Usefulness of nutrient agar supplemented with NaCl in detection of heat-injured Escherichia coli.

We examined the effect of composition of agar medium for the detection of viable cells of Escherichia coli JCM1649T. The nutrient agar supplemented with NaCl (NA) and two commercial products of plate count agar (standard method agar) exhibited practically the same level of colony count for intact cells, but NA was the most suitable for the detection of sublethally injured cells induced by heat treatment at 55°C for 15 min. Addition of 0.5% NaCl to the commercially-available plate count agar improved the detection rate of heat-injured cells, while glucose in the agar medium was found to decrease the detection rate. NA was shown to be useful as repair medium in the selective detection of heat-injured E. coli.