Reason Why coliform organism is primary and most reliable indicator for water quality from parks textbook
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I now have all the content needed. Here is the complete answer directly from Park's Textbook of Preventive and Social Medicine:
Coliform Organisms as the Primary Indicator of Water Quality
- Park's Textbook of Preventive and Social Medicine, p. 821
What Are Coliform Organisms?
The "coliform" organisms include all aerobic and facultatively anaerobic, gram-negative, non-sporing, motile and non-motile rods capable of fermenting lactose at 35-37°C in less than 48 hours.
The group includes both faecal and non-faecal organisms:
- Faecal group: e.g., Escherichia coli
- Non-faecal group: e.g., Klebsiella aerogens
From a practical standpoint, all coliforms are assumed to be of faecal origin unless a non-faecal origin can be proved.
Why Coliform Is the PRIMARY and MOST RELIABLE Indicator
Park's gives 4 specific reasons why coliform organisms are chosen as indicators of faecal pollution rather than testing for waterborne pathogens directly:
1. Constantly Present in Great Abundance
- Coliform organisms are always present in huge numbers in the human intestine.
- An average person excretes 200-400 billion coliform organisms per day.
- Because they are foreign to potable waters, their presence in water is taken as direct evidence of faecal contamination.
2. Easily Detected by Culture Methods
- They can be detected by relatively simple culture techniques, identifying as few as 1 bacterium in 100 mL of water.
- By contrast, methods for detecting pathogenic organisms are complicated and time-consuming.
- This makes coliform testing practical for routine large-scale water quality monitoring.
3. Survive Longer Than Pathogens in Water
- Coliform bacilli survive longer in water than the actual pathogens, which tend to die out more rapidly.
- Therefore, if pathogens were used as the indicator, their absence could give a false sense of safety even when contamination had recently occurred - coliforms remain detectable longer, making them a more conservative (safer) signal.
4. Greater Resistance to Natural Purification Forces
- Coliform bacilli have greater resistance to the forces of natural purification than waterborne pathogens.
- This means: if coliforms are still present after natural purification, pathogens likely were present before.
- Conversely, if coliform count is zero, pathogens would certainly have been eliminated too.
Logical conclusion stated by Park's:
"If the coliform organisms are present in a water sample, the assumption is the probable presence of intestinal pathogens."
Supplementary Indicators (Not Primary)
| Indicator | Role |
|---|---|
| Faecal streptococci | Confirmatory evidence of recent faecal pollution; resistant to drying; useful after repairs in distribution systems |
| Cl. perfringens (spores) | Suggests faecal contamination at a remote time; spores survive longer and resist chlorination; their presence without coliforms = old contamination; useful to detect filtration deficiency |
Summary Table: Properties of an Ideal Indicator - Why Coliform Fits
| Ideal Indicator Property | Coliform Status |
|---|---|
| Present in large numbers when faecal contamination exists | YES - 200-400 billion/day excreted |
| Absent in uncontaminated water | YES - foreign to potable water |
| Easy and cheap to detect | YES - simple culture methods |
| More resistant than pathogens (conservative indicator) | YES - survives and resists purification longer |
| Correlates with presence of pathogens | YES - probable presence of intestinal pathogens implied |
The WHO-recommended standard is zero coliform organisms per 100 mL for treated drinking water supply.
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