Isolation and characterization of conditional adherent and non‐type 1 fimbriated Salmonella typhimurium mutants

Mutations in the genes encoding the type 1 fimbriae of Salmonella typhimurium were isolated by selecting for the deletion of Tn10 inserted adjacent to the chromosomal fim⁺ genes and screening for the loss of mannose‐sensitive haemagglutination (HA) activity. S. typhimurium strains with Tn10 insertions in ahp were hypersensitive to peroxides, and tetracycline‐sensitive derivatives of ahp::Tn10 mutants displayed two fim mutant phenotypes. The predominant class of fim mutants did not synthesize type 1 fimbriae. A second type of fim mutant synthesized type 1 fimbriae and exhibited a conditional lipoic acid requirement for HA. A fim–lip conditional mutant synthesized type 1 fimbriae when grown in Mueller‐Hinton broth but the haemagglutinating activity of the fimbriae was dependent upon the addition of lipoic acid to the growth medium. Independentiy isolated lip mutations did not demonstrate a similar pleiotropic effect on HA. Western blots of fimbriae extracted from a fim–lip conditional mutant that was grown under permissive and restrictive conditions indicated the presence of 33 and 36.6 kDa proteins in HA⁺ fimbriae that were absent in HA^‐ fimbriae. The HA⁺ phenotype of both conditional and non‐fimbriafed mutants was restored by transformation with cloned genes encoding S. typhimurium type 1 fimbriae.