Acid tolerance and gad mRNA levels of Escherichia coli O157:H7 grown in foods

We examined the acid tolerance and gad mRNA levels of Escherichia coli O157:H7 (three strains) and nonpathogenic E. coli (strains K12, W1485, and B) grown in foods. The E. coli cells (approximately 30,000 cells) were inoculated on the surface of 10 g of solid food samples (asparagus, broccoli, carrot, celery, cucumber, eggplant, ginger, green pepper, onion, potato, radish, tomato and beef) and in 10 ml of cowʹs milk, cultured statically at 10–25 °C for 1–14 days, and subjected to an acid challenge at 37 °C for 1 h in LB medium (pH 3.0). When grown at 20 and 25 °C in all foods, except for tomato and ginger, the strains showed a stationary-phase specific acid tolerance. The acid tolerance of the O157 strains changed depending on the types of foods (3–10% survival), but was clearly lower than that of the cells grown in EC medium (more than 90% survival). Tomato and ginger induced relatively high acid tolerances (10–30% survival) in the O157 strains irrespective of the growth phase, probably because of their acidity. No remarkable difference was observed in the acid tolerance between the O157 and nonpathogenic strains grown in all foods. When grown at 10 and 15 °C in the foods and EC medium, none of the strains showed the stationary-phase specific acid tolerance. In beef, broccoli, celery, potato and radish, the acid tolerance showed a tendency to decrease with the prolonged cultivation time. In other foods, the acid tolerance was almost constant (about 0.1% survival) irrespective of the growth stage. The mRNA level of glutamate decarboxylase genes (gadA and gadB) correlated to the acid tolerance level when the E. coli cells were grown at 25 °C, but was very low even in the stationary phase when the E. coli cells were grown at 15 °C or below.