Comparison of methods for determining the numbers and species distributionof coliform bacteria in well water samples

Aims: Enumeration of coliform bacteria and Escherichia coli is the most wid ely used method in the estimation of hygienic quality of drinking water. Th e yield of target bacteria and the species composition of different populat ions of coliform bacteria may depend on the method. Three methods were comp ared. Methods and Results: Three membrane filtration methods were used for the en umeration of coliform bacteria in shallow well waters. The yield of confirm ed coliform bacteria nas highest on Differential Coliform agar, followed by LES Endo agar. Differential Coliform agar had the highest proportion of ty pical colonies, of which 74% were confirmed as belonging to the Enterobacte riaceae. Of the typical colonies on Lactose Tergitol 7 TTC agar, 75% were c onfirmed as Enterobacteriaceae, whereas 92% of typical colonies on LES Endo agar belonged to the Enterobacteriaceae. LES Endo agar yielded many Serrat ia strains, Lactose Tergitol 7 TTC agar yielded numerous strains of Rahnell a aquatilis and Enterobacter, whereas Differential Coliform agar yielded th e widest range of species. Conclusions: The yield of coliform bacteria varied beta een methods. Each m ethod compared had a characteristic species distribution of target bacteria and a typical level of interference of non-target bacteria. Identification with routine physiological tests to distinct species was hampered by the s light differences between species. High yield and sufficient selectivity ar e difficult to achieve simultaneously, especially if the target group is di verse. Significance and Impact of the Study: The results showed that several aspec ts of method performance should be considered, and that the target group mu st be distinctly defined to enable method comparisons.