Review of Coliforms - National Health and Medical Research Council

MICROBIAL INDICATORS OF DRINKING WATER QUALITY
4. THE CHANGING FACE OF COLIFORMS AND INDICATORS
4.1 CHANGES IN COLIFORM DEFINITION
The last two decades in microbiology have seen a move away from selective growth mediabased
recovery methods for faecal bacteria to enzymatic and molecular methods. With these
advances, the definition of what is considered a coliform has expanded, leading to increased
scrutiny of the role that total coliforms play in water quality assessment and the validity of the
information assumed by their presence or absence in drinking water systems.
Pre 1994 – acid and gas from lactose
Until the 1990s it was accepted that a coliform was a member of the Enterobacteriaceae,
which displayed the biochemical characteristics of acid and gas production from lactose
within 24–48 hours at 36±2°C. Thermotolerant or faecal coliforms were those that fitted
the basic definition, but were able to grow and ferment lactose at 44.5±0.2°C. The
vast majority of thermotolerant coliforms are E. coli and to a lesser extent Klebsiella,
Enterobacter and Citrobacter. E coli are differentiated by their thermotolerance plus
the ability to produce indole from tryptophan.
Report 71 (1994) – acid only from lactose
In 1994, the sixth edition of the UK, ‘Bacteriological Examination of Drinking Water
Supplies 1982’ was published (HMSO, 1994). This report is referred to as Report 71.
Report 71 altered a component of the biochemical definition of a coliform from:
acid and gas production from lactose
to
acid-only production from lactose.
This change in definition resulted in an increase in the number of bacterial species
considered to be coliforms. Species of Enterobacter and Citrobacter, which do not
produce gas from lactose were also included in the new definition. The ongoing review
of the Australian Drinking Water Guidelines (NHMRC-ARMCANZ, 1996) has proposed
adoption of this coliform definition.
Current and Future - presence of specific enzymes
With the advent of new technologies for bacterial analyses, the working definition of
a coliform has again changed. Lactose fermentation is one of the key criteria in the
coliform definition and fermentation of lactose is determined, in part, by the presence
of a specific enzyme, ß-galactosidase. The presence of ß-galactosidase in a member
of the Enterobacteriaceae is considered specific to coliforms. Many water companies
in the UK, USA, Europe and Australasia use commercial kits for total coliform analyses
based on specific enzymes. The USEPA has adopted this technology and the associated
coliform definition. It is understood that the next Report 71 will also include this more
specific coliform definition and it has been proposed that it will be included in the
revised Australian Drinking Water Guidelines (NHMRC-ARMCANZ, 1996).
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