---
title: "Indicator organisms"
subject: "Water Supply Engineering"
chapter: 4
key: "semester_5/water_supply_engineering:indicator-organisms-definition"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/indicator-organisms-definition
site: BCT Engineering
---

# Indicator organisms

Water Supply Engineering · Chapter 4: Quality of Water

- Asked in: 2067 Asar (old course)
- Video: [Indicator organism (fecal coliform, total coliform)](https://www.youtube.com/watch?v=DUXIWKGCKyU) — Shomu's Biology
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/indicator-organisms-definition
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-4

## Question

What do you understand by 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.

## Answer

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Indicator organisms are:
   A. organisms always found in faeces, easily detected, showing faecal contamination
   B. pathogens that cause all waterborne disease
   C. algae causing taste
   D. chemicals showing hardness
   Answer: A. They indicate possible pathogens. (Key point: Definition)

2. A good indicator does NOT:
   A. occur in large numbers in faeces
   B. survive at least as long as pathogens
   C. stay absent in unpolluted water
   D. multiply in water
   Answer: D. It must not multiply in water. (Key point: Good indicator)

3. The best indicator of recent faecal pollution is:
   A. Clostridium perfringens
   B. faecal coliform / E. coli
   C. algae
   D. iron bacteria
   Answer: B. C. perfringens spores suggest older pollution. (Key point: Recent pollution)

## Related in Water Supply Engineering

- [Indicator organisms and membrane tube fermentation](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aindicator-organisms-mtf) — Water Supply Engineering, chapter 4, asked 3×
- [Indicator organism; membrane filter technique](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aindicator-organisms-membrane-filter) — Water Supply Engineering, chapter 4, asked 2×