Evaluation of the methods for enumerating coliform bacteria from water samples using precise reference standards.

AIMS To use BioBall cultures as a precise reference standard to evaluate methods for enumeration of Escherichia coli and other coliform bacteria in water samples. METHODS AND RESULTS Eight methods were evaluated including membrane filtration, standard plate count (pour and spread plate methods), defined substrate technology methods (Colilert and Colisure), the most probable number method and the Petrifilm disposable plate method. Escherichia coli and Enterobacter aerogenes BioBall cultures containing 30 organisms each were used. All tests were performed using 10 replicates. The mean recovery of both bacteria varied with the different methods employed. CONCLUSIONS The best and most consistent results were obtained with Petrifilm and the pour plate method. Other methods either yielded a low recovery or showed significantly high variability between replicates. SIGNIFICANCE AND IMPACT OF THE STUDY The BioBall is a very suitable quality control tool for evaluating the efficiency of methods for bacterial enumeration in water samples.