A factor that plays a role in acid resistance is the mechanism of cross-protection between the different stresses microorganisms are exposed to. For example, exposure of the microorganism to the various stresses they encounter in the course of food production, e.g., heat processing, dehydration, or acidification, can elicit a higher tolerance to stresses encountered passing through the stomach. A study of the response of B. cereus to low pH revealed that the strains which were initially incubated for 40 min at pH 6.3, exhibited a marked increase in survival when exposed later to pH 4.6 for 20 min [19]. Indeed, a sublethal acidic environment can
trigger an adaptive response that protects the bacterium during subsequent incubations at lethal acidic pH. This mechanism is known as acid tolerance response (ATR) and plays an important
role in the adaptation of intestinal pathogens to the pH of the stomach [20e22]. B. cereus vegetative cells are also able to induce ATR [23,24]. The ATR of B. cereus may involve (i) F 0 F 1 ATPase and/or glutamate decarboxylase (implicated in pH i homeostasis), (ii) modifications of metabolism and (iii) synthesis of proteins which act as protect and/or repair factors