AWWA WQTC58790

This paper compares five methods for the detection of coliforms and E.coli in water. The study was performed in two parts, one specifically to examine the specificity of the methods, particularly with regard to Aeromonas spp., and the other investigating the sensitivity, particularly with regard to E.coli. Considerable differences were seen between some of the methods. Colitag£ (CPI, CA) was found to be extremely non-specific, giving high numbers of false positives when Aeromonas was present in the water. This occurred even at low levels of Aeromonas (<10 cfu/ml) in some samples. The remaining media were generally fairly specific, although all media produced occasional false positive coliform results. ReadyCult£ (Merck) gave increased numbers of false positive coliform results when it was incubated for 29 h, but this length of incubation was required to facilitate acceptable levels of E.coli detection. Occasional false positive E.coli results were seen with ReadyCult£ apparently due to the presence of fluorescent pseudomonads. Furthermore, the size of vessel used for incubation affected the ReadyCult£ result. When incubated in vessels of 120 ml volume, some samples which contained coliforms and/or E.coli failed to give a positive coliform reaction. In these situations, the reaction was weak but detectable in 150 ml vessels and clearly detectable in 250 ml vessels. Removing the lid from 120 ml vessels and leaving the sample on the bench for 10-30 minutes allowed color to develop. Colilert 18£ (IDEXX, ME) was the most specific and most sensitive of the five methods tested. Includes tables.