Use of two formulations of Colilert and QuantiTray™ for assessment of the bacteriological quality of water

The use of standard Colilert (IDEXX) for the detection of coliforms and E. coli in potable water has been studied. In a large survey, Colilbert recovered E. coli from similar numbers of samples to membrane filtration whilst recovery of coliforms was higher with Colilert. Further experiments to study the sensitivity of the two methods using pure cultures, showed that there was no significant difference in their ability to recover E. coli. Studies on a newly developed form of Colilert (Colilert 18) which is said to give results within 18 h showed that there was no significant difference between the two forms of the product. Use of the Colilert 18 in a quantitative format using QuantiTray™ showed that the product gave similar quantitative results to membrane filtration, but was easier to read and was less time consuming. Subculture of a proportion of positive samples demonstrated that there was no need to confirm results obtained with the Colilert/QuantiTray™ system. The results of this study suggest that both formulations of Colilert are suitable alternatives to the United Kingdom membrane filtration method for monitoring the bacteriological quality of drinking water.