Minimal medium recovery of heat-treated Salmonella typhimurium LT-2 has been expressed as the reduced viability on trypticase soy agar supplemented with 0.5% yeast extract (TSY) relative to a glucose-salts (M-9) agar. Incubation of S. typhimurium LT-2 in water at 50 C for 15 min did not change the sedimentation patterns of deoxyribonucleic acid (DNA) in alkaline sucrose gradients. The same pattern was obtained if the heated cells were further incubated for 15 min at 37 C in M-9 broth. However, a marked increase in DNA single-strand breakage accompanied by a loss of viability was observed after a similar incubation of heated bacteria in TSY broth. If heated bacteria were incubated in M-9 broth before TSY broth, there was a decrease in the single-strand breakage occurring in the TSY broth. This decrease is believed to be the result of repair of heat-induced damage. We conclude that minimal medium recovery after heat treatment is due to DNA damage caused by sequential exposure to heat and TSY medium, such damage not occurring after sequential exposure to heat and M-9 medium.
