Effect of Heat Shock on Recovery of Escherichia coli from Drinking Water

Abstract

Recent studies on the survival of bacteria in the aquatic environment have suggested that bacteria in water may enter a ‘viable non-culturable' phase in response to stress. Such bacteria are a cause of concern if pathogenic, because they cannot be detected using traditional culture methods. It has been found that mild heat shock causes viable non-culturable Legionella pneumophila to grow on laboratory media. The survival and culturability of Escherichia coli in sterile drinking water was investigated. The isolates used were from the environment rather than clinical. As expected, the count by culture on nutrient agar declined with time while the microscopy count remained approximately constant. Under laboratory conditions the E. coli isolates survived up to three months or more. After three months a portion of each suspension was heat shocked at 35°C for 20 minutes and then assayed by culture and microscopy immediately. An average increase of three log cycles was noted in the count by culture. There was no corresponding increase in the count by microscopy. Thus E. coli appears to exhibit viable non-culturable behaviour. Heat shock causes non-culturable bacteria to regain their ability to grow on artificial media. Such a finding may have several implications for the water industry and microbiology in general.