Salmonella enterica Serovar Typhimurium Resistance to Bile: Identification and Characterization of the tolQRA Cluster

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Abstract
Salmonella enterica serovar Typhimurium is resistant to the action of bile salts, and resistance to bile is enhanced in strains in which the PhoP-PhoQ (PhoPQ) two-component regulatory system has been activated. To identify genes necessary for bile resistance, MudJ transposon mutagenesis was performed on a strain containing a phoP mutation that results in constitutive expression of PhoP-activated genes. After screening >10,000 mutants for the loss of growth on Luria-Bertani broth-bile plates, 14 bile-sensitive mutants were identified. Of these 14 mutants, 3 were found to retain the bile sensitivity phenotype upon P22 transduction, to possess wild-type growth characteristics, and to contain a smooth lipopolysaccharide. Southern hybridization experiments showed that all three strains contained unique insertions. DNA sequencing of the transposon-chromosomal-DNA fusion junctions of these strains showed all to be linked to the putative Salmonella orf1-tolQRA operon, with insertions in tolQ , orf1 , and a gene upstream of the orf1-tolQRA operon not previously associated with Tol function ( orfX ). Through the use of transcriptional fusions, none of the putative tol (or tol -associated) genes were shown to be regulated by PhoPQ, bile, or the RcsC-RcsB two-component system; however, all of the genes ( orfX , orf1 , tolQRA ) are predicted to be cotranscribed. This is the first identification of Salmonella serovar Typhimurium Tol homologs and the first demonstration of their role in bile resistance in this organism. In addition, the observed regulation, operon arrangement, and phenotypes associated with these tol genes demonstrate significant differences from their Escherichia coli homologs.