Cross-protection between controlled acid-adaptation and thermal inactivation for 48 Escherichia coli strains

Given the importance of pH reduction and thermal treatment in food processing and food preservation strategies, the cross-protection between acid adaptation and subsequent thermal inactivation for 48 Escherichia coli strains was investigated. Those strains were selected among 188 E. coli strains according to their odds of growth under low pH conditions as determined by Haberbeck et al. (2015) [Haberbeck, L.U., Oliveira, R.C., Vivijs, B., Wenseleers, T., Aertsen, A., Michiels, C., Geeraerd, A.H., 2015. Variability in growth/no growth boundaries of 188 different Escherichia coli strains reveals that approximately 75% have a higher growth probability under low pH conditions than E. coli O157:H7 strain ATCC 43888. Food Microbiol. 45, 222-2301. E. coli cells were acid and nonacid-adapted during overnight growth in controlled acidic pH (5.5) and neutral pH (7.0), respectively, in buffered Lysogenic Broth (LB). Then, they were heat inactivated at 58 degrees C in non-buffered LB adjusted to pH 62 and 7.0. Thus, four conditions were tested in total by combining the different pH values during growth/thermal inactivation: 5.5/6.2, 5.5/7.0, 7.0/6.2 and 7.0/7.0. Acid adaptation in buffered LB at pH 5.5 increased the heat resistance of E. coli strains in comparison with nonacid-adaptation at pH 7.0. For instance, the median D-58-value of strains inactivated at pH 7.0 was approximately 6 and 4 min for acid-adapted and nonacid-adapted strains, respectively. For the nonacid-adapted strains, the thermal inactivation at pH 6.2 and 7.0 was not significantly (p = 0.06) different, while for the acid-adapted strains, the thermal treatment at pH 6.2 showed a higher heat resistance than at pH 7.0. The correlation between the odds of growth under low pH previously determined and the heat resistance was significant (p < 0.05). Remarkably, a great variability in heat resistance among the strains was observed for all pH combinations, with D-58-values varying between 1.0 and 69.0 min. In addition, highly heat resistant strains were detected. Those strains exhibited 1358-values between 17.6 and 69.0 min, while E. coli O157:H7 (ATCC 43888) showed D-58-values between 12 and 3.1 min. In summary, results clearly showed that adaptation of E. coil cells to constant acidic pH results in cross-protection against thermal inactivation. (C) 2016 Elsevier B.V. All rights reserved.