Effect of the DNA replication velocity on the response of Salmonella typhimurium to mild heating.

Changes in the DNA replication velocity of Salmonella typhimurium following mild heat stress (52 degrees C) were studied independently of the major physiological parameter of growth rate, using thymine-requiring mutant strains derived from Salm. typhimurium LT2. The isolated mutant strains BM1 or BM2, grown either as batch or chemostat cultures, showed a greater sensitivity to 52 degrees C heat stress when grown on a minimal medium containing near-limiting concentrations of thymine, compared with growth in the presence of excess thymine. Radiolabelling experiments provided evidence for alterations in the velocity of DNA replication upon growth on different thymine concentrations, independent of the growth rate. Thus, replicating DNA was implicated as the major site of injury after mild moist heat stress.