The result is induction of overlapping arrays of acid shock proteins that protect the cell against acid and other environmental stresses. Most notable among these pH-response regulators are RpoS, Fur, PhoP and OmpR. In addition, we will review three dedicated acid resistance systems of E. coli, not present in Salmonella, that allow this organism to survive extreme (pH 2) acid challenge.

Lack of evidence of an association between the carriage of virulence plasmid and the bacteremia of Salmonella typhimurium in humans

The involvement of the virulence plasmid (pSTV) of Salmonella typhimurium in human salmonellosis was examined. Most of the 224 clinical strains isolated from the blood (53) and nonblood samples (171) contained a 90 kb or larger plasmid, most of which were pSTV. The rates of pSTV carriage in the isolates showed no statistically significant difference between those derived from the blood and those from other sources (87% vs. 83%; chi2=0.49, 0.1<P<0.9), suggesting that pSTV may not play a critical role in promoting S. typhimurium bacteremia in humans. Nine strains with representative plasmid profiles were tested for the mouse virulence. The result revealed that these clinical isolates contained all three virulent types known: the avirulent, the highly virulent when a pSTV was present, and the moderately virulent regardless of the presence or absence of pSTV. This indicated that mouse virulence of S. typhimurium did not correlate their virulence in humans. Clinical data showed that most patients with primary bacteremia had underlying immunosuppressive diseases, whereas only a few patients with secondary bacteremia had preexisting diseases (87% vs. 13%; chi2=22.73, P<0.005). It is suggested that the contribution of pSTV to S. typhimurium bacteremia in humans is likely to be limited, and both the host factor and the microbial virulence determinants on the chromosome are more important than virulence plasmid in predisposing patients to bacteremia.

Constitutively expressed phoP inhibits mouse-virulence of Salmonella typhimurium in an Spv-dependent manner

In Salmonella typhimurium, the transcription of several virulence genes including spvB is regulated by the PhoP/PhoQ regulatory system. To further examine the relationship between the PhoP/PhoQ and Spv systems for virulence in mice, we examined a non-polar phoP mutation combined with different virulence plasmid genotypes for effects on virulence of S. typhimurium in the mouse model. PhoP-/Spv+ and PhoP-/Spv- mutants were not detectably recovered from the spleens of subcutaneously or orally inoculated mice. The phoP gene constitutively expressed from the lacZ promoter of a low copy number vector (phoP(C)) only partially complemented the non-polar phoP mutation for mouse-virulence in both the Spv+ and Spv- backgrounds; both PhoP(C) strains exhibited virulence equal only to a PhoP+/Spv- strain. Interestingly, in a PhoP+ background, the phoP(C) gene reduced splenic infection of the Spv+ but not Spv- salmonellae after subcutaneous or oral inoculation compared with the PhoP+ parents. Additionally, the phoP(C) gene in an Spv+ background reduced the net growth of salmonellae in macrophages in vitro; phoP(C) in an Spv- background was without effect. These data suggest that the constitutive expression of the phoP gene attenuates the virulence of S. typhimurium in mice in an Spv-dependent manner.

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.

Salmonella typhimurium mutants that downregulate phagocyte nitric oxide production

To examine the potential and strategies of the facultative intracellular pathogen Salmonella typhimurium to increase its fitness in host cells, we applied a selection that enriches for mutants with increased bacterial growth yields in murine J774-A.1 macrophage-like cells. The selection, which was based on intracellular growth competition, rapidly yielded isolates that out-competed the wild-type strain during intracellular growth. J774-A.1 cells responded to challenge with S. typhimurium by mounting an inducible nitric oxide synthase (iNOS) mRNA and protein expression and a concomitant nitric oxide (NO) production. Inhibition of NO production with the use of the competitive inhibitor N-monomethyl-L-arginine (NMMA) resulted in a 20-fold increase in bacterial growth yield, suggesting that the NO response prevented bacterial intracellular growth. In accordance with this observation, five out of the nine growth advantage mutants isolated inhibited production of NO from J774-A.1 cells, despite an induction of iNOS mRNA and iNOS protein. Accompanying bacterial phenotypes included alterations in lipopolysaccharide structure and in the profiles of proteins secreted by invasion-competent bacteria. The results indicate that S. typhimurium has the ability to mutate in several different ways to increase its host fitness and that inhibition of iNOS activity may be a major adaptation.

Salmonella enterica serovars Typhimurium and Dublin can lyse macrophages by a mechanism distinct from apoptosis

Salmonella enterica serovars Typhimurium and Dublin lysed primary bovine alveolar macrophages and immortalized J774.2 macrophage-like cells in the absence of either the morphological changes or DNA fragmentation characteristic of apoptosis. Macrophage lysis was dependent on a subset of caspases and an intact sipB gene.

The Salmonella virulence plasmid spv genes are required for cytopathology in human monocyte-derived macrophages

The pathogenesis of serious systemic Salmonella infections is characterized by survival and proliferation of bacteria inside macrophages. Infection of human monocyte-derived macrophages in vitro with S. typhimurium or S. dublin produces cytopathology characterized by detachment of cells that contain large numbers of proliferating bacteria. This cytopathology is dependent on the expression of the bacterial spv genes, a virulence locus previously shown to markedly enhance the ability of Salmonella to produce systemic disease. After 24 h of infection, macrophage cultures contain two populations of bacteria: (i) proliferating organisms present in a detached cell fraction; and (ii) a static bacterial population in macrophages remaining attached to the culture well. Mutations in either the essential transcriptional activator SpvR or the key SpvB protein markedly reduce the cytopathic effect of Salmonella infection. The spv-dependent cytopathology in macrophages exhibits characteristics of apoptosis, with release of nucleosomes into the cytoplasm, nuclear condensation and DNA fragmentation. The current findings suggest that the mechanism of the spv effect is through induction of increased cytopathology in host macrophages.

Salmonella pathogenicity islands: big virulence in small packages

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

Host and bacterial factors involved in the innate ability of mouse macrophages to eliminate internalized unopsonized Escherichia coli

In an effort to better understand genetic and cellular factors that influence innate immunity, we examined host and bacterial factors involved in the nonopsonic phagocytosis and killing of Escherichia coli K-12 by mouse macrophages. Unelicited (resident) peritoneal macrophages from five different mouse strains, BALB/c, C57BL/6, CD-1, C3H/HeJ, and C3H/HeN, were employed. Additional macrophage populations were obtained from CD-1 mice (bone marrow-derived macrophages). Also, for BALB/c and C57BL/6 mice, peritoneal macrophages elicited with either thioglycolate or proteose peptone, bone marrow-derived macrophages, and macrophage-like cell lines derived from the two strains were employed. Two E. coli K-12 strains that differed specifically in their abilities to produce type 1 pili containing the adhesive protein FimH were examined. The parameters used to assess macrophage bacteriocidal activity were (i) the killing of internalized (gentamicin-protected) E. coli during the approximately 4-h assay and (ii) the initial rate at which internalized E. coli were eliminated. Data on these parameters allowed the following conclusions: (i) unelicited or proteose peptone-elicited peritoneal macrophages were significantly better at eliminating internalized bacteria than thioglycolate-elicited peritoneal macrophages, bone marrow-derived macrophages, or macrophage cell lines; (ii) the host genetic background had no significant effect upon the ability of unelicited peritoneal macrophages to kill E. coli (even though the mouse strains differ widely in their in vivo susceptibilities to bacterial infection); and (iii) the FimH phenotype had no significant effect upon E. coli survival once the bacterium was inside a macrophage. Additionally, there was no correlation between the bacteriocidal effectiveness of a macrophage population and the number of bacteria bound per macrophage. However, macrophage populations that were the least bacteriocidal tended to bind higher ratios of FimH(+) to FimH(-) E. coli. The effect of gamma interferon, fetal calf serum, and the recombination proficiency of E. coli were examined as factors predicted to influence intracellular bacterial killing. These had no effect upon the rate of E. coli elimination by unelicited peritoneal macrophages.

Intracellular Salmonella typhimurium induce lysis of human polymorphonuclear leukocytes which is not associated with the Salmonella virulence plasmid

The interaction between Salmonella typhimurium and human polymorphonuclear leukocytes (PMNs) was analyzed in vitro. Three S. typhimurium strains, the wild-type strain OU5043, its isogenic virulence plasmid-cured strain OU5048, and LT2, which represented the types that exhibited three mouse virulence levels, respectively, were used in this study. There was no correlation between the recovery of intracellular S. typhimurium from PMNs and the presence or absence of the virulence plasmid, or the strains' mouse virulence level. When the oxygen-dependent response of PMNs upon phagocytosis of S. typhimurium was examined by checking the intracellular reduction of nitroblue tetrazolium (NBT), the fraction of PMNs that reduced NBT on phagocytosis of the three strains was around 80%, whereas it was 58% with Escherichia coli, 95% with phorbol 12-myristate 13-acetate and 15% with a negative control. Thus there were no significant differences among the three Salmonella strains in terms of their ability to induce the oxidative response in PMNs. Microscopic analysis of Salmonella-infected PMNs indicated that the intracellular Salmonella induced lysis of PMNs. Both OU5043 and OU5048 exhibited a significant intracellular cytotoxic effect on PMNs after 24 hr of infection and this effect was not associated with the presence or absence of the virulence plasmid. On the other hand, lysis of PMNs was related to the intracellular survival of Salmnonella, as ofloxacin, an antibiotic, appeared to be able to protect human PMNs from Salmonella-induced cytotoxicity when this agent was added into the medium to inactivate the intracellular organism. The ability to induce lysis of PMNs by either wild-type or plasmid-cured strains of S. typhimurium may play a crucial role in the pathogenesis of non-typhoid Salmonella. The contribution of pSTV to human salmonellosis is likely to be limited. Furthermore, early institution of antibiotics with a high intracellular activity against Salmonella, such as fluoroquinolones, may be useful to prevent the dissemination of Salmonella infection.

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.

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.

The spv genes on the Salmonella dublin virulence plasmid are required for severe enteritis and systemic infection in the natural host

The pathogenic role of the spv (Salmonella plasmid virulence) genes of Salmonella dublin was determined in the natural, bovine host. Since the lack of overt signs of enteritis or enterocolitis due to Salmonella infections in mice has limited the development of a convenient experimental system to study enteric disease, we used calves to study the contribution of the spv genes to S. dublin-induced salmonellosis. Since the SpvR transcriptional regulator is required for expression of the spvABCD operon, we constructed an spvR knockout mutation in a calf-virulent strain of S. dublin. Calves were infected with the wild-type strain, an spvR mutant, and an spvR mutant containing a complementing plasmid. Calves that were infected with the wild type or the complemented spvR mutant rapidly developed severe diarrhea and became moribund. Calves that were infected with the spvR mutant showed little or no clinical signs of systemic salmonellosis and developed only mild diarrhea. The survival and growth of the wild-type strain and the spvR mutant were determined by using blood-derived bovine monocytes. Wild-type S. dublin survived and grew inside cells, while the spvR mutant did not proliferate. These results suggest that the spv genes of S. dublin promote enhanced intracellular proliferation in intestinal tissues and at extraintestinal sites in the natural host.

The Salmonella virulence plasmid enhances Salmonella-induced lysis of macrophages and influences inflammatory responses

The Salmonella dublin virulence plasmid mediates systemic infection in mice and cattle. Here, we analyze the interaction between wild-type and plasmid-cured Salmonella strains with phagocytes in vitro and in vivo. The intracellular recovery of S. dublin from murine peritoneal and bovine alveolar macrophages cultured in the presence of gentamicin in vitro was not related to virulence plasmid carriage. However, the virulence plasmid increased the lytic activity of S. dublin, Salmonella typhimurium, and Salmonella choleraesuis for resident or activated mouse peritoneal macrophages. Lysis was not mediated by spv genes and was abolished by cytochalasin D treatment. Peritoneal and splenic macrophages were isolated from mice 4 days after intraperitoneal infection with wild-type or plasmid-cured S. dublin strains. The wild-type strain was recovered in significantly higher numbers than the plasmid-cured strain. However, the intracellular killing rates of such cells cultured in vitro for both S. dublin strains were not significantly different. Four days after infection, there was a lower increase of phagocyte numbers in the peritoneal cavities and spleens of mice infected with the wild-type strain compared with the plasmid-cured strain. The virulence plasmid influenced the survival of macrophages in vitro following infection in vivo as assessed by microscopy. Cells from mice infected with the plasmid-cured strain survived better than those from mice infected with the wild-type strain. This is the first report demonstrating an effect of the virulence plasmid on the interaction of Salmonella strains with macrophages. Plasmid-mediated macrophage dysfunction could influence the recruitment and/or the activation of phagocytic cells and consequently the net growth of Salmonella strains during infection.

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.

The Salmonella dublin virulence plasmid does not modulate early T-cell responses in mice

The virulence plasmid in Salmonella dublin mediates systemic infection in mice and cattle. The role of gamma delta T cells or hepatic extrathymic T cells has recently been reported to be important in the control of the early stage of Salmonella choleraesuis infections of mice. Here, we report on T-cell responses in conventional mice after challenge with a virulent strain of S. dublin carrying a virulence plasmid or with a strain cured of the plasmid. Over a period of 4 days postinfection, when both strains could be compared, similar changes in alpha beta and gamma delta T-cell subsets in peritoneal cavities, livers, and spleens were recorded, demonstrating no clear role of the virulence plasmid in modulation of early T-cell responses. To investigate further the role of the virulence plasmid in pathogenesis, the growth of the plasmid-cured strain was assessed in SCID, SCID bg, and irradiated mice. During the first 6 days after infection, there was no statistically difference in the net growth of Salmonella cells in the livers and spleens of SCID and SCID bg mice compared with conventional BALB/mice. This observation excludes a key role for a T- or B-cell-mediated immune response in controlling the initial growth of the plasmid-cured S. dublin strain. Thereafter, the immunocompromised mice were no longer able to control infection, although SCID mice were more efficient at controlling net bacterial multiplication than SCID bg mice, potentially implicating NK cells in the control of infection in SCID mice. The early control of net bacterial multiplication in the spleens and livers of BALB/c mice was ablated by whole-body X-irradiation. Both wild-type and plasmid-cured strains multiplied significantly more rapidly in irradiated than in conventional BALB/c mice. However, the numbers of wild-type bacterial still increased more rapidly than in the numbers of the cured strains. These results are consistent with a role of the S. dublin virulence plasmid in promoting in vivo growth of Salmonella cells.

The Salmonella dublin virulence plasmid mediates systemic but not enteric phases of salmonellosis in cattle

Plasmid-bearing and plasmid-free isolates and a plasmid-cured strain of Salmonella dublin were compared for virulence in calves. The plasmid-bearing strains were highly virulent, causing severe enteric and systemic disease with high mortality. In contrast, the plasmid-free strains caused diarrhea but only low mortality. The infection kinetics of a wild-type and a derivative plasmid-cured strain were compared. Both strains were isolated in high numbers from intestinal sites at 3 and 6 days after oral challenge and were isolated at comparable frequencies from systemic sites at 3 days, but not at 6 days, when the wild-type strain was predominant. The strains were equally invasive in intestinal epithelia with and without Peyer's patch and elicited comparable secretory and inflammatory responses and intestinal pathology in ligated ileal loops. The effect of the virulence plasmid on growth kinetics and on the outer membrane protein profile was assessed in an in vivo growth chamber. The virulence plasmid did not influence either extracellular growth or the expression of major outer membrane proteins. These observations demonstrate that the virulence plasmid is not involved in either the enteric phase of infection or the systemic dissemination of S. dublin but probably mediates the persistence of S. dublin at systemic sites.

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.

A low-pH-inducible, stationary-phase acid tolerance response in Salmonella typhimurium

Acid is an important environmental condition encountered by Salmonella typhimurium during its pathogenesis. Our studies have shown that the organism can actively adapt to survive potentially lethal acid exposures by way of at least three possibly overlapping systems. The first is a two-stage system induced in response to low pH by logarithmic-phase cells called the log-phase acid tolerance response (ATR). It involves a major molecular realignment of the cell including the induction of over 40 proteins. The present data reveal that two additional systems of acid resistance occur in stationary-phase cells. One is a pH-dependent system distinct from log-phase ATR called stationary-phase ATR. It was shown to provide a higher level of acid resistance than log-phase ATR but involved the synthesis of fewer proteins. Maximum induction of stationary-phase ATR occurred at pH 4.3. A third system of acid resistance is not induced by low pH but appears to be part of a general stress resistance induced by stationary phase. This last system requires the alternative sigma factor, RpoS. Regulation of log-phase ATR and stationary-phase ATR remains RpoS independent. Although the three systems are for the most part distinct from each other, together they afford maximum acid resistance for S. typhimurium.

Expression of the Salmonella virulence plasmid gene spvB in cultured macrophages and nonphagocytic cells

Certain serotypes of salmonellae carry virulence plasmids that greatly enhance the pathogenicity of these bacteria in experimentally infected mice. This phenotype is largely attributable to the 8-kb spv regulon. However, spv genes are not expressed while bacteria grow in vitro. We now show that spvB, which is required for virulence, is expressed rapidly after Salmonella dublin is ingested by cultured J774 and murine peritoneal macrophages and that expression is not affected by the alkalinization of intracellular vesicles. The level of induction of spvB is reduced when macrophages are pretreated with gamma interferon. spvB is also expressed in human and canine epithelial cell lines and a human hepatoma cell line. In all cases, spvB expression is dependent on the spvR gene, just as it is in stationary-phase cultures in vitro. These data suggest that spv virulence genes are expressed by intracellular salmonellae in vivo in response to a signal that is common to the intracellular compartments of cells that are invaded by salmonellae.

Role of gamma interferon in late stages of murine salmonellosis

Gamma interferon (IFN-gamma) is an important mediator of natural resistance of mice to Salmonella species during the first week of infection, when it restricts the rate of intracellular growth of the bacteria but does not lead to their killing (A. Muotiala and P. H. Mäkelä, Microb. Pathog. 8:135-141, 1990). We used the experimental mouse salmonellosis model to investigate the role of IFN-gamma in the later stages of a sublethal infection and the ensuing specific immunity. When anti-IFN-gamma was administered starting 6 days after challenge, it did not prevent the cessation of intracellular bacterial growth and the formation of the plateau stage in the second week of infection. In addition, anti-IFN-gamma given 14 and 16 days after challenge did not alter the elimination of the bacteria in the clearance stage in the third week of infection. When mice immunized 2 months previously with live vaccine were infected with virulent salmonellae, depletion of IFN-gamma enhanced the early growth of the bacteria in the same manner as that seen in naive mice. However, when the immunized mice were infected with attenuated aroA bacteria, their clearance started immediately and was unaffected by IFN-gamma depletion, demonstrating that IFN-gamma is not required for the clearance. We conclude that IFN-gamma restricts the rate of intracellular bacterial multiplication in the first week of Salmonella infection in both naive and immune mice but is not a mediator of bacterial clearance in either naive or immunized mice.

Transcriptional regulation of Salmonella enterica virulence plasmid genes in cultured macrophages

The plasmid-carried spv genes promote virulence of salmonellae in mice by allowing bacterial growth in the reticuloendothelial tissue. When the bacteria are cultivated under normal laboratory conditions the spv genes appear dormant. This communication explores the transcriptional regulation of spv genes within murine macrophage-like J774-A.1 cells utilizing a new reporter system. Transcriptional fusions were constructed between promoter elements of the Salmonella enterica var. Typhimurium spv genes and the KS71A fimbrial gene cluster. The expression of KS71A fimbriae in fusion-carrying Escherichia coli strains was found to be under the control of the transcriptional activator gene spvR. In strains overproducing SpvR, KS71A fimbriae were assembled on the bacterial cell surface and could be detected by bacterial agglutination or immunofluorescence of intact bacteria; the reporter activity was quantified by estimating the percentage of fluorescent bacteria and by immunoblotting of cell lysates. The activity of the reporters, when transformed into the parent Typhimurium TML R66, was low and revealed less than 0.3% fimbriated cells under in vitro culture conditions. A 15-30-fold increase in fimbriation was observed when the bacteria were cultivated within J774-A.1 cells. No such increase occurred when the reporter fusions were transformed into TML R66 cured of the virulence plasmid. Insertional inactivation of the spvR gene of the virulence plasmid in Typhimurium TML R66 also abolished induction, whereas corresponding inactivation of spvA or spvB did not reduce induction. No increase in reporter activity was obtained in Typhimurium of line Q1, which is naturally avirulent for mice, although the strain was provided with virulence plasmid pEX102 of line TML R66. We conclude that the intracellular environment of J774-A.1 cells induces the spv genes and that this induction requires gene functions of both the bacterial chromosome and the virulence plasmid.