Behaviour of cold adapted, log phase Escherichia coli at temperatures near the minimum for growth

The behaviour of cold adapted, log phase Escherichia coli at temperatures near 7°C, the minimum for growth, was investigated because discrepancies were found in the relationship between increases in absorbance values and the number of colony forming units (cfu). At suitable intervals, absorbance readings (A600) were determined, cell length was determined by microscopy and/or flow cytometry, and colonies were enumerated on plate count agar. Cells elongated to form filaments during incubation at constant temperatures >4°C and <15°C. At constant temperatures >7°C, numbers of colony forming units (cfu) and A600 increased with time, but at constant temperatures ≤7°C, numbers of cfu decreased while A600 increased. When cells incubated at <7°C were incubated at temperatures >7°C, elongated cells lengthened further before dividing into cells of normal size. Cells elongated when cultures were incubated at 4 or 2°C with increases to 10°C for 35 min at 6 h intervals. Elongated cells were able to divide when temperatures were raised from 6°C to >7°C for <45 min. at ≤12 h intervals. Such temperature fluctuations may be experienced by chilled foods during defrosting cycles of retail display cases and may have important implications for understanding of appropriate temperatures for safe storage, assessment of microbiological risks, and for predictive modeling of bacterial growth for chilled foods.; The mechanism for filamentation at low temperatures is not known. Therefore, the proteomic response of E. coli at temperatures just below the minimum for growth was examined. A number of proteins that were upregulated are involved in protein folding and degradation, carbohydrate metabolism, electron transport, and the TCA cycle and downregulated proteins were involved in protein synthesis or anaerobic carbon metabolism and energy generation. Cells appear to invoke the stringent response due to insufficient energy to maintain growth. Polymerization of the cell division protein FtsZ requires guanosine triphosphate (GTP), which may be limiting for cell division because GTP is diverted to guanosine pentophosphate (pppGpp) during the stringent response. DNA replication generally ceases under unfavourable conditions but glucose inhibited division protein B (GidB) was upregulated at 6°C, which may allow DNA replication to continue. These studies provide insight into the physiological response of E. coli at temperatures near the minimum for growth.