Demonstration of different mutational sites controlling rhamnose fermentation in FIRN and non-FIRN rha− strains of Salmonella typhimurium: an essay in bacterial archaeology

Observations of mutation and phage-mediated transduction of rhamnose-fermentation properties were made on two groups of wild-type rhamnose-non-fermenting (rha−) strains of Salmonella typhimurium isolated in several different countries. The ‘FIRN’ group included twenty-three strains in the biotypes 15, 16, 17 and 18, and the ‘non-FIRN rha−’ group included fourteen strains in a new biotype, provisionally designated 19 X d. The two groups showed different patterns of mutational behaviour and the pattern was uniform for the different strains in each group. Non-FIRN rha− strains gave rise directly to colonies of rha+ mutant bacteria in platings on rhamnose minimal media. FIRN strains were unable to do this, but produced rha+ bacteria by two successive mutations, the first of which yielded weakly fermenting (rhaw) bacteria capable of fermenting rhamnose in peptone water, but not of utilizing it for growth on minimal medium. Rha+ recombinant bacteria were produced in transductional crosses between any one FIRN strain and any one non-FIRN rha− strain, but not in any cross between one FIRN strain and another FIRN strain or between one non-FIRN rha− strain and another non-FIRN rha− strain. These findings suggest that although the sites of the rha− mutations are different between the FIRN and non-FIRN rha− groups, they are located at identical sites on the chromosome in all strains in either group. It is concluded that all FIRN strains have probably descended from a single ancestral strain which underwent mutations at two sites in the rhamnose region of the chromosome and that all non-FIRN rha− strains have descended from a single ancestral bacterium which underwent a mutation at a third site in the rhamnose region.