Identification of a Salmonella typhimurium invasion locus by selection for hyperinvasive mutants.

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Abstract
Salmonella typhimurium penetrate intestinal epithelial cells during infection. In vitro studies reveal that the availability of oxygen during bacterial growth decreases their capacity to adhere to and enter cultured epithelial cells. To identify S. typhimurium genes involved in epithelial cell entry, mutants were selected that entered HEp-2 cells when grown under repressing, aerobic culture conditions. Two types of transposons were used to generate bacterial mutations--transposons that disrupt genes (Tn10 and Tn5) and one transposon (Tn5B50) that, in addition to disrupting genes, can cause constitutive expression of genes from the neo promoter at one end of the transposon. Three classes of mutations were found that increased the ability of aerobically grown S. typhimurium to enter HEp-2 cells. One class of mutations disrupts the che operons and results in a nonchemotactic phenotype. The second class of mutations revealed that defects in rho, which encodes an essential transcription termination factor, result in hyperinvasiveness. The third class of mutations was obtained only from mutagenesis with Tn5B50, suggesting that their increased invasiveness is due to constitutive expression of a gene(s) from the exogenous neo promoter. Analysis of this third class of mutations identified a S. typhimurium locus hil (hyperinvasion locus), which is essential for bacterial entry into epithelial cells. The results suggest that hil encodes an invasion factor or an activator of invasion factor expression. hil maps between srl and mutS near minute 59.5 of the S. typhimurium chromosome, a region adjacent to other loci that have been identified as required for S. typhimurium invasiveness and virulence.

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