This part of ISO 9308 specifies a method for the enumeration of Escherichia coli (E. coli) and coliform bacteria. The method is based on membrane filtration, subsequent culture on a chromogenic coliform agar medium, and calculation of the number of target organisms in the sample. Due to the low selectivity of the differential agar medium, background growth can interfere with the reliable enumeration of E. coli and coliform bacteria, for example, in surface waters or shallow well waters. This method is not suitable for these types of water.

This part of ISO 9308 is especially suitable for waters with low bacterial numbers that will cause less than 100 total colonies on chromogenic coliform agar (CCA). These may be drinking water, disinfected pool water, or finished water from drinking water treatment plants.

Some strains of E. coli which are ß-D-glucuronidase negative, such as Escherichia coli O157, will not be detected as E. coli. As they are ß-D-galactosidase positive, they will appear as coliform bacteria on this chromogenic agar.