Virulence and mutation rates of Salmonella typhimurium strains with increased mutagenic strength in a mouse model

Two strains of Salmonella typhimurium presenting increased mutation rates, either spontaneous or mediated by DNA damage, have been constructed. One of the strains carries a null mutS mutation, while the other harbors plasmid pRW30, which contains the Escherichia coli umuDC operon. The virulence of t hese strains has been determined by inoculating BALB/c or Swiss mice. The 5 0% lethal dose of both strains is identical to that obtained for th wild-ty pe. Likewise, the two strains and th wild-type contribute equally to animal death in mixed infections. The frequency of Nal(R) mutants recovered from animals inoculated with either wild-type or MutS(-) cells was not affected by the presence of pRW30. These results indicate that the DNA damage which S. typhimurium cells can suffer during the infectious process by host cell metabolites does not cause induction of the SOS response at levels able to trigger the error-prone DNA repair pathway. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights r eserved.