The effect was studied of supplementing the recovery medium with lysozyme, glucose, NaCl, and MgSO4 on the apparent heat resistance of four strains of Bacillus cereus . The composition of an optimal medium (allowing the germination and growth of all surviving spores) and of a selective medium (inhibiting injured spores) was then determined: the optimal medium consisted of nutrient agar supplemented with 50 × 10-6 g of lysozyme and 0.5 g of MgSO4 per liter. The selective medium had the same composition but was supplemented with 15 g of NaCl per liter. The injury and lethality were analyzed of heat treatment of spores of four Bacillus cereus strains in phosphate buffer and in mechanically separated poultry meat. The four strains tested exhibited great differences in injury and death rates, and considerable differences could be observed in survivor counts when spores were suspended in buffer or in meat. Moreover, variability in behavior when the strains were heat treated in buffer could not be extrapolated to the same strains suspended in food. These data showed that it is necessary to be very careful when evaluating potential hazards from B. cereus spores in various foods; heating medium and strain selection must be considered to evaluate the efficiency of heat treatments.
Keywords: Spores; heat treatments; injury; recovery media.