The Impact of Organic Acids and pH on the Virulence Factor Expression of E. coli O157:H7.

Abstract

Acidification is used as a hurdle in many minimally processed foods. Decreased pH (pH 5.5) may enhance survival and virulence factor expression of E. coli O157:H7 (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 EC. Gene fusions containing the lacZ gene 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 into tryptic soy broth acidified with citric, malic, lactic, or hydrochloric acid at pH 6.0, 5.5, 5.0, or 4.5 or apple juice (pH 3.5). Cell growth characteristics were characterized, and &#946;-galactosidase activity of stressed or control cells (neutral pH, no acid) was subsequently determined to follow virulence factor production. Production of all three virulence factors was increased at pH 5.5 or 5.0 compared to production at neutral pH (p<0.05). Acid type impacted production of EaeA and StxII, but had no effect on HlyA. Production of StxII and HlyA was not detected in apple juice. At pH 5.5, cell growth was slowest in lactic acid, followed by malic and citric acids then HCl. At pH 5.0, the slowest growth was observed in citric acid, followed by malic acid, lactic acid and HCl. At pH 4.5, no growth occurred in citric, malic and lactic acids, and cell numbers decreased over a period of 5 days. In HCl at pH 4.5, cells grew slowly and increased by 2 logs over a 5-day period. Sublethal acid stress impacts virulence factor expression of E. coli O157:H7 and these effects are impacted by pH and acid type.