29F-22 |
Influence of acetic, citric, and lactic acid on E. coli O157:H7 membrane lipid composition, verotoxin secretion, and acid resistance in simulated gastric fluid |
H. G. YUK and D. L. Marshall. Dept. of Food Science & Technology, Mississippi State Univ., Campus Mailstop 9805, Mississippi State, MS 39762-9805 Organic acids are used to decontaminate animal carcasses and as antimicrobials in fermented foods. Acid adapted pathogens have more acid resistance, but the combined relationship between organic acid exposure and membrane fluidity, verotoxin secretion, and acid resistance in E. coli O157:H7 is unreported. The objective of this study was to determine the influence of acetic, citric, and lactic acid adaptation on changes in membrane lipid composition, verotoxin secretion, and acid resistance of E. coli O157:H7 (HEC) and its rpoS mutant (HEC-RM). Acid adapted cells were prepared by incubating (37oC) cultures at pH 7.3 for 18 hr without acids (control), then transferring to pH 6.4 for 18 hr, followed by an additional 24 hr at pH 5.4. Each pH was achieved using the individual acids. After incubation at each pH, cells were measured for acid resistance in simulated gastric fluid (pH 1.5), membrane lipid composition by gas chromatography, and verotoxin concentration by ELISA. The ratio of palmitic to cis-vaccenic acids increased (membrane fluidity decreased) in citric and lactic acid adapted cells, and decreased (fluidity increased) in acetic acid adapted cells. Extracellular verotoxin concentration decreased with decreasing pH. Lactic acid adapted HEC cells had highest D-value (29.0 min) and acetic acid adapted cells had lowest value (12.6 min) in gastric fluid. D-values of citric and acetic acid adapted HEC-RM cells were similar to control cells but D-value increased to 27.9 from 14.2 min in lactic acid adapted HEC-RM cells. Changes in membrane lipid composition, verotoxin concentration, and acid resistance in HEC and HEC-RM were organic acid dependent but not pH dependent. The deletion of rpoS gene did not affect these changes, but it did affect acid resistance in citric acid adapted cells. These results showed that acetic acid was the most effective to minimize populations of E. coli O157:H7 at equivalent pH.
Session 29F, Food Microbiology: Control of foodborne microorganisms by antimicrobials
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