Demonstration of different mutational sites controlling rhamnose fermentation in FIRN and non-FIRN rha⁻strains ofSalmonella 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 (rha w ) 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.