[Show abstract][Hide abstract]ABSTRACT: The ability of pH-dependent, stationary phase acid resistance to cross-protect Escherichia coli O157:H7 against a subsequent lethal thermal stress was evaluated using microbiological media and three liquid foods. Three strains were grown for 18 h at 37°C in acidogenic (TSB+G, final pH 4·6–4·7) and non-acidogenic (TSB-G, final pH 7·0–7·2) media to provide stationary phase cells with and without induction of pH-dependent acid resistance. The cells were then heated in BHI broth (pH 6·0) at 58°C, using a submerged coil apparatus. The TSB+G grown strains had greatly increased heat resistance, with the heating time needed to achieve a five-log inactivation, being increased two- to four-fold. The z -values of TSB+G and TSB-G grown cells were 4·7°C and 4·3°C, respectively. Increases in heat resistance with TSB+G-grown E. coli O157:H7 were also observed using milk and chicken broth, but not with apple juice. However, cross-protection was restored if the pH of the apple juice was increased from 3·5 to 4·5. The data indicate that pH-dependent acid resistance provides E. coli O157:H7 with cross-protection against heat treatments, and that this factor must be considered to estimate this pathogen's thermal tolerance accurately.
Full-text · Article · Oct 1999 · Food Microbiology
[Show abstract][Hide abstract]ABSTRACT: The extent and location of Escherichia coli O157:H7 contamination after intact apples were immersed in cold (2 degrees C) 1% peptone water containing approximately 3x10(7) CFU/ml was assessed using four apple varieties, Golden Delicious, McIntosh, Red Delicious, and Braeburn. Room temperature and refrigerated apples were used to determine the effect of temperature differential on E. coli infiltration. The highest levels of E. coli were associated with the outer core region of the apple, followed by the skin. Apples were subsequently treated by immersing them for 1 min in 2,000 mg/liter sodium hypochlorite, followed by a 1-min tapwater rinse. This treatment reduced pathogen levels by 1- to 3-log cycles but did not eliminate the microorganism, particularly from the outer core region. While E. coli was not detected in the inner core of most apples, warm fruit immersed in cold peptone water occasionally internalized the pathogen. The frequency and extent of internalization of the pathogen was less when cold apples were immersed in cold peptone water. Subsequent dye uptake studies with Golden Delicious apples indicated that approximately 6% of warm apples immersed into a cold dye solution accumulated dye via open channels leading from the blossom end into the core region. However, dye uptake did not occur when the dye solution was warmer than the apple.
Full-text · Article · Jun 1999 · Journal of food protection
[Show abstract][Hide abstract]ABSTRACT: The effects of pH and the induction of pH-dependent stationary-phase acid resistance on the radiation resistance of Escherichia coli were determined for seven enterohemorrhagic strains and one nonenterohemorrhagic strain. The isolates were grown in acidogenic or nonacidogenic media to pH levels of approximately 4.7 and 7.2, respectively. The cells were then transferred to brain heart infusion (BHI) broth adjusted to pH 4.0, 4.5, 5.0, and 5.5 (with HCl) that was preequilibrated to 2 degrees C, and cultures were then irradiated using a 137Cs source. Surviving cells and the extent of injury were determined by plating on BHI and MacConkey agars both immediately after irradiation and after subsequent storage at 2 degrees C for 7 days. Decreasing the pH of the BHI in which E. coli was irradiated had relatively little effect on the microorganism's radiation resistance. Substantial differences in radiation resistance were noted among strains, and induction of acid resistance consistently increased radiation resistance. Comparison of E. coli levels immediately after irradiation and after 7 days of refrigerated storage suggested that irradiation enhanced pH-mediated inactivation of the pathogen. These results demonstrate that prior growth under conditions that induce a pH-dependent stationary phase cross-protects E. coli against radiation inactivation and must be taken into account when determining the microorganism's irradiation D value.
[Show abstract][Hide abstract]ABSTRACT: 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 reference strain) were cultured individually for 18 h at 37 degrees C in tryptic soy broth (TSB) plus 1% dextrose and in TSB without dextrose to yield acid resistance induced and noninduced stationary-phase cells, respectively. These cultures were then inoculated into brain heart infusion broth (BHI) supplemented 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 previously 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 least damaging to cells; lactic acid was the most detrimental. The relative activity of the other acids was strain dependent. Inducing pH-dependent acid resistance increased the already substantial acid tolerance of stationary-phase E. coli. The extent of injury also varied with acid and strain, with as much as a 5-log-cycle differential between BHI agar and MA CFU counts. The accurate determination of the survival of enterohemorrhagic E. coli in acidic foods must take into account the biological variability of the microorganism with respect to its acid resistance and its ability to enhance survival through the induction of physiological stress responses.
[Show abstract][Hide abstract]ABSTRACT: Three strains (932, Ent-C9490, and SEA13B88) of Escherichia coli O157:H7 were used to determine the effectiveness of low-dose gamma irradiation for eliminating E. coli O157:H7 from apple juice or cider and to characterize the effect of inducing pH-dependent, stationary-phase acid resistance on radiation resistance. The strains were grown in tryptic soy broth with or without 1% dextrose for 18 h to produce cells that were or were not induced to pH-dependent stationary-phase acid resistance. The bacteria were then transferred to clarified apple juice and irradiated at 2 degrees C with a cesium-137 irradiator. Non-acid-adapted cells had radiation D values (radiation doses needed to decrease a microbial population by 90%) ranging from 0.12 to 0.21 kGy. D values increased to 0.22 to 0.31 kGy for acid-adapted cells. When acid-adapted SEA13B88 cells were tested in five apple juice brands having different levels of suspended solids (absorbances ranging from 0.04 to 2.01 at 550 nm), radiation resistance increased with increasing levels of suspended solids, with D values ranging from 0.26 to 0.35 kGy. Based on these results, a dose of 1.8 kGy should be sufficient to achieve the 5D inactivation of E. coli recommended by the National Advisory Committee for Microbiological Criteria for Foods.
Full-text · Article · Dec 1998 · Applied and Environmental Microbiology
[Show abstract][Hide abstract]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 degrees 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 to 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.
Full-text · Article · Nov 1996 · Applied and Environmental Microbiology