--- title: "E-coli: harmful? why and how tested" subject: "Water Supply Engineering" chapter: 4 key: "semester_5/water_supply_engineering:ecoli-harmful-tested" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ecoli-harmful-tested site: BCT Engineering --- # E-coli: harmful? why and how tested Water Supply Engineering · Chapter 4: Quality of Water - Asked in: 2070 Chaitra, 8 marks; 2076 Chaitra, 2+1+1 marks; 2079 Baisakh, 4 marks (asked 3 times) - Video: [Step-by-Step Guide: How to Test for Coliform and E. coli in Water Using Ready-to-Use Media](https://www.youtube.com/watch?v=HtAYtOqMR5g) — Water Life - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ecoli-harmful-tested - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-4 ## Question What is E-coli (and coliform)? Are they harmful to human beings? Why and how is their presence tested in water for drinking purposes? ## Short answer E. coli is a faecal coliform living in intestines; most strains are harmless, but some cause diarrhoea, dysentery, and kidney failure. It is tested as an indicator of faecal contamination because it is easy to detect and signals possible pathogens. Tests include MPN, membrane filter, presence-absence, and chromogenic methods; safe water requires zero E. coli per 100 ml. ## Answer #### What are E. coli and coliform? **Coliform bacteria** are Gram-negative, non-spore-forming, rod-shaped bacteria that ferment lactose with gas and acid at 35 to 37 degrees C within 48 hours. They live in soil, vegetation and in the intestines of warm-blooded animals. **Escherichia coli (E. coli)** is the main faecal coliform. It lives only in the intestines of humans and warm-blooded animals and is passed in large numbers (about $10^{8}$ to $10^{9}$ per gram) in faeces. It can ferment lactose at 44.5 degrees C and produces indole. #### Are they harmful? - Most E. coli strains are harmless and even help digestion in the gut. - Some strains (for example E. coli O157:H7, enterotoxigenic and enteropathogenic strains) cause diarrhoea, bloody dysentery, kidney failure (haemolytic uremic syndrome) and can be fatal, especially to children. - Coliform bacteria in general are usually harmless, but their presence shows that water may have been polluted. - The main danger is what they stand for: faecal contamination, which means pathogens such as Salmonella, Vibrio cholerae, Shigella, viruses and protozoa may also be in the water. #### Why they are tested (indicator organisms) - Testing for each pathogen is difficult, costly and slow, as many pathogens exist only in small numbers. - E. coli and coliforms are always present in faeces, easy to detect, survive longer than most pathogens in water and do not multiply in clean water. - Their absence therefore shows that the water is bacteriologically safe. The standard is **zero E. coli per 100 ml** (Nepal Drinking Water Quality Standards, WHO). #### How they are tested 1. **Multiple tube fermentation (MPN) technique:** presumptive, confirmed and completed tests in lactose broth, BGLB and EC broth; the result is given as MPN per 100 ml. 2. **Membrane filter technique:** 100 ml of water is passed through a 0.45 micron filter, which is placed on selective media (m-Endo, m-FC) and incubated; colonies are counted as CFU per 100 ml. 3. **Presence-absence test:** a simple test that shows only whether coliforms are present or absent in 100 ml. 4. **Chromogenic substrate tests (for example Colilert):** a colour change (and fluorescence under UV light) shows coliforms and E. coli. Field kits such as the **H2S strip test** turn black when faecal contamination is present. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which describes coliform bacteria? A. Gram-positive spore-forming cocci B. Gram-negative, non-spore-forming rods that ferment lactose with gas and acid C. Viruses that infect gut cells D. Algae producing toxins Answer: B. This is the standard description of the coliform group. (Key point: Coliform definition) 2. How harmful is E. coli? A. Most strains are harmless, but some (e.g. O157:H7) cause serious disease B. All strains are fatal C. No strain can cause disease D. Only harmful in chlorinated water Answer: A. Some strains cause bloody dysentery and kidney failure; most are harmless gut residents. (Key point: Harm) 3. Why is E. coli used instead of testing each pathogen? A. It is more dangerous than all pathogens B. Pathogens are always present in large numbers C. It is always in faeces, easy to detect and does not multiply in clean water D. It multiplies quickly in clean water Answer: C. Pathogen tests are difficult, costly and slow; E. coli indicates faecal contamination. (Key point: Why indicator) 4. The real significance of finding coliform in water is that: A. it proves cholera is present B. it makes water taste bitter C. it shows high hardness D. it signals faecal contamination and possible pathogens Answer: D. Coliform presence shows faecal pollution, so pathogens such as Salmonella may also be present. (Key point: Main danger) 5. The drinking water standard for E. coli is: A. 10 per 100 ml B. 0 per 100 ml C. 1 per ml D. 100 per 100 ml Answer: B. Nepal standards and WHO require zero E. coli per 100 ml. (Key point: Standard) 6. In the membrane filter technique, how much water is passed through a 0.45 micron filter and how is the result expressed? A. 100 ml; colonies counted as CFU per 100 ml B. 1 ml; MPN per 100 ml C. 10 ml; mg/l D. 1000 ml; NTU Answer: A. Bacteria retained on the filter grow on selective media into countable colonies. 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