Delayed incubation as an alternative method to sample storage for enumeration of E. coli and culturable bacteria in water

The effects of sample storage on enumeration of Escherichia coli in marine bathing water and culturable bacteria in drinking water were evaluated. Results showed that overnight storage at 0−5°C significantly reduced the counts of E. coli in bathing water (p=0.0001) with a mean reduction of 25%. A similar effect of sample storage was observed for the enumeration of culturable bacteria in drinking water at 22±2°C for 66±4 h (p=0.0074; mean REDUCTION=25%) or at 36±2°C for 44±4 h (p=0.0353; mean REDUCTION=6%). The use of a delayed incubation method, i.e. overnight storage at 0−5°C of inoculated agar plates prior to incubation, did not significantly affect the counts of culturable bacteria when plates were incubated at 22±2°C for 66±4 h, whereas it resulted in a significant increase of the bacterial numbers when plates were incubated at 36±2°C for 44±4 h (p=0.0002; mean INCREASE=32%). Based on these results, it is suggested to avoid the use of overnight or longer sample storage for the enumeration of E. coli in bathing water samples, as well as for the enumeration of culturable bacteria in drinking water. The delayed incubation method appears to be a reliable procedure for the enumeration of culturable bacteria and could represent a valid alternative to sample storage in order to overcome problems associated with the performance of bacteriological counts during weekends or statutory holidays. However, a multi-laboratory study is needed to evaluate the reproducibility of the delayed incubation method for the enumeration of culturable bacteria and its possible use for the enumeration of E. coli by membrane filtration.