1 Scope
This part of ISO 9308 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. This method can be applied to all types of water, including
those containing an appreciable amount of suspended matter and high background counts
of heterotrophic bacteria. However, it must not be used for the enumeration of coliform
bacteria in marine water. When using for the enumeration of E. coli in marine waters, a 1→10 dilution in sterile water is typically required, although
the method has been shown to work well with some marine waters that have a lower than
normal concentration of salts. In the absence of data to support the use of the method
without dilution, a 1→10 dilution is used.
This method relies upon the detection of E. coli based upon expression of the enzyme β-D-glucuronidase and consequently does not detect
many of the enterohaemorhagic 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.
The choice of tests used in the detection and confirmation of the coliform group of
bacteria, including E. coli , can be regarded as part of a continuous sequence. The extent of confirmation with
a particular sample depends partly on the nature of the water and partly on the reasons
for the examination. The test described in this part of ISO 9308 provides a confirmed result with no requirement for further confirmation of positive
wells.
Open preview (PDF)Water quality. Enumeration of Escherichia coli and coliform bacteria - Most probable number method