Role of IS 1 in the conversion of virulence (Vi) antigen expression in Enterobacteriaceae

When Escherichia coli HB101 harbors pWR127, a plasmid comprising the viaB gene from Citrobacter freundii WR7004 and the ColEl-derived pACKC1, the strain produces the virulence (Vi) antigen. Vi antigen expression is abolished (Vi^- phenotype), however, when an IS 1 or IS 1-like DNA element inserts into the viaB region. To determine the sites of IS 1 insertion, pWR127 DNAs extracted from 95 independently isolated Vi^- strains were analyzed by digestion with the restriction endonuclease PstI and agarose gel electrophoresis. Ten insertion sites were found distributed nonrandomly in an area of about 1.3 kb. Nine Vi⁺ strains (two Citrobacter, two E. coli, and five Salmonella strains), four of which contain pWR127, were then tested for the presence of IS 1 by DNA-DNA hybridization. Of the nine strains, five were stable Vi⁺ and did not contain IS 1. The other four which generated Vi^- strains, contained IS 1. When pRR134, a plasmid that contains IS 1 was transferred into a stable Vi⁺Salmonella typhimurium strain carrying pWR127 (OU5140), Vi^- strains were produced from which pWR127 derivatives carrying IS 1 inserts could be isolated. It appears, therefore, that the presence of an IS 1I or IS 1-like element in a strain is required for conversion of the Vi⁺ expression state to the Vi^- expression state.