Abstract
Treatment by heat (130 to 155 C) or by nitrous acid induces stable mutants and also unstable lactose-fermentation mutants in E. coli and unstable auxotrophs in both E- coli and B. subtilis. The unstable fermentation mutants were isolated from some of the sectored colonies on a medium (Penassay-tetrazolium-lactose agar) which also permits detection of mutants with low levels of [beta]-galactosidase. The back-mutation rates of unstable auxtrophs were 10-4 to 4 x 10-2. The unstable strains can be maintained indefinitely.