Identification and characterization of a Salmonella typhimurium oxygen-regulated gene required for bacterial internalization.

Growth of Salmonella typhimurium in a low-oxygen environment induces the ability of these bacteria to enter mammalian cells. We have carried out a search for invasion genes that are expressed under low-oxygen conditions by using Tn5lacZY transcriptional fusions. Several noninvasive oxygen-regulated lacZY insertion strains have been identified. The invasion defect in one of these noninvasive S. typhimurium strains, BJ66, has been complemented by introduction of a cosmid (pBDJ125) from an S. typhimurium SL1344 gene bank. A 1.9-kb EcoRV DNA fragment subcloned from this cosmid, containing a single open reading frame (orgA), restores the ability of BJ66 to invade mammalian cells. Comparative searches of the GenBank and EMBL sequence data banks with the nucleotide sequence of the gene and deduced amino acid sequence of the protein reveal no significant similarities. Interestingly, hybridization of an orgA gene probe with a P22 chromosomal mapping library demonstrated that the orgA gene maps to a region on the chromosome between 57.5 and 60 min where other Salmonella invasion genes have been mapped. Other enteroinvasive bacteria (Shigella flexneri, Escherichia coli, Yersinia spp., and Listeria monocytogenes) lack sequences which cross hybridize to the probe. We have compared the virulence of S. typhimurium SL1344 and an isogenic orgA mutant in a mouse model of typhoid fever. The orgA mutant was as virulent as the wild-type strain was when inoculated intraperitoneally but is significantly reduced (> 60-fold) in its ability to cause disease by an oral route of infection.