Construction of plasmid-free derivatives of Salmonella typhimurium LT2 using temperature-sensitive mutants of pKZ1 for displacement of the resident plasmid, pSLT

Dose-Response Analysis of Chemotactic Signaling Response in Salmonella typhimurium LT2 upon Exposure to Cysteine/Cystine Redox Pair

The chemotaxis system enables motile bacteria to search for an optimum level of environmental factors. Salmonella typhimurium senses the amino acid cysteine as an attractant and its oxidized dimeric form, cystine, as a repellent. We investigated the dose-response dependence of changes in chemotactic signaling activity upon exposure to cysteine and cystine of S. typhimurium LT2 using in vivo fluorescence resonance energy transfer (FRET) measurements. The dose-response curve of the attractant response to cysteine had a sigmoidal shape, typical for receptor-ligand interactions. However, in a knockout strain of the chemoreceptor genes tsr and tar, we detected a repellent response to cysteine solutions, scaling linearly with the logarithm of the cysteine concentration. Interestingly, the magnitude of the repellent response to cystine also showed linear dependence to the logarithm of the cystine concentration. This linear dependence was observed over more than four orders of magnitude, where detection started at nanomolar concentrations. Notably, low concentrations of another oxidized compound, benzoquinone, triggered similar responses. In contrast to S. typhimurium 14028, where no response to cystine was observed in a knockout strain of chemoreceptor genes mcpB and mcpC, here we showed that McpB/McpC-independent responses to cystine existed in the strain S. typhimurium LT2 even at nanomolar concentrations. Additionally, knocking out mcpB and mcpC did not affect the linear dose-response dependence, whereas enhanced responses were only observed to solutions that where not pH neutral (>100 μM cystine) in the case of McpC overexpression. We discuss that the linear dependence of the response on the logarithm of cystine concentrations could be a result of a McpB/C-independent redox-sensing pathway that exists in S. typhimurium LT2. We supported this hypothesis with experiments with defined cysteine/cystine mixed solutions, where a transition from repellent to attractant response occurred depending on the estimated redox potential.

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

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

Utilization of orotate as a pyrimidine source by Salmonella typhimurium and Escherichia coli requires the dicarboxylate transport protein encoded by dctA

Mutants deficient in orotate utilization (initially termed out mutants) were isolated by selection for resistance to 5-fluoroorotate (FOA), and the mutations of 12 independently obtained isolates were found to map at 79 to 80 min on the Salmonella typhimurium chromosome. A gene complementing the mutations was cloned and sequenced and found to possess extensive sequence identity to characterized genes for C4-dicarboxylate transport (dctA) in Rhizobium species and to the sequence inferred to be the dctA gene of Escherichia coli. The mutants were unable to utilize succinate, malate, or fumarate as sole carbon source, an expected phenotype of dctA mutants, and introduction of the cloned DNA resulted in restoration of both C4-dicarboxylate and orotate utilization. Further, succinate was found to compete with orotate for entry into the cell. The S. typhimurium dctA gene encodes a highly hydrophobic polypeptide of 45.4 kDa, and the polypeptide was found to be enriched in the membrane fraction of minicells harboring a dctA+ plasmid. The DNA immediately upstream of the deduced -35 region contains a putative cyclic AMP-cyclic AMP receptor protein complex binding site, thus affording an explanation for the more effective utilization of orotate with glycerol than with glucose as carbon source. The E. coli dctA gene was cloned from a lambda vector and shown to complement C4-dicarboxylate and orotate utilization in FOA-resistant mutants of both E. coli and S. typhimurium. The accumulated results demonstrate that the dctA gene product, in addition to transporting C4-dicarboxylates, mediates the transport of orotate, a cyclic monocarboxylate.

Strain typing with IS200 fingerprints in Salmonella abortusovis

A collection of Salmonella abortusovis isolates was examined for the presence of insertion element IS200. All proved to contain three or four copies of the element. One IS200 hybridization band of approximately 9 kb was found in all isolates, indicating that all S. abortusovis strains carry an IS200 element in similar or identical locations; this band can be potentially useful for serovar identification. S. abortusovis collection isolates from distinct geographic areas were highly polymorphic, suggesting that IS200 fingerprints might provide information on the geographic origin of S. abortusovis strains. Isolates obtained from the same geographic area (the island of Sardinia, Italy) were less polymorphic: all shared three constant IS200 hybridization bands, indicating that they derive from a single ancestor. Most strains analyzed contained an additional copy of IS200 in the variable region of the virulence plasmid. Certain Sardinian flocks proved to be infected by only one S. abortusovis strain, while others harbored two strains. Strain typing with IS200 fingerprints proved to be more reliable than plasmid analysis, because the latter yielded a high degree of polymorphism, even among isolates from the same flock.

Functional complementation between chromosomal and plasmid mutagenic DNA repair genes in bacteria

The umuDC operons of Escherichia coli and Salmonella typhimurium and the analogous plasmid operons mucAB and impCAB have been previously characterized in terms of their roles in DNA repair and induced mutagenesis by radiation and many chemicals. The interrelationships of these mutagenic DNA repair operons were examined in vivo in functional tests of interchangeability of operon subunits in conferring UV resistance and UV mutability phenotypes to wild-type S. typhimurium and umu mutants of E. coli. This approach was combined with DNA and protein sequence comparisons between the four operons and a fifth operon, samAB, from the S. typhimurium LT2 cryptic plasmid. Components of the E. coli and S. typhimurium umu operons were reciprocally interchangeable whereas impCA and mucA could not function with umuC in either of these species. mucA and impB could also combine to give a mutagenic response to UV. These active combinations were associated with higher degrees of conservation of protein sequence than in other heterologous gene combinations and related to specific regions of sequence that may specify subunit interactions. The dominance of the E. coli umuD44 mutation over umuD was revealed in both wild-type E. coli and S. typhimurium and also demonstrated against impCAB. Finally interspecies transfer showed that the apparently poor activity of the S. typhimurium umuD gene in situ is not the result of an inherent defect in umuD but is due to the simultaneous presence of the S. typhimurium umuC sequence. It is suggested that the limitation of umuD activity by umuC in S. typhimurium is the basis of the poor induced mutability of this organism.