Performance validation of chromogenic coliform agar for the enumeration of Escherichia coli and coliform bacteria

The performance of chromogenic coliform agar (CCA) for the enumeration of Escherichia coli and coliform bacteria was validated according to ENV ISO 13843 using pure cultures and naturally contaminated water samples. The results indicate that for the detection of E. coli and coliform bacteria, respectively, the method is sensitive (94 and 91%), specific (97 and 94%), selective (selectivity -0·78 and -0·32) and efficient (96 and 92%). Relative recovery of E. coli and coliform bacteria on CCA in comparison with tryptone soy agar (TSA) was good (104 and 94% in mean, >80 and >70% in all cases), and repeatability and reproducibility were sufficient. The linear working range was defined for 10-100 total target colonies per 47-mm membrane filter. A high precision of the method was confirmed by low overdispersion in comparison with Poisson distribution. The robustness of the method with respect to the variable incubation time of 21 ± 3 h was found to be low, because an incidental increase in presumptive colonies especially between 18 and 21 h was observed. In conclusion, the CCA method was proved as a reliable method for the quantification of E. coli and coliform bacteria.

The international standard for the detection and enumeration of E. coli and coliform bacteria by membrane filtration (ISO 9308-1) is currently under revision and will be published in 2014. In the new standard, lactose-triphenyl tetrazolium chloride (TTC) agar will be replaced by a CCA. A performance validation of this revised method according to ENV ISO 13843 is presented in this study to determine fundamental data on its applicability and to provide reference data for secondary validation by users of this method.