SUMMARY: The production of fimbriate (Fim+) recombinants was observed in transductional crosses between different pairs of wild-type strains of different biotypes of Salmonella typhimurium. Fim+ recombinants were readily produced in transductions from Fim+ donor strains to Fim− recipient strains and, less frequently, between some pairs of Fim− strains, for example, between almost any strain of the FIRN biogroup (Fim− Inl− Rha− Bxyl−) and many strains of the non-FIRN Fim− biogroup. None of numerous crosses between different pairs of FIRN strains gave Fim+ recombinants, suggesting that the fim mutation was present at the same intragenic site in all FIRN strains. FIRN strains are thought to have descended from a single ancestral FIRN bacterium which originated by a series of mutations from a strain of the common biotype la (Fim+ Inl+ Rha+ Bxyl+). Two FIRN-like (Fim− Inl+ Rha− Bxyl−) strains that did not yield Fim+ recombinants in crosses with FIRN strains were probably wild-type Inl+ mutants from FIRN strains.
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