Changes in heat tolerance of Escherichia coli O157: H7 after exposure to acidic environments

Acid-adapted bacterial cells are known to have enhanced tolerance to various secondary stresses However, a comparison of heat tolerance of acid-adapted and acid-shocked cells of Escherichia coli O157:H7 has not been reported. D- and z-values of acid-adapted acid-shocked, and control cells of an unusually heat-resistant strain (E0139) of E. coli O157:H7 as well as two other strains of E. coli O157:H7 were determined based upon the number of cells surviving heat treatment at 52, 54 or 56 degrees C in tryptic soy broth (pH 7.2) for 0, 10, 20 or 30 min. The unusual heat tolerance of E. coli O157:H7 strain E0139 was confirmed. D-values for cells from 24-h cultures were 100.2, 28.3, and 6.1 min at 52, 54 and 56 degrees C, respectively, with a z-value of 3.3 degrees C. The highest D-values of other E. coli O157:H7 strains were 13.6 and 9.2 min at 52 and 54 degrees C, respectively, whereas highest D-values of non-O157:H7 strains were 78.3 and 29.7 min at 52 and 54 degrees C. D-values of acid-adapted cells were significantly higher than those of unadapted and acid-shocked cells at ail temperatures tested in a previous study, we observed that both acid-adapted cells and acid-shocked cells of strain E0139 had enhanced acid tolerance. This suggest that different mechanisms protect acid-adapted and acid-shocked cells against subsequent exposure to heat or an acidic environment. The two types of cells should be considered separately when evaluating survival and growth characteristics up on subsequent exposure to different secondary stress conditions. (C) 1999 Academic Press.