Prolonged environmental stress via a two step process selects mutants of Escherichia, Salmonella and Pseudomonas that grow at 54 degrees C.

A prolonged incubation of Escherichia, Salmonella or Pseudomonas at 48 degrees C with nalidixic acid selected mutants (T48) able to grow at 48 degrees C. A prolonged incubation at 54 degrees C of the T48 mutants selected mutants (T54) able to grow at 54 degrees C. These mutants were susceptible to the same bacteriophages as the original mesophilic strains. Auxotrophic phenotypes of Escherichia coli and Salmonella typhimurium mesophilic parents were demonstrated by these mutants if they were cultivated on minimal agar with cellobiose at 48 degrees C or 54 degrees C or on a minimal agar with glucose at 37 degrees C. The T48 alleles mapped in the gyrA region of E. coli or S. typhimurium chromosome. In S. typhimurium the T54 alleles, which permit growth at 54 degrees C, were shown by cotransductional analysis to be linked to gyrA.