Thermal Resistance of Single Strains of Shiga Toxin-Producing Escherichia coli O121 :H19 and O157:H7 Based on Culture Preparation Method and Osmolyte-Reduced Water Activity

摘要

Bacterial exposure to stress, such as reduced water activity (a_w), can increase thermal resistance. Pathogen thermal resistance studies on low-a_w foods use a variety of methods to inoculate food, as well as strategies to reduce a_w, which can influence observations. This study investigated effects of culture preparation method and osmolyte-induced a_w on thermal resistance of two Shiga toxin-producing Escherichia coli (STEC) strains (O121 :H19 and O157:H7) challenged with isothermal conditions, determining D- and z-values for each isolate (56, 59, and 62°C). Tryptic soy broth (TSB) and agar (lawn cultures) were compared. D-values of broth cultures were significantly and consistently larger than those of lawn cultures, and O121 was significantly more resistant than O157, but only at 56°C (P ＜ 0.05). To compare potential effects of a_w on STEC thermal resistance, cells were suspended in osmolyte solutions with varying a_w: high (TSB, a_w 0.99), intermediate (61% glycerol or 26% NaCl, a_w 0.75), and low (82% glycerol, a_w 0.5). In most instances, STEC strains in high-a_w broth exhibited greater heat resistance compared to reduced-a_w solutions, with the exception of the glycerol intermediate-a_w solution (a_w 0.75). Magnitudes varied with strain and temperature. The z-values of lawn cultures were significantly lower than those of broth cultures (P ＜ 0.05), but there were few differences between high-a_w and reduced-a_w samples. There were no significant differences of z-values based on strain type. These results highlight that thermal resistance can be affected by culture preparation and that osmolyte-induced changes to a_w influence thermal inactivation of STEC by varying magnitudes. These results emphasize the challenges of extrapolating results from laboratory inactivation kinetic experiments to determine the inactivation of low-a_w foods, especially those considered dry in nature.