Influence of growth temperature on inactivation and injury of Escherichia coli O157:H7 by heat, acid, and freezing

The influence of growth temperature on heat-, lactic acid-, and freeze-induced inactivation and injury of Escherichia coli O157:H7 in 0.1% peptone water was investigated. Three strains of E. coli O157:H7 isolated respectively from salami, apple cider, and ground beef were evaluated. Growth of strains at 10 degrees C compared with growth at 37 degrees C had a significant impact on reducing (P < 0.01) D values obtained for heating (D(H) value), acid exposure (D(A) value), and freezing (D(F) value), with the exception of the cider strain stored in lactic acid solutions. When strains were cultivated at 10 and 37 degrees C and heated at 54 and 56 degrees C, the salami strain possessed the highest (P < 0.01) D(H) values (5.9 to 59.7 min). When grown at 10 degrees C, the beef strain had the lowest (P < 0.01) D(H) values after heating at 52, 54, and 56 degrees C (11.2, 4.1, and 2.5 min, respectively). The salami strain grown at 10 degrees C had the highest (P < 0.01) D(A) values in all concentrations of lactic acid. When grown at 37 degrees C, the salami strain had the highest (P < 0.01) D(A) values after storage in 0.1 and 0.25% lactic acid, while D(A) values for the salami and beef strains did not differ (P > 0.05) when stored in 0.5% lactic acid. Portions of strain populations were sublethally injured by heat and lactic acid treatments, as evidenced by the inability of injured organisms to form colonies on tryptone soya agar containing 2% NaCl. Strains cultured at 10 degrees C were more susceptible to sublethal heat injury than strains cultured at 37 degrees C. Storage of test strains at -20 degrees C for 7 months resulted in a 4- to 6-log CFU/ml reduction in viable population, but induced only minimal sublethal injury. After 5 months at -20 degrees C, strains cultured at 10 degrees C were more sensitive to freeze inactivation than strains cultured at 37 degrees C. When grown at 10 and 37 degrees C and stored at -20 degrees C for 7 months, the cider strain possessed higher (P < 0.01) D(F) values than the beef and salami strains.