pH-dependent stationary-phase acid resistance response of enterohemorrhagic Escherichia coli in the presence of various acidulants

The effect of acidulant identity on the pH-dependent stationary-phase acid resistance response of enterohemorrhagic Escherichia coli was studied. Nine strains of E. coli (seven O157:H7, one O111:H-, and one biotype 1 referenc e strain) were cultured individually for 18 h at 37 degrees C in tryptic so y broth (TSB) plus 1% dextrose and in TSB without dextrose to yield acid re sistance induced and noninduced stationary-phase cells, respectively. These cultures were then inoculated into brain heart infusion broth (BHI) supple mented with 0.5% citric, malic, lactic, or acetic acid and adjusted to pH 3 .0 with HCl. The BHI tubes were incubated at 37 degrees C for up to 7 h and samples were removed after 0, 2, 5, and 7 h and plated for counting CFU on BHI agar and MacConkey agar (MA). The results were compared to data previo usly obtained with HCl only. Acid resistance varied substantially among the isolates, being dependent on the strain, the acidulant, and the induction of pH-dependent acid resistance. Hydrochloric acid was consistently the lea st damaging to cells; lactic acid was the most detrimental. The relative ac tivity of the other acids was strain dependent. Inducing pH-dependent acid resistance increased the already substantial acid tolerance of stationary-p hase E. coli. The extent of injury also varied with acid and strain, with a s much as a 5-log-cycle differential between BHI agar and MA CFU counts. Th e accurate determination of the survival of enterohemorrhagic E. coli in ac idic foods must take into account the biological variability of the microor ganism with respect to its acid resistance and its ability to enhance survi val through the induction of physiological stress responses.