Induction of a lag phase by chiller temperatures in Escherichia coli growing in broth or on pork

Abstract

When log phase cultures of Escherichia coli in brain–heart infusion (BHI) were cooled from 12°C to temperatures below 7°C (the minimum required for growth), the optical densities of cultures increased at declining rates for up to 6 days during incubation at temperatures between 6 and 3°C inclusive, but did not increase at temperatures ≤2°C. However, the numbers of E. coli recovered from cultures on selective or non-selective agars were similar at all times of incubation up to 8 days at temperatures below the minimum for growth. From optical density measurements it appeared that when log phase cultures were cooled to 2°C, then returned to 15°C, a lag developed with time at 2°C to reach a maximum of about 1 h after about 4 h. When cultures were returned to 12°C after times at 2°C between 0·5 and 8 days, the time during which optical densities did not increase was constant at about 2 h, but the initial rate of optical density increase declined with time at 2°C. On pork fat tissue the lag time, determined by increases in the numbers of E. coli cfu on tissues inoculated with cells returned from incubation at 2°C for 16 h to a growth permitting temperature between 7 and 30°C, was longer than the lag time determined from optical density measurements of broth cultures subjected to the same temperature regime. Lag times for E. coli adapted to and growing on normal pH pork lean tissue were longer than for E. coli on fat tissue, while unadapted E. coli did not initiate growth on lean tissue, after incubation at 2°C, at 12°C or lower temperatures. The observations indicate that lag phase development in log phase E. coli subjected to chiller temperatures is complex, and that prediction of lag resolution in log phase cells exposed to temperatures that fluctuate around the minimum for growth will require modeling of lag induction as well as lag resolution.