Culturing enterohemorrhagic Escherichia coli in the presence and absence of glucose as a simple means of evaluating the acid tolerance of stationary- phase cells

Abstract

Prior growth of seven enterohemorrhagic and one nonenterohemorrhagic strains of Escherichia coli in tryptic soy broth with (TSB+G) and without (TSB-G) 1% glucose was evaluated for its effect on acid tolerance. The final pHs of 18-h TSB+G and TSB-G cultures were 4.6 to 5.2 and 6.9 to 7.0, respectively. Cells were then transferred to brain heart infusion broth adjusted to pH 2.5 or 3.0 with HCl, incubated at 37°C for up to 7 h, and assayed periodically for viable populations with brain heart infusion and MacConkey agars. All enterohemorrhagic strains were acid resistant (<0.5 log decline after 7 h) when initially cultured in TSB+G, but substantial differences in acid tolerance were observed among strains cultured in TSB-G (log declines ranged from <0.3 to >3.8). The results indicated that prior growth in a medium with and without a fermentable carbohydrate is a convenient way of studying the induction of acid tolerance, that acid inactivation is preceded by a period of acid injury, and that pH-independent and pH-dependent stationary-phase acid tolerance phenotypes may exist among strains of enterohemorrhagic E. coli.