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

Abstract

The behaviour of cold-adapted, log phase Escherichia coli broth cultures during incubation at 2°C or 6°C for upto 8 days, and during subsequent incubation at 12°C, was determined by measurement of absorbance values at 600 nm (A600), 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 A600 values and the mean length of cells remained constant for cultures incubated at 2°C; however, numbers of cfu recovered on PCA declined by about 1 log cfu over 8 days, while the numbers of cfu recovered on VRBA declined by about 1 log cfu during the first day, and by about a further log cfu by day 8. For cultures incubated at 6°C, A600 values increased about 0·6 log A600 units during the first 4 days and declined by less than 0·1 log A600 unit during the next 4 days. The numbers of cfu recovered on PCA increased by about 0·5 log cfu unit during the first day at 6°C 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 day at 6°C, 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 μ m during the first day at 6°C, but increased to 8 μ m between the fourth and eighth days, with the mean length of the longest 10% of cells increasing from 6 to 18 μ m. For cultures incubated at 12°C after incubation at 2°C or 6°C 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°C proceeded to sustained exponential growth, while cells in cultures that had been incubated at 6°C elongated during incubation at 12°C between 10 and 30 h. 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°C. The observations may have implications for the control of mesophilic pathogen proliferation in raw meats and other chilled foods.