99D-33 |
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S. D. ADHIKARI, D. Elhanafi, and M. A. Drake. Dept. of Food Science, North Carolina State Univ., Schaub Hall, Box 7624, Raleigh, NC 27695-7624 E. coli O157:H7 (EC) has emerged as a food and waterborne pathogen of major public health concern. Several studies have documented the stress response of EC to sublethal stresses. Acidification is used as a hurdle in many minimally processed foods. Previous work has indicated that decreased pH (pH 5.5) may enhance survival and virulence factor expression of EC. The objective of this research was to determine the effect of different organic acids and pH on the expression of three virulence factor genes (stx2, hlyA, eaeA) in E. coli O157:H7 and the implication of the stress response regulator RpoS. Gene fusions containing lacZ inserted into the stx2, eaeA or hlyA genes were created in E. coli O157:H7 with and without a functional rpoS gene. Overnight cultures were inoculated (10E3 CFU/mL) into tryptic soy broth acidified with citric, maleic, lactic, or hydrochloric acid at pH 6.0, 5.5, 5.0, or 4.5. â-galactosidase activity of stressed or control cells (neutral pH, no acid) was determined at time 0, 2, 4, 8, 12, 18, 24, 32, 48 and 56 hours. Viable cells were enumerated at each time point using tryptic soy agar pour plates. Experiments were conducted in duplicate with repeated measures. Exposure to decreased pH enhanced eaeA expression (p<0.05) and effects were larger for citric and maleic acid compared to lactic or hydrochloric acid (p<0.05). Expression of eaeA was also increased under acidic conditions in the absence of RpoS. The effect of decreased pH with different organic acids on stx2 and hlyA expression were significant but growth phase dependent. Sublethal acid stress impacts virulence factor expression of E. coli O157:H7 and these effects are impacted by pH and acid type.
Session 99D, Food Microbiology: General
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