Coliform bacteria are used as an indicator of sanitary quality in water because

• A. They are commonly present among other disease-causing organisms
• B. They are a major disease-causing organism
• C. They are easily removed by filtration
• D. They never occur in treated water

Answer: (A)

Coliforms are used as an indicator because their presence signals fecal contamination, which raises the likelihood that other disease-causing organisms may be present in the water. They come from the intestines of warm-blooded animals, are plentiful and easy to detect with simple tests, so they provide a practical, cost-effective way to assess sanitary quality. Their role is to warn that the water might harbor pathogens, not to be the main pathogens themselves. The other ideas don’t fit: coliforms aren’t chosen because they’re the primary disease-causing organisms, their removal by filtration isn’t the defining measure, and they can be present even in water that has been treated.