Genetic mapping of novel virulence determinants of Salmonella typhimurium to the region between trpD and supD

Regulation of igaA and the Rcs system by the MviA response regulator in Salmonella enterica

IgaA is a membrane protein that prevents overactivation of the Rcs regulatory system in enteric bacteria. Here we provide evidence that igaA is the first gene in a sigma(70)-dependent operon of Salmonella enterica serovar Typhimurium that also includes yrfG, yrfH, and yrfI. We also show that the Lon protease and the MviA response regulator participate in regulation of the igaA operon. Our results indicate that MviA regulates igaA transcription in an RpoS-dependent manner, but the results also suggest that MviA may regulate RcsB activation in an RpoS- and IgaA-independent manner.

Bacterial phenotypes mediated by mviA and their relationship to the mouse virulence of Salmonella typhimurium

The focus of this study was the phenotypic characterization of Salmonella typhimurium mutants lacking the function of the response regulator mviA. The inactivation of mviA+ (mviA::kan) is shown to induce a significant change in the growth of most virulent strains, as reflected in the size of the colonies formed on agar plates. The colony phenotype observed in these strains has been designated as the small colony morphology (Scm+) phenotype. Mutants exhibiting the Scm+ phenotype are shown to be significantly attenuated for virulence in susceptible (ItyB) mice. The Scm+ phenotype therefore provides an in vitro phenotypic marker for mviA+ activity. Further examination of Scm+ mutants has revealed that they lack expression of a 55 kDa periplasmic protein which is detected in isogenic mviA+ strains. This protein has been designated mviA+ related protein A (MrpA) and was expressed in direct correlation with virulence in all S. typhimurium strains examined.

Contribution of TonB- and Feo-mediated iron uptake to growth of Salmonella typhimurium in the mouse

We examined the role of iron(II) and iron(III) uptake, mediated by FeoB and TonB, respectively, in infection of the mouse by Salmonella typhimurium. The S. typhimurium feoB gene, encoding a homolog of an Escherichia coli cytoplasmic membrane iron(II) permease, was cloned, and a mutant was generated by allelic exchange. In addition, an S. typhimurium tonB mutant was constructed. Together these two mutations inactivate all known iron uptake systems of S. typhimurium. We examined the abilities of these mutants to grow in vitro and in different compartments of the host. Mutants in feoB were outcompeted by the wild type during mixed colonization of the mouse intestine, but the feoB mutation did not attenuate S. typhimurium for oral or intraperitoneal infection of mice. The tonB mutation attenuated S. typhimurium for infection of mice by the intragastric route but not the intraperitoneal route, and the mutant was recovered in lower numbers from the Peyer's patches and mesenteric lymph nodes than the wild type. These results indicate that TonB-mediated iron uptake contributes to colonization of the Peyer's patches and mesenteric lymph nodes but not the liver and spleen of the mouse. The tonB feoB double mutant, given intraperitoneally, was able to infect the liver and spleen at wild-type doses, indicating that additional iron acquisition systems are used during growth at systemic sites of infection.

The predicted amino acid sequence of the Salmonella typhimurium virulence gene mviAA(+) strongly indicates that MviA is a regulator protein of a previously unknown S. typhimurium response regulator family

The Salmonella typhimurium virulence gene mviA+ has a predicted amino acid sequence with homology to the N-terminal 112-amino-acid sequence of response regulator proteins. A previously described mutant allele (mviA), which restores virulence to avirulent LT2 strains, was shown to contain a point mutation which would be predicted to cause a single amino acid change, V-102-->G (W. H. Benjamin, Jr., J. Yother, P. Hall, and D. E. Briles, J. Exp. Med. 1,74:1073-1083, 1991). A comparison of the nucleotide sequence of mviA+ with that of the Escherichia coli and Salmonella typhi genes revealed a high degree of conservation.

A 'safe-site' for Salmonella typhimurium is within splenic polymorphonuclear cells

Following oral or systemic infection with Salmonella typhimurium, the focus of infection is in the liver and spleen. The majority of Salmonella surviving in the liver and spleen by 4 h post infection are already in an environment where they are largely protected from subsequent killing. Previous studies have shown that the majority of surviving Salmonella are intracellular. In the present study we sought to determine the cell type containing most of the cell-associated Salmonella liberated from the spleen. We enriched for Salmonella-containing cells by Ficoll-Hypaque separation followed by fluorescence-activated cell sorting. Approximately 85% of the total intracellular Salmonella were found in Mac-1+/J-11d+ cell fractions of the Ficoll-Hypaque band and pellet. By microscopic examination of stained cells from the sorted cell populations, it was evident that virtually all of the Salmonella were in polymorphonuclear cells (PMN). The numbers of Salmonella observed microscopically were similar in numbers to Salmonella colony forming units detected by plating. Salmonella containing PMN in the Ficoll band generally contained a single bacterium, while those from the probably less healthy cells in the Ficoll pellet generally contained several Salmonella.

The Salmonella typhimurium locus mviA regulates virulence in Itys but not Ityr mice: functional mviA results in avirulence; mutant (nonfunctional) mviA results in virulence

The virulent Salmonella typhimurium strain WB600 carries the mviA allele of the gene mouse virulence A. As shown here, the virulent phenotype of WB600 is the result of a nonfunctional mviA gene. As compared to the functional allele mviA+, mviA increases virulence in Itys mice, but not in Ityr mice. A specific BglII site, mviA4185, between osmZ and galU, located at approximately 35 min on the salmonella chromosome, was within mviA. Insertion of an antibiotic cassette in the mviA4185 site of mviA+ or the homologous mviA4093 site of mviA DNA resulted in virulence when either cassette was recombined into the chromosome. When mviA and mviA+ were both expressed in the same strain with one carried in the chromosome and the other on a plasmid, avirulence was dominant. Replacement of the mviA allele of strain WB600 using P22 transductions of linked antibiotic cassettes cloned into the chromosome of virulent S. typhimurium strains (SR-11, TML, SL1344, C5, ATCC14028, W118-2, and WB600) showed that all but WB600 contained the avirulent mviA+ allele. Southern hybridizations provided no evidence for a second mviA allele anywhere in the genome of the six non-WB600 strains.

A hemA mutation renders Salmonella typhimurium avirulent in mice, yet capable of eliciting protection against intravenous infection with S. typhimurium

The hemA mutation reduces the virulence of Salmonella typhimurium for mice by at least 10(7)-fold, as measured by change in LD50. The hemA mutation does not appear to affect killing of salmonella in mice. The salmonella with the hemA mutation persist in the spleen and liver for 2 to 3 weeks following intravenous injection. The most likely effect of the hemA mutation is to block, or retard, growth of S. typhimurium in an aerobic in vivo environment. Intravenous vaccination of susceptible ltys mice with hemA salmonella was able to elicit about 4 logs of protection against invasive infection with wild-type S. typhimurium 78 days after vaccination, at a time when the vaccine strain was no longer detectable in the spleen and liver.

A 'safe-site' for Salmonella typhimurium is within splenic cells during the early phase of infection in mice

Salmonella typhimurium infection in mice is focused on the spleen and liver, and prolonged infection can lead to sepsis and death. After intravenous infection with a moderate dose of S. typhimurium, the few bacteria that survive in the spleen and liver grow in a 'safe-site' where they are protected from immune destruction. In this study, we demonstrated that the lack of killing of resident salmonella in the spleen and liver was not because the salmonella were transformed within the host and became resistant to killing, or because the infected mice lost the ability to kill salmonella. We showed that the salmonella were within an intracellular 'safe-site' that protected them from killing. Brief treatment of salmonella-infected mice with gentamicin reduced the numbers of salmonella in the blood but had no effect on the numbers in the liver and spleen, suggesting an intracellular location of the salmonella. After dissociation of spleen cells from recently infected mice, 60% of the salmonella remained cell associated. These cell-associated salmonella, unlike cell-free salmonella, were resistant to killing by gentamicin. The cell-associated salmonella were rendered susceptible to gentamicin after sonication, providing confirmation of their previous intracellular location.

Strain differences in expression of virulence by the 90 kilobase pair virulence plasmid of Salmonella serovar Typhimurium

A number of plasmidless strains were obtained by curing the 90 kilobase pair (kb) virulence plasmid from six strains, C5, TML, W118, SR11, LT2 and Fisher, of Salmonella serovar Typhimurium. A number of transposon (Tn5) tagged 90 kb plasmids, also derived from these Typhimurium strains, were then transferred back into these plasmidless strains. Plasmid-cured strains, reconstituted strains, and the parental strains were tested for their virulence in BALB/c mice. There were two groups of Typhimurium strains: one required the 90 kb plasmid to express high virulence (LD50 less than 50 bacteria), and the other, regardless of the presence or absence of the 90 kb plasmid, maintained the same level of virulence at LD50 = 10 to 7 x 10(5) bacteria. Among the plasmidless strains, there were strains with a virulence level as low as LD50 = 10(7) bacteria, which was unaffected by the presence of the 90 kb plasmid.

Mouse hepatitis virus strain UAB infection enhances resistance to Salmonella typhimurium in mice by inducing suppression of bacterial growth

We have previously shown that intranasal infection of mice with mouse hepatitis virus (MHV) strain UAB (MHV-UAB) increases their resistance to Salmonella typhimurium injected intravenously 6 days later. To study how salmonella resistance was induced, BALB/cAnNCr mice were infected with salmonella strains carrying specific genetic alterations. One set of studies compared the effect of MHV infection on subsequent salmonella infections with AroA- (avirulent) and Aro+ (virulent) salmonellae. Unlike its effect on Aro+ salmonellae, MHV failed to reduce the number of AroA- salmonellae recovered from mice. Because AroA- S. typhimurium shows almost no growth in vivo, this failure indicated that the effect of MHV on salmonella resistance required growth of the infecting salmonellae. In other studies, the effect of MHV infection on both growth and killing were monitored simultaneously in mice with growing salmonellae carrying a single copy of the temperature-sensitive pHSG422 plasmid, which is unable to replicate in vivo. MHV infection reduced salmonella growth but caused no increase in salmonella killing. MHV infection of mice given wild-type salmonellae also resulted in no increase in salmonella killing 4 h after salmonella challenge. These studies demonstrate that MHV-UAB infection increases host resistance to salmonellae by enhancing suppression of bacterial growth instead of by increasing the amount of salmonella killing.

Protein H1: a role for chromatin structure in the regulation of bacterial gene expression and virulence?

There has been a recent revival of interest in one of the most abundant Escherichia coli proteins, H1 (also called H-NS). This protein was first identified many years ago as a major component of the bacterial nucleoid, and has been characterized biochemically by several groups. However, no clear function for the protein emerged from these studies. Our thinking has been transformed by recent findings which complement the biochemistry with genetic data. Several mutations, selected over many years by virtue of their diverse effects on gene expression, have turned out to be allelic and to fall within the structural gene for H1. Bringing together the genetics and the biochemistry has demonstrated that the whole is worth more than the sum of the parts! These findings have far-reaching implications for the mechanisms by which gene expression is regulated and also, perhaps, for the control of bacterial virulence.

A Salmonella typhimurium virulence gene linked to flg

Isogenic pairs of strains of Salmonella typhimurium which differed only in whether or not they were flagellate were found to be equally virulent in C57BL/6J mice infected orally, intravenously, or intraperitoneally. Therefore, we investigated the genetic basis for our previous observation that in this mouse model, nonflagellate delta flagABCDE25 strains were reduced in virulence compared with isogenic wild-type flagellate strains. The recombinant plasmid pMH6, which contains several flg+ genes and a segment of the S. typhimurium chromosome adjacent to the flg genes, was introduced into a delta flgABCDE25 mutant. This restored virulence in mice challenged intraperitoneally, which suggested that a virulence gene occurs adjacent to the flg genes. When plasmid pMH64, which lacks the chromosomal segment adjacent to the flg genes, was introduced into the same delta flgABCDE25 mutant, virulence was not restored. In contrast, the introduction of pMH71, a plasmid which retains the chromosomal segment adjacent to the flg genes, restored virulence. We concluded that a hitherto unknown virulence gene, which we have named mviS, occurs adjacent to the flg genes and that its absence in delta flgABCDE25 mutants, rather than the nonflagellate phenotype of the delta flgABCDE25 mutants, caused the previously reported attenuation of such mutants.

Isolation of orally attenuated Salmonella typhimurium following TnphoA mutagenesis

One hundred fifty Tn5 IS50L::phoA (TnphoA) mutants of a mouse-virulent, nalidixic acid-resistant (Nalr), prototrophic Salmonella typhimurium strain, C5 Nalr, were isolated. None of the mutants were auxotrophs. Groups of 8 to 10 BALB/c mice were infected orally with each of 95 mutants with a dose equivalent to 20-fold the 50% lethal dose of the wild-type C5 Nalr strain, and deaths were counted over the next 28 days. Fifteen of the mutants failed to kill any mice, whereas all mice died following challenge with the other mutants. Nine of the 15 attenuated mutants exhibited a defect in lipopolysaccharide biosynthesis. The remaining six mutants were smooth. The TnphoA transposon of each of the smooth attenuated mutants was moved, using P22-mediated transduction, into a fresh C5 background, and all retransductants were still attenuated. Analysis of the membrane proteins of the attenuated mutants failed to reveal any alterations in detectable major outer membrane proteins, although colonies of two of the mutants exhibited a mucoid phenotype following growth on L-agar plates. Individual attenuated mutants differed in their abilities to translocate to livers and spleens of mice following oral infection. All of the smooth TnphoA mutants exhibited increased 50% lethal doses with respect to the wild type following intravenous infection of BALB/c mice. Southern analysis of DNA prepared from each of the mutants suggested that TnphoA had inserted into a number of different sites in the S. typhimurium genome. None of the TnphoA mutants had inserts in the virulence-associated plasmid.