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Indicator Organisms

Microbial Monitoring

Monitoring for specific bacterial pathogens is usually complex, expensive and time consuming. The AWQC tests for a select group of organisms known as indicator organisms, which are used to verify water quality.

Indicator organisms can be divided into two groups, those which are used as indicators of faecal contamination and those used as indicators of the efficacy of treatment.

Thermotolerant Coliforms and Escherichia coli – Indicators of Faecal Contamination

Most waterborne pathogens are introduced into drinking water supplies via contamination with human or animal faeces. These pathogens cause a range of conditions ranging from mild to severe gastroenteritis, diarrhoea, dysentery, hepatitis and cholera.

Thermotolerant coliforms (faecal coliforms) are always present in high numbers in human and animal faeces. E.coli is the most common thermotolerant coliform present in faeces (typically >90%) and is regarded as the most specific indication of recent faecal contamination. While most thermotolerant coliforms are non pathogenic, there are some pathogenic subspecies of E. coli that can cause gastrointestinal illness.

E. coli (or thermotolerant coliforms ) should not be detected in a minimum 100mL sample of drinking water. If detected, immediate action should be taken as it is an indication of faecal contamination and suggests a potentially serious fault in the quality of the water. Treatment by chlorination or other acceptable forms of disinfection inactivates these microorganisms in water, provided the turbidity is low.

Total Coliforms and Heterotrophic Plate Count – Indicators used in Operational Monitoring

Coliforms are a diverse group of bacteria including Escherichia coli (E. coli) and other thermotolerant coliforms. Coliforms are always present in the digestive tracts of animals, including humans, and are found in their wastes. They are also found in plant and soil material.

Total coliforms (excluding E. coli) are no longer recommended as indicators for faecal contamination. As coliform bacteria occur widely in soil and water environments there are many environmental coliforms that are not of faecal origin.

Total coliforms can be used as indicator organisms for monitoring the efficiency of water treatment and disinfection process. The presence of these organisms may indicate inadequate treatment, breakdowns in system integrity (leaks, fractures or repair work) or the presence of biofilms.

Total coliforms can also be used as a reasonable indication of the presence of pathogenic bacteria. Coliforms are relatively easy to identify, are usually present in larger numbers than pathogenic bacteria. They also respond to the environment, water and wastewater treatment similarly to many pathogens.

To find out more about Indicator organisms and sample requirements review the  Indicator Organisms Fact Sheet

To find out more about Molecular Source Tracking review the  Microbial Source Tracking Fact Sheet