Growth of Escherichia coli O157:H7 at Fluctuating Incubation Temperatures

Temperature abuse of foods is often transitory and little information is available describing the response of the foodborne pathogen, Escherichia coli O157:H7, to nonisothermal and/or fluctuating temperature storage. Growth responses were determined for a mixture of three E. coli O157:H7 strains in brain heart infusion (BHI) broth as a function of temperature (static and fluctuating), initial pH (5, 6, and 7), and NaCl content (0.5, 1, 2, and 3%). Five 6-h "square-wave" fluctuating temperature regimes were used: 4 to 12, 4 to 19, 4 to 28, 8 to 19, and 12 to 28°C and compared with growth at 8, 10, 12, 19, and 28°C. The growth curves obtained from fitting the Gompertz equation for the fluctuating temperatures were compared to those obtained for the static temperatures. Increased NaCl concentration decreased growth temperature both for the fluctuating temperature growth curves and the static growth data. The cells grew or remained viable for up to 21 days under all conditions and fluctuating temperatures. Growth kinetics at fluctuating temperatures more closely approximated the higher temperature than the midpoint temperature of each cyclic range. The results indicate that transitory abuse could lead to more rapid growth than expected of E. coli O157:H7 in foods and that given sufficient time E. coli O157:H7 can grow at as low as 8°C.