The behaviour of log phase Escherichia coli at temperatures below the minimum for sustained growth

The behaviour of cold-adapted log phase Escherichia coli broth cultures during incubation at 2 degreesC or 6 degreesC for upto 8 days, and during subsequent incubation at 12 degreesC, was determined by measurement of absorbance values at 600 nm (A(600)), enumeration of colony forming units (cfu) on plate count agar (PCA) and violet red bile agar (VRBA), and measurement of the length of cells viewed under phase contrast illumination. The A(600) values and the mean length of cells remained constant for cultures incubated at 2 degreesC; however, numbers of cfu recovered on PCA declined by about 1 log cfu over 8 clays, while the numbers of cfu recovered on VRBA declined by about 1 log cfu during the first day, and by about a further log ctu by day 8. For cultures incubated at 6 degreesC A600 values increased about 0.6 log A(600) units during the first 4 days and declined by less than 0-1 log A(600) unit during the next 4 Clays. The numbers of cfu recovered on PCA increased by about 0.5 log cfu unit during the first day at 6 degreesC and declined by about 1 log cfu during the subsequent 7 days. The numbers of cfu recovered on VRBA did not increase during the first clay at 6 degreesC, and at that and subsequent times were between 0.3 and 0.8 log cfu less than the log numbers recovered on PCA. The mean lengths of cells declined from 5 to less than 4 pm during the first day at 6 degreesC, but increased to 8 mum between the fourth and eighth days, with the mean length of the longest 10% of cells increasing from 6 to 18 AM For cultures incubated at 12 degreesC after incubation at 2 degreesC or 6 degreesC for 4 and 8 days, both A600 values and enumeration of colonies on PCA indicated the initiation of growth after about 15 h. However cultures that had been incubated at 2 degreesC proceeded to sustained exponential growth, while cells in cultures that had been incubated at 6 degreesC elongated during incubation at 12 degreesC between 10 and 30h. The division of elongated cells to cells of normal size resulted in numbers of cfu increasing at rates greater than the exponential growth rate at 12 degreesC. The observations may have implications for the control of mesophilic pathogen proliferation in raw meats and other chilled foods. (C) 2002 Elsevier Science Ltd.