Coliform bacteria are used as indicator organisms because they:

Question

Coliform bacteria are used as indicator organisms because they:

1. A. Have very high treatment cost
2. B. Are present in human/animal faeces and are easy to detect (Correct answer)
3. C. Are resistant to disinfectants
4. D. Indicate only physical pollution

Correct Answer

Option B — Are present in human/animal faeces and are easy to detect

Detailed Solution & Explanation

The correct answer is Are present in human/animal faeces and are easy to detect.

Key Points

• Coliforms are used as indicator organisms, not because they are themselves dangerous, but because they signal the possible presence of faecal contamination and therefore of pathogens.
• They qualify because they are abundant in the intestines of humans and warm-blooded animals, are present in far greater numbers than pathogens, are easy and cheap to detect, and survive in water for a similar period to the pathogens of concern without multiplying in it.
• The remaining options invert the logic — cost, resistance to disinfectants and physical pollution have nothing to do with why an indicator is chosen.

Additional Information

• Testing directly for every pathogen would be slow, expensive and unreliable, since pathogens are present intermittently and in small numbers. A single indicator that is always present when faecal matter is present solves the problem.
• Escherichia coli is the most specific indicator of recent faecal pollution, because unlike other coliforms it does not occur naturally in soil or vegetation.
• The Indian standard requires that treated drinking water contain no coliforms in any 100 mL sample; enumeration is by the Most Probable Number (MPN) or membrane-filtration technique.
• A good indicator organism should be non-pathogenic, easily cultured, and at least as resistant to disinfection as the pathogens it stands for — which is why coliform survival characteristics matter.