Standards Development

Scope

This document specifies a method for the enumeration of E. coli and coliform bacteria in water. The method is based on the growth of target organisms in a liquid medium and calculation of the “Most Probable Number” (MPN) of organisms by reference to MPN tables or by use of appropriate formulae 1]. This method can be applied to a range of types of water (for example drinking water, groundwater, surface water, recreational water and wastewater),[2-5] including those containing a considerable amount of suspended matter and high background counts of heterotrophic bacteria. Users should satisfy themselves that any product used has been validated for its intended use (e.g. chlorinated drinking water) and should generate performance characteristics using ISO13843.[6] Performance characteristic data generated prior to 2017 using ISO TR 13843[7] is also acceptable. If a water sample has some background colour, the incubated sample should be compared to a blank control of the same water sample.

This method relies upon the detection of E. coli based upon expression of the enzyme β‑D‑glucuronidase and consequently does not detect some enteropathogenic strains of E. coli, which do not typically express this enzyme. Additionally, there are a small number of other E. coli strains that do not express β‑D‑glucuronidase. As they are β‑D‑galactosidase positive, they will appear as coliform bacteria.