Understanding Indicator Organisms for Water Pollution

When assessing water quality, especially for contamination by faeces, scientists often use 'indicator organisms' rather than searching directly for specific disease-causing pathogens (like Salmonella or Shigella). The question asks why coliform organisms are chosen as indicators of faecal pollution. Let's analyze the given statements:

Statement I Analysis: Abundance in Faeces

Statement I says that coliform organisms are present in great abundance in the human intestine and excreted in large numbers in faeces. This is a crucial characteristic for an indicator organism. If an organism is consistently present in high numbers in faecal matter, its presence in water suggests faecal contamination. This makes tracking contamination more feasible.

Statement IV Analysis: Ease of Detection

Statement IV mentions that coliform organisms are easily detected by culture methods. Practicality is key in water testing. Indicator organisms should be relatively simple and quick to identify using standard laboratory techniques. This allows for routine monitoring of water supplies efficiently.

Statements II & III Analysis: Survival and Resistance

Statement II suggests coliforms have lower resistance to natural purification forces, and Statement III states they survive lesser than pathogens. While coliforms do die off in the environment, the ideal indicator organism should survive long enough in water to be detected, but not so long that it indicates past contamination long after the immediate risk has passed. If coliforms died significantly faster than the actual pathogens, their absence might not necessarily mean pathogens are also absent. Conversely, if they survived much longer, they might be detected long after the source of contamination was removed. The key is that their survival pattern is *similar enough* to pathogens to be a reliable signal, but the primary reasons for choosing them are abundance and ease of detection. Lower resistance itself isn't the *reason* for choosing them; rather, their predictable die-off rate relative to pathogens is considered.

Conclusion on Coliforms as Indicators

Based on the analysis:

• Statement I is correct because high numbers make them detectable signals of contamination.
• Statement IV is correct because ease of testing is vital for practical water quality monitoring.
• Statements II and III are less critical as primary reasons compared to I and IV. While survival rates matter, they don't represent the core advantages of coliforms over direct pathogen detection in the same way abundance and detectability do.

Final Answer Selection

Therefore, the primary reasons for choosing coliform organisms as indicators of faecal pollution are their presence in great abundance (I) and their ease of detection by culture methods (IV).

The correct option is the one that includes both statements I and IV.