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Indicator organisms

Short answer

Indicator organisms are faecal bacteria (e.g., E. coli, faecal coliforms) whose presence in water indicates faecal contamination and potential pathogens. They are used because they are easy to detect, survive similarly to pathogens, and are absent in unpolluted water. Drinking water should have zero E. coli per 100 mL.

What do you understand by indicator organisms?

Indicator organisms are micro-organisms (usually bacteria) that are always found in faeces of humans and warm-blooded animals, easily detected in water, and used to show that the water has been contaminated with faecal matter and may contain pathogens.

Testing for each pathogen (cholera, typhoid, viruses, protozoa) is difficult, slow and costly, so an indicator is tested instead.

Qualities of a good indicator

  • Present in large numbers in faeces, and present whenever pathogens are present.
  • Absent in unpolluted water.
  • Survives in water at least as long as pathogens and is not killed more quickly by disinfection.
  • Does not multiply in water.
  • Easy and cheap to detect and count.

Examples

  • Total coliform group bacteria.
  • Faecal coliform and E. coli (best indicator of recent faecal pollution).
  • Faecal streptococci (enterococci) and Clostridium perfringens (for older pollution).

The drinking water standard is zero E. coli / faecal coliform per 100 ml.

✓ Answer checked by Claude Opus 5.5 (high effort)

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