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Environmental Microbiology
Indicator Organisms & Pathogens
Coliforms, MPN and faecal indicators — how microbiological water safety is assessed.
PART 1
Topic Breakdown & Traps
The Engineering Principle
Direct testing for every waterborne pathogen is impractical, so indicator organisms signal faecal contamination. Total and faecal (thermotolerant) coliforms, especially *Escherichia coli*, are the classic indicators: abundant in faeces, easily detected, and non-pathogenic themselves. Counts are reported as the Most Probable Number (MPN) per 100 mL. Drinking water must contain zero *E. coli* per 100 mL.
The Core Formula Matrix
Indicator criteria: present whenever pathogens are, abundant in faeces, easily measured, survives like pathogens, non-pathogenic.
Reporting unit: MPN (Most Probable Number) per 100 mL.
Drinking-water standard: 0 *E. coli* / 100 mL (must be absent).
Faecal coliform tests incubate at 44.5 °C (thermotolerant).
Reporting unit: MPN (Most Probable Number) per 100 mL.
Drinking-water standard: 0 *E. coli* / 100 mL (must be absent).
Faecal coliform tests incubate at 44.5 °C (thermotolerant).
The ‘IIT Traps’
- ⚠The indicator is non-pathogenic — it flags risk, it is not the disease agent.
- ⚠MPN is a statistical estimate, not a direct plate count.
- ⚠*E. coli* is the most specific faecal indicator; total coliforms include environmental species.
📚 Standard references
- Environmental Microbiology — Ian L. Pepper et al. · Indicator Microorganisms
PART 2
Progressive 3-Tier Question Suite
Q1BASIC1 Mark · MCQ
The most widely used indicator organism for faecal contamination of drinking water is:
Q2MEDIUM1 Mark · MCQ
Microbiological water-quality results for coliforms are commonly expressed as:
Q3HARD2 Marks · MCQ
Which property disqualifies an organism from being a good faecal indicator?