Multiple insertions of fimbrial operons correlate with the evolution of Salmonella serovars responsible for human disease.

On centisome 7, Salmonella spp. contain a large region not present in the corresponding region of Escherichia coli. This region is flanked by sequences with significant homology to the E. coli tRNA gene aspV and the hypothetical E. coli open reading frame yafV. The locus consists of a mosaic of differentially acquired inserts forming a dynamic cs7 region of horizontally transferred inserts. Salmonella enterica subspecies I, responsible for most Salmonella infections in warm-blooded animals, carries a fimbrial gene cluster (saf) in this region as well as a regulatory gene (sinR). These genes are flanked by inverted repeats and are inserted in another laterally transferred region present in most members of Salmonella spp. encoding a putative invasin (pagN ). S. enterica subspecies I serovar Typhi, the Salmonella serovar that causes the most severe form of human salmonellosis, contains an additional insert of at least 8 kb in the sinR-pagN intergenic region harbouring a novel fimbrial operon (tcf ) similar to the coo operon encoding the CS1 fimbrial adhesin expressed by human-specific enterotoxigenic E. coli. It is suggested that the multiple insertions of fimbrial genes that have occurred in the cs7 region have contributed to phylogenetic diversity and host adaptation of Salmonella spp.

Expression of thin aggregative fimbriae promotes interaction of Salmonella typhimurium SR-11 with mouse small intestinal epithelial cells.

The factors that mediate binding of Salmonella typhimurium to small intestinal epithelial cells have not been fully characterized. In this paper we demonstrate that elimination of production of thin aggregative fiber by a transposon insertion within the gene encoding the subunit protein of the fiber reduced binding of S. typhimurium SR-11 to a conditionally immortalized proximal small intestinal epithelial cell line established from transgenic mice. This binding defect could be overcome by transcomplementation with a wild-type allele. The conditionally immortalized cell line should prove useful in identifying the epithelial cell receptor for bacterial attachment since expression of its bacterial binding activity can be induced by manipulating the line's proliferative status.

Development of a murine model of chronic Salmonella infection.

The invasive disease caused by Salmonella typhimurium in mice resembles the acute phase of human typhoid fever caused by Salmonella typhi, and experimental murine salmonellosis is a widely used experimental model for systemic salmonellosis. In this paper we demonstrate that murine S. typhimurium infection can also be used to model the development of the chronic carrier state that develops in humans after infection with S. typhi. We describe a virulent variant of S. typhimurium that has decreased expression of AgfA fibers under all environmental conditions studied and that causes a chronic carrier state in BALB/c mice after peroral inoculation. The chronic carrier state is associated with persistence of bacteria in the small intestine, spleen, and liver, and chronic infection continues despite the development of protective immunity to challenge with virulent Salmonella.

The RpoS sigma factor relieves H-NS-mediated transcriptional repression of csgA, the subunit gene of fibronectin-binding curli in Escherichia coli.

Curli encoded by the curlin subunit gene, csgA, are fibronectin- and laminin-binding fibres expressed by many natural Escherichia coli and E. coli K-12 strains in response to low temperature, low osmolarity and stationary-phase growth conditions. Curli expression is dependent on RpoS, a sigma factor that controls many stationary phase-inducible genes. Many commonly used K-12 strains carry an amber mutation in rpoS. Strains able to form curli carry an amber suppressor whereas curli-negative E. coli K-12 strains, in general, are sup0. Introduction of SupD, SupE, or supF suppressors into sup0 strains resulted in expression of temperature-regulated curli. In curli-deficient, RpoS- E. coli K-12 strains, csgA is transcriptionally activated by mutations in hns, which encodes the histone-like protein H-NS. Curli expression, fibronectin binding, and csgA transcription remain temperature- and osmoregulated in such double mutants. Our data suggest that RpoS+ strains, and hence curli-proficient strains of E. coli K-12, are relieved for the transcriptional repression mediated by the H-NS protein upon accumulating RpoS as cells reach stationary phase.

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

The virulence plasmid, characteristic of many serovars of Salmonella sp., and specifically its spv genes, promote intracellular growth of the bacteria in the liver and spleen and are essential for the virulence of these Salmonella serovars in the mouse. In an attempt to establish an in vitro model for studying its function, we evaluated its effect on the intracellular growth of the bacteria in macrophages in culture. We used a number of different macrophage-like cell lines (J774-A.1, IC-21 and PU5-1.8), as well as peritoneal or splenic macrophages from genetically Salmonella-sensitive (Itys, BALB/c) or resistant (Ityr, C3H/HeN) mice, and at different states of activation, stimulated in vivo or in vitro with lipopolysaccharide and/or recombinant gamma interferon. These were found to differ in their ability to suppress or sustain intracellular growth of several Salmonella serovars, but in all cases the growth was independent of the spv genes.

A new alpha-helical coiled coil protein encoded by the Salmonella typhimurium virulence plasmid.

A new protein of Salmonella typhimurium was identified and characterized. The gene (tlpA) encoding this protein (TlpA) was isolated from the large virulence-associated plasmid of S. typhimurium and sequenced in order to predict the primary structure of TlpA. tlpA encodes a 371-amino acid soluble protein with a calculated M(r) of 41600 and pI of 4.63. Secondary structure predictions and sequence statistics of TlpA indicated a predominant alpha-helical configuration and presence of heptapeptide repeat motifs characteristic of coiled coil proteins. Purified TlpA was shown to have biochemical properties similar to those of coiled coil proteins, including adoption of an alpha-helical configuration and a tendency to form homodimers. Furthermore, TlpA possessed heat resistance, evidence for a chain register and altered mobility in urea/sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels which are characteristics of tropomyosins. TlpA shows 32% overall sequence similarity with rat cardiac myosin and 36% similarity with horse platelet beta-tropomyosin over 226 residues, whereas selected regions possessed significant sequence identities with myosins, tropomyosins, and alpha-helical surface proteins of Streptococcus pyogenes. Our results indicate that TlpA represents a new member of prokaryotic coiled coil proteins.

Plasmid-mediated serum resistance in Salmonella enterica.

The ability of Salmonella enterica to survive an incubation of 2 h in fresh, pooled guinea-pig serum was determined comparing strain sets of serovariants Dublin and Typhimurium, harbouring or lacking the virulence plasmid. All strains showed marked serum resistance, which was slightly decreased among cured strains of Typhimurium. However, when introduced into a rough Escherichia coli strain, all Typhimurium plasmids substantially increased the resistance of the host strain to guinea-pig serum, whereas the Dublin plasmid did not. The traT gene, previously shown to affect serum resistance, was identified on all Typhimurium plasmids, but not on the Dublin plasmid. Mutational inactivation of the traT gene on the Typhimurium plasmids eliminated the serum resistance mediated by the plasmids.

Outer membrane permeability of Escherichia coli K12: isolation, cloning and mapping of suppressors of a defined antibiotic-hypersensitive mutant.

We have previously described defined mutants of the TraT protein, an outer membrane lipoprotein specified by F-like plasmids, which sensitize Escherichia coli and Salmonella typhimurium to antibiotics that are normally excluded from the cell. In this paper, the isolation, characterization and molecular cloning of suppressors of one such mutant (pDOC40) is reported. The suppressors, which were isolated by selection for vancomycin-resistant revertants, also restored resistance to several hydrophobic antibiotics although there were no detectable changes in lipopolysaccharides (LPS), phospholipids or outer membrane proteins. Three suppressor loci, provisionally designated sip, for suppression of increased permeability, were cloned in cosmids and mapped by a novel approach involving random sequencing of cloned DNA to identify flanking genes with known map positions. Our results indicate that the sipB locus is located in the 11 min region (485-510 kb) whereas sipC and sipD both map to 82 min (3850-3885 kb). Additionally, the previously sequenced nlpA gene was also mapped to the 82 min region. The cloned suppressor loci were specific for the permeability phenotype caused by the mutant R6-5 TraT protein and had no effect on the permeability phenotype caused by a related TraT mutant of S. typhimurium.

The mkaC virulence gene of the Salmonella serovar typhimurium 96 kb plasmid encodes a transcriptional activator.

The intracellular growth and virulence of Salmonella serovar Typhimurium for mice is dependent on a plasmid-borne gene cluster termed mka. We studied the regulatory interactions of the genes mkaA, mkaB, mkaC and mkaD using lacZ gene fusions. Complementation experiments with cloned DNA fragments encoding each of the four MKa proteins indicated that mkaC enhances the expression of beta-galactosidase from the mkaA-, mkaB- and mkaC-lacZ gene fusions in trans. An mkaD-lacZ fusion or mkaA-lacZ fusion that did not contain DNA proximal to mkaB was not inducible with MkaC, indicating that at least mkaB and mkaA are induced together as an operon. MkaC is thus the first virulence protein whose function has been resolved.

Amino-terminal sequence analysis of four plasmid-encoded virulence-associated proteins of Salmonella typhimurium.

We have expressed four virulence-associated proteins encoded by the Salmonella typhimurium plasmid pEX102 in Escherichia coli. The genes coding for the proteins MkaA, MkaB, MkaC and MkaD subcloned in the vectors pUC19 or Bluescript KS+ directed substantial production of these proteins in E. coli. The same host harbouring pEX102 did not produce these proteins in detectable amounts. The proteins were exclusively found in the particulate fraction. The amino-terminal sequence analysis showed that the N-termini of these proteins corresponded to the ones predicted from the open reading frames found previously in the DNA sequence of the virulence determinant and that no N-terminal signal sequence processing of the proteins occurred.

TraT lipoprotein, a plasmid-specified mediator of interactions between gram-negative bacteria and their environment.

The TraT protein is a cell-surface-exposed, outer membrane lipoprotein specified by large, usually conjugative, F-like plasmids. Two biological activities have been associated with the protein: (i) prevention of self-mating of cells carrying identical or closely related conjugative plasmids, by blocking the formation of stable mating aggregates; and (ii) resistance to the bactericidal activities of serum, possibly by inhibiting the correct assembly or efficient functioning of the terminal membrane attack complex of complement. The protein therefore interacts not only with components of the outer membrane but also with specific external agents. In conjugative plasmids the traT gene lies within the region necessary for the conjugal transfer of DNA (tra), although its expression is not necessarily dependent on the expression of other tra genes. Recently, however, the gene has been discovered in isolation from other tra genes in nonconjugative virulence-associated plasmids, providing further evidence that the TraT protein may have a role in pathogenesis. The nucleotide sequences of several traT genes have been determined, and comparison of the corresponding amino acid sequences suggests that a central region of five amino acid residues flanked by hydrophobic domains determines the specificity of the protein in surface exclusion. Additionally, studies of mutants with different amino acid alterations within the hydrophobic domains have shown that insertion of charged residues disrupts normal outer membrane integrity. This review considers our current knowledge of the distribution, structure, and biological role(s) of the protein. Recent applications of the protein in studies of the unusual permeability properties of the outer membrane and for the transport of foreign antigenic determinants to the bacterial cell surface are also discussed.

Characterization of the traT gene and mutants that increase outer membrane permeability from the Salmonella typhimurium virulence plasmid.

The nucleotide sequence of the traT gene present in the virulence-associated plasmid of Salmonella typhimurium was determined. The predicted TraT protein encoded by this gene was found to consist of 243 amino acids and to resemble the known TraT proteins of the plasmids of the F incompatibility group. Thus it contains a signal sequence of 20 amino acids, an amino-terminal lipid attachment site, and two strongly hydrophobic regions close to each other in the mature protein. A mutation leading to increased permeability of the outer membrane to hydrophobic agents, previously localized to the traT gene, was shown to change a glycine residue to arginine within one of these hydrophobic regions. The same principle was found to apply to TraT of R6-5: the introduction, by site-directed mutagenesis, of either positively or negatively charged amino acids or the helix-disrupting proline in the corresponding hydrophobic region led to increased hydrophobic permeability of the outer membrane.

rfaP mutants of Salmonella typhimurium.

Salmonella typhimurium rfaP mutants were isolated and characterised with respect to their sensitivity towards hydrophobic antibiotics and detergents, and their lipopolysaccharides were chemically analysed. The rfaP mutants were selected after diethylsulfate mutagenesis or as spontaneous mutants. The mutation in two independent mutants SH7770 (line LT2) and SH8551 (line TML) was mapped by cotransduction with cysE to the rfa locus. The mutants were sensitive to hydrophobic antibiotics (clindamycin, erythromycin and novobiocin) and detergents (benzalkoniumchloride and sodium dodecyl sulfate). Analysis of their lipopolysaccharides by chemical methods and by sodium dodecyl sulfate/polyacrylamide gel electrophoresis revealed that their saccharide portion was, to a large extent, of chemotype Rc with small proportions of material containing a more complete core oligosaccharide and O-specific chains. Only 2.5 mol phosphate/mol lipopolysaccharide was found whereas the phosphate content of the lipopolysaccharide of a galE mutant strain was 4.8 mol. Thus the rfaP mutant lipopolysaccharides lacked more than two phosphate residues. Assessment of the location of phosphate groups in rfaP lipopolysaccharides revealed the presence of at least 2 mol phosphate in lipid A, indicating that the core oligosaccharide was almost devoid of phosphate. The chemical, physiological and genetic data obtained for these mutants are in full agreement with those reported earlier for rfaP mutants of Salmonella minnesota.

Occurrence of Salmonella typhimurium virulence plasmid-specific sequences in different serovars of Salmonella.

We have subcloned the 96-kilobasepair (kb) virulence plasmid, pLT2, of Salmonella typhimurium line LT2 into 7 subfragments. Using these subclones as probes, 35 independent Salmonella isolates were tested for complementary DNA sequences Sequences homologous to pLT2 were present in 15 of the isolates. All of these contained sequences homologous to at least one specific probe representing 15 kb of pLT2. The traT gene from pLT2 was absent in 6 of these 15 isolates. Three strains reported to be cured of the plasmid were shown to harbour some sequences with homology to the pLT2 plasmid. Seven isolates were shown to contain homologous sequences with pBR322 but not with the pLT2 plasmid.

The role of the traT gene of the Salmonella typhimurium virulence plasmid for serum resistance and growth within liver macrophages.

The role of the traT gene of the Salmonella typhimurium virulence plasmid for the serum resistance of the bacteria and their growth within mouse liver macrophages was investigated. The gene product, the TraT protein, increased the serum resistance in E. coli HB101, which naturally does not carry traT. It also contributed to the serum tolerance of S. typhimurium. The capacity of an isogenic S. typhimurium TML strain triplet, differing in their ability to express TraT and in the quality of the traT gene expressed, to grow in vivo in the mouse liver indicated that, although TraT was dispensable for the net growth of the bacteria within the liver, the expression of a mutated traT gene reduced the growth rate. The traT gene was mapped on the virulence plasmid outside previously defined virulence determinants suggesting that other regions of the virulence plasmid are mainly responsible for the growth within mouse liver macrophages in S. typhimurium.

Plasmid-associated virulence of Salmonella enteritidis.

Plasmid-associated virulence of Salmonella enteritidis was studied using plasmid-cured and plasmid-reintroduced strains. The plasmidless strain was unable to grow in the liver of mice after intravenous inoculation. Reintroduction of the plasmid pEX106 from the original S. enteritidis fully restored its capacity to grow in the mice. The plasmid pEX102 of Salmonella typhimurium introduced into the plasmid-cured S. enteritidis had a similar effect. The smooth lipopolysaccharide character of the S. enteritidis strains was not affected by the presence or absence of the plasmid, and the same was true of their high resistance to complement in guinea-pig serum as such or with added antibody.

Amino acid alterations in a hydrophobic region of the TraT protein of R6-5 increase the outer membrane permeability of enteric bacteria.

Two insertion mutations, each introducing a new negatively charged amino acid residue into a hydrophobic region of the TraT protein coded by the F-like plasmid R6-5 caused significant alterations in outer membrane permeability of Escherichia coli and Salmonella typhimurium, so that strains carrying plasmids with either of these mutations became sensitive to hydrophobic antibiotics.

The traT protein is able to normalize the phenotype of a plasmid-carried permeability mutation of Salmonella typhimurium.

The isolation of different classes of antibiotic-supersensitive outer membrane permeability mutants of Salmonella typhimurium has been described previously (Sukupolvi et al., 1984, Journal of Bacteriology 159, 704-712). One of these, the SS-A mutation, sensitizes the bacteria to gentian violet and to hydrophobic antibiotics. The phenotype of the SS-A mutant was restored to normal when a cloned fragment of the F plasmid, or the R plasmid R6-5, carrying the genes traS, T and D was introduced on a multicopy plasmid. The introduction of a plasmid carrying only the traT gene showed that this gene was sufficient to restore the phenotype. Only clones with functioning traT (irrespective of copy number) restored the normal antibiotic-resistant phenotype in the SS-A mutant. An incompatibility test using a donor strain which carried transposon Tn10 in the 60 MDa plasmid of S. typhimurium and a recipient in which Tn5 was placed close to the SS-A mutation indicated that the SS-A mutation was located in the 60 MDa virulence plasmid (previously called the cryptic plasmid) of S. typhimurium. The introduction of the large virulence plasmid carrying the SS-A mutant allele into wild-type S. typhimurium or Escherichia coli resulted in strains with a phenotype identical to that of the original SS-A mutant.

New Salmonella typhimurium mutants with altered outer membrane permeability.

We describe three new classes of Salmonella typhimurium mutants with increased sensitivity to hydrophobic agents. In contrast to many previously described mutants, the phage sensitivity pattern of these mutants did not give any indication of defective lipopolysaccharide. Furthermore, they had no detectable changes in their phospholipid or outer membrane protein composition, and their growth rate and cell morphology were normal. Class B mutants were nearly as sensitive to novobiocin, fusidic acid, erythromycin, rifampin, and clindamycin as are deep rough (heptoseless) mutants; in addition they were sensitive to methicillin, penicillin (to which heptoseless mutants are resistant), gentian violet, and anionic and cationic detergents. Class A and C mutants had less sensitive, but characteristic phenotypes. None of the three classes were sensitive to serum bactericidal action. The class B mutation mapped between map positions 7 and 11 on the S. typhimurium chromosome, and the class C mutation mapped between positions 5 and 7. The map position for the class A mutation remained undefined, but it was separate from the class B and C mutations and, like those, did not correspond to any gene loci known to participate in the synthesis of major outer membrane constituents.