| Salmonella typhimurium strain 798 has been shown to exist in two distinct phenotypes and to persistently infect pigs. One phenotype, 1519, expresses type 1 fimbriae and is adhesive to porcine enterocytes. The other phenotype, 1518, does not express type 1 fimbriae and is non-adhesive. 1519 is resistant to killing by serum complement whereas 1518 is sensitive due to differences in O-antigen repeat lengths. The studies in this thesis describe the development of assays to differentiate the two phenotypic phases and the identification of more genes whose expression is controlled by phase variation. Colony color differences on Evans blue-Uranine plates allowed for differentiation of the two phenotypes. 1519 colonies appeared as light-colored while 1518 colonies were dark blue in color. The two phenotypes showed differential expression of type III secretion proteins (hilA, invF, sigE) with 1519 expressing these proteins while 1518 did not. Invasion profiles of the two phenotypes reflect the difference in expression of these genes. 1519 cells invaded Henle 407 cells approximately 4 times better than 1518 cells. Two independent genetic screens were used to identify genes controlling phase variation. lrhA was found in both screens. Increased expression of lrhA led to increased rates of phase variation from the 1518 phenotype to the 1519 phenotype but not of 1519 to 1518. 1518 was shown to produce more RpoS compared to 1519. We previously hypothesized that phase variation was important in the development of persistent asymptomatic infections in swine. Since S. typhimurium strain 798 is not highly virulent and causes persistent infection, two highly virulent strains were analyzed to determine if they undergo phase variation. S. typhimurium strains SL 1344 and 14028 were found to phase vary to blue colored colonies at rates similar to 1519. Phase variation from blue colonies back to light-colored was 100-fold faster in these strains. The laboratory strain LT2 was found not to produce blue-colored colonies. A murine model of infection was used in a competition study between 1518 and 1519. 1518 was out competed by 1519 in all tissues examined. |
