RECOVERY OF HEAT-INJURED LISTERIA-MONOCYTOGENES

Concern has been expressed that the ability of heat-injured Listeria monocytogenes cells to resuscitate during refrigerated storage of food may lead to underestimation of their heat resistance. The recovery of heat-injured L. monocytogenes was therefore examined as a function of incubation temperature and composition of recovery medium. Heat-injured cells exhibited a broad optimum temperature for recovery centered around 20-25-degrees-C. The best recovery medium of those tested was blood agar. Incubation of cells in broth or chicken slurry at 5-degrees-C (cold-enrichment) did not allow repair of potentially lethal injury i.e. it did not allow recovery of cells that would otherwise have died if incubated at a higher temperature. In some cases incubation of heat-injured cells at 5-degrees-C resulted in death of a proportion of the population. Repair of sublethal heat-injury, measured as the time of incubation in tryptone soya broth needed to regain the ability to grow on Listeria selective agar, was slower and less complete at 25-degrees-C than at 2-degrees-C; repair took 10-15 h at 25-degrees-C compared with 8-12 days at 5-degrees-C. Refrigeration of heat-treated foods should not therefore increase the risk that heat-injured cells will recover from the heat treatment.