The behaviour of log phase Escherichia coli at temperatures that fluctuate about the minimum for growth.

AIMS

To investigate the behaviour of cold-adapted, log phase Escherichia coli exposed to temperatures that fluctuate below and above the minimum for growth.

METHODS AND RESULTS

Log phase E. coli cultures were incubated at a constant temperature of 2, 4 or 6 degrees C or with temperatures allowed to increase from those temperatures for 35 min, to 10 degrees C, at 6-, 12- or 24-h intervals, as commonly occurs during retail display of chilled foods. At suitable intervals for each culture, the optical absorbance value was determined using a spectrophotometer, the forward angle light scatter was determined using a flow cytometer, and portions were spread on plate count agar for enumeration of colony forming units (CFU). Numbers of CFU decreased by 3 log units or increased by 1 log unit for cultures incubated at 6 degrees C for 17 days without or with temperatures fluctuations at < or =12-h intervals, respectively. Cells elongated when cultures were incubated at 4 or 2 degrees C with temperatures fluctuating at 6-h intervals, and at 6 degrees C at constant or fluctuating temperatures, but cells did not elongate in cultures incubated at a constant temperature of 2 or 4 degrees C.

SIGNIFICANCE AND IMPACT OF THE STUDY

The minimum growth temperature of E. coli is assumed to be > or =7 degrees C. Elongated cells were able to divide when temperatures rose from 6 degrees C to above 7 degrees C for <45 min at < or =12-h intervals. Such temperature fluctuations may be experienced by chilled foods during defrosting cycles of retail display cases. The finding that cells behave differently under fluctuating than at constant temperatures may significantly affect understanding of appropriate temperatures for the safe storage of chilled foods and for predictive modelling of bacterial growth in such foods.