M00001439
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ISO 9308-2 2nd Edition, July 1, 2012 Water quality - Enumeration of Escherichia coli and coliform bacteria - Part 2: Most probable number method
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Availability date: 07/13/2021
Description / Abstract:
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 ß-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 ß-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.
NOTE While this method describes the use of an enumeration
device that is commercially available, the medium described here
can also be used in a standard MPN format.