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Abstract: This study compared the Quanti-Disc™ most probable number (MPN) test for heterotrophic bacteria from drinking water with the widely used yeast extract agar (YEA) pour plate method. The Quanti-Disc™ test module contains 50 reaction wells in which a medium has been pre-deposited. The medium contains a suite of three fluorogenic enzyme substrates selected for the detection of enzymes expressed widely by heterotrophic bacteria. The MPN of heterotrophic bacteria is calculated from the number of fluorescing reaction wells after incubation of a sample. Quanti-Disc™ and the YEA pour plate method were compared according to guidance on comparing methods given in United Kingdom national guidance and ISO 17994:2004. The two methods were also challenged with reference strains and isolates of heterotrophic bacteria from drinking water. This indicated that heterotrophic bacteria commonly encountered in drinking water are detected by both the YEA pour plate method and Quanti-Disc™. Analysis of data from split water samples (723 for 37°C tests and 872 for 22°C tests) from nine geographically diverse laboratories in England and Wales demonstrated that the Quanti-Disc™ method is equivalent to the YEA pour plate method for the analysis of heterotrophic bacteria from drinking and similar waters at 37°C, and superior to YEA for the analysis at 22°C. The Quanti-Disc™ method is a simple and efficient alternative method for the enumeration of heterotrophic bacteria from drinking water. [Copyright &y& Elsevier]

The count of general or heterotrophic bacterial populations in treated drinking water in the UK has been undertaken since the 1880s. Counts of heterotrophic bacteria at 22 and 37 °C are used widely as part of an overall assessment of treated drinking water quality. There were no legislated standards for water quality in the UK until adoption of the first EU Directive in 1989. The UK has, however, never stipulated numerical standards or guidelines for heterotrophic bacteria, although their enumeration has long been part of the assessment of ‘wholesome’ water, on which advice regarding microbiological quality was given in a series of documents known as ‘Report 71’. The current regulations stipulate only that there should be ‘no abnormal change’ in numbers normally associated with a given supply. This paper reviews the historical context regarding the enumeration, and interpretation of results, of heterotrophic bacteria from treated drinking water, and information regarding current practices by UK water suppliers. The appropriateness of using heterotrophic bacteria counts as an operational tool or as a health parameter is briefly discussed in the light of the UK experience. [Copyright &y& Elsevier]