Comparison of sublethal injury induced in Salmonella enterica serovar Typhimurium by heat and by different nonthermal treatments

We have studied sublethal injury in Salmonella enterica serovar Typhimurium caused by mild heat and by different emerging nonthermal food preservation treatments, i.e., high-pressure homogenization, high hydrostatic pressure, pulsed white light, and pulsed electric field. Sublethal injury was determined by plating on different selective media, i.e., tryptic soy agar (TSA) plus 3% NaCl, TSA adjusted to pH 5.5, and violet red bile glucose agar. For each inactivation technique, at least five treatments using different doses were applied in order to cover an inactivation range of 0 to 5 log units. For all of the treatments performed with a technique, the logarithm of the viability reductions measured on each of the selective plating media was plotted against the logarithm of the viability reduction on TSA as a nonselective medium, and these points were fitted by a straight line. Sublethal injury between different techniques was then compared by the slope and the y intercept of these regression lines. The highest levels of sublethal injury were observed for the heat and high hydrostatic pressure treatments. Sublethal injury after those treatments was observed on all selective plating media. For the heat treatment, but not for the high-pressure treatment, sublethal injury occurred at low doses, which were not yet lethal. The other nonthermal techniques resulted in sublethal injury on only some of the selective plating media, and the levels of injury were much lower. The different manifestations of sublethal injury were attributed to different inactivation mechanisms by each of the techniques, and a mechanistic model is proposed to explain these differences.