Aims: To compare the use of MI agar, Membrane Lactose Glucoronide Agar (MLGA), CM1046 agar and Colilert-18 (Defined Substrate Technology, IDEXX Laboratories Pty. Ltd., Sydney) on Australian potable water.
Methods and results: Both potable (n = 369) and nonpotable waters (n = 35) were analysed by membrane filtration using chromogenic agars as well as Colilert-18 over a period of 12 months. Recoveries of stressed organisms on these chromogenic media were also investigated. Agar-based chromogenic technologies compared favourably to Colilert-18 for chlorinated waters, but there are possible limitations when using these agars for chloraminated waters. Additionally, the breakthrough of problematic organisms, especially oxidase positive organisms, may lead to misrepresentation or over-estimation of E. coli and total coliforms, particularly on MLGA and CM1046. The recovery of stressed organisms was favoured in the Colilert-18 system when compared to chromogenic agars.
Conclusions: MI agar performed better than the other chromogenic agars with respect to recovery and colour identification and discrimination of organisms, and compared favourably with Colilert-18. The use of chromogenic agars in chloraminated waters should be done cautiously.
Significance and impact of the study: This study provides comparison data for laboratories looking to adopt chromogenic technologies, and is especially important for Australian laboratories wanting to uptake the use of MI agar (as used in USEPA method 1604) for routine use and for gaining accreditation. Additionally, to the best of our knowledge, this is the first reported evaluation of these agars in chloraminated waters and is especially timely as the use of this disinfection agent is increasing.