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Aims: The aim of the study was to evaluate the effect of habituation at different pH conditions on the acid resistance of Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella enterica serotype Typhimurium, and to identify potential differences between the adaptive responses of the three pathogens. Methods: Stationary phase cells of L. monocytogenes, E. coli O157:H7 and S. Typhimurium, grown in glucose-free media, were exposed to pH 3AE5 broth directly or after habituation for 90 min at various pH conditions from 4AE0 to 6AE0. Survivors at pH 3AE5 were determined by plating on tryptic soy agar and incubating at 30°C for 48 h. The kinetics (death rate) of the pathogens at pH 3AE5 was calculated by fitting the data to an exponential model. Results: Habituation to acidic environments provided protection of the pathogens against lethal acid conditions. This acid protection, however, was found to be pH dependent. For example, for E. coli O157:H7 an increased acid resistance was observed after habituation at a pH range from 4AE0 to 5AE5, while the maximum acid tolerance was induced at pH 5AE0. Furthermore, the effect of low pH habituation was different among pathogens. For L. monocytogenes, E. coli O157:H7 and S. Typhimurium, the pH range within which habituation resulted to increased acid resistance was 5AE0-6AE0, 4AE0-5AE5 and 4AE0-5AE0, respectively, while the maximum acid tolerance was induced after habituation at pH 5AE5, 5AE0 and 4AE5, respectively. Significance: Acid stress conditions are common within current food processing technologies. The information on adaptive responses of L. monocytogenes, E. coli O157:H7 and S. Typhimurium after habituation to different pH environments provided in the present study, could lead to a more realistic evaluation of food safety concerns and to a better selection of processes in order to avoid adaptation phenomena and to minimize the potential for food safety risks.

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