Role of nalidixic acid in isolation of Salmonella typhimurium strains capable of growth at 48 degrees C.

Salmonella typhimurium thermotolerant mutants dependent on the presence of nalidixic acid for growth at 48 degrees C were isolated and designated nalidixic acid-dependent, thermotolerant mutants, naldttl. Genetic mapping revealed that naldttl alleles map within the gyrA gene. When S. typhimurium strain Q was plated in the dark on nutrient agar containing nalidixic acid (20 micrograms/ml) as a photosensitizer and briefly exposed to white light or near VU light prior to incubation at 42 degrees C, nalidixic acid-resistant mutants arose in about 16 h at frequencies of 5 x 10(-8) for white light and 1 x 10(-6) for near UV light. About 10% of these nalidixic acid-resistant mutants derived from photodynamic mutagenesis exhibited the thermotolerant characteristic.