A comparison of the International Standards Organisation reference method for the detection of coliforms and Escherichia coli in water with a defined substrate procedure

Aims: This study investigated the use of the International Standards Organisation(ISO) procedure for the comparison of microbiological methods. Using this procedure the ISO reference procedure for the detection of coliforms and Escherichia coli in water was compared with a defined substrate method (Colilert(TM)). Methods and Results: A total of 20 laboratories from 13 European countries compared the use of Colilert/Quanti-Tray(TM), a quantitative defined substrate test(DST) for the presence of coliforms and E. coli with the ISO reference procedure, which utilizes tergitol-TTC medium. Results of the study showed that DST detected significantly more coliforms and E. coli than did the reference procedure. In the case of E. coli the recoveries were also higher when using DST and the difference seen was statistically significant. The confirmation rate obtained when using the DST product suggested that no confirmation of wells positive for E. coli was necessary during routine use. Conclusions: Colilert is a suitable alternative to the ISO reference procedure for the detection of coliforms and E. coli in water. The methods used during the comparison study indicated that confirmation of all colonies/positive wells led to the most accurate information and it is recommended that for future comparison studies this should become standard practice. Confirmation of a small proportion of colonies led to misleading conclusions and should be avoided when comparing microbiological methods. Significance and Impact of Study: It has been demonstrated that the ISO reference procedure fails to detect a significant proportion of coliforms and E. coli in drinking water. Colilert/QuantiTray(TM) is a more suitable alternative.