---
title: "Bacteriological examination of water"
subject: "Water Supply Engineering"
chapter: 4
key: "semester_5/water_supply_engineering:bacteriological-examination"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bacteriological-examination
site: BCT Engineering
---

# Bacteriological examination of water

Water Supply Engineering · Chapter 4: Quality of Water

- Asked in: 2080 Baisakh, 6 marks
- Video: [Bacteriological Analysis of Water](https://www.youtube.com/watch?v=yk0YzqFK7P0) — I 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/bacteriological-examination
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-4

## Question

Describe and explain the bacteriological examination of water.

## Short answer

Bacteriological examination tests for indicator organisms (coliforms, E. coli) to assess faecal contamination. Samples are collected in sterile bottles with sodium thiosulphate if chlorinated, kept cool, and tested within 6 hours. Methods include multiple tube fermentation (MPN), membrane filtration, and plate counts. Drinking water must have zero E. coli per 100 mL.

## Answer

The **bacteriological examination** of water tests for the presence of faecal bacteria to find if the water is safe to drink. As testing for each pathogen is not practical, **indicator organisms** (coliform and E. coli) are used.

#### 1. Sampling
Collect the sample in a sterile, clean glass bottle (with sodium thiosulphate if the water is chlorinated to remove residual chlorine). Flame the tap, let water run for 2 to 3 minutes, fill the bottle leaving an air space, and keep it cool (below 10 degrees C). Test within 6 hours (maximum 24 hours).

#### 2. Tests
- **Standard plate count (total bacterial count):** 1 ml of sample is placed on nutrient agar and incubated at 37 degrees C for 24 hours (and 22 degrees C for 72 hours); colonies are counted. Treated water should have a low count (less than 100 per ml).
- **Multiple tube fermentation test (MPN):** presumptive test in lactose broth at 35 degrees C, confirmed test in BGLB or EC broth, and completed test on EMB agar. The result is given as MPN of coliform per 100 ml.
- **Membrane filter technique:** 100 ml is filtered through a 0.45 micron membrane and incubated on m-Endo or m-FC medium; coliform colonies are counted.
- **Presence-absence test and field kits (H2S strip test, Colilert):** quick tests for screening.

#### 3. Interpretation
Zero E. coli per 100 ml must be found in drinking water (Nepal Drinking Water Quality Standards and WHO). A positive result shows faecal contamination, and the water should be treated (chlorination) and the source protected.

## Check yourself

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

1. Indicator organisms are used in bacteriological examination because:
   A. testing each pathogen is impractical
   B. pathogens are always harmless
   C. indicators are visible by eye
   D. it avoids sampling
   Answer: A. Coliform and E. coli represent faecal contamination. (Key point: Purpose)

2. Sodium thiosulphate is added to the sample bottle when:
   A. water is hard
   B. the water is chlorinated, to remove residual chlorine
   C. water is turbid
   D. water is cold
   Answer: B. Residual chlorine would kill bacteria after collection. (Key point: Sampling)

3. A sample should be tested within:
   A. 1 week
   B. 3 days
   C. 6 hours (maximum 24 hours)
   D. 60 days
   Answer: C. Kept cool below 10 degrees C meanwhile. (Key point: Holding time)

4. Standard plate count uses:
   A. 100 ml on m-FC
   B. 1 ml in EC broth at 44.5
   C. 5 tubes of BGLB
   D. 1 ml on nutrient agar incubated at 37 degrees C for 24 hours
   Answer: D. It gives total bacterial count. (Key point: Plate count)

5. A positive E. coli result means:
   A. faecal contamination; treat water and protect source
   B. water is safe
   C. water is too hard
   D. only taste problems
   Answer: A. Drinking water must have zero E. coli per 100 ml. (Key point: Interpretation)