Water samples are checked for the presence of which organism to indicate biological contamination?

• A. E. coli
• B. Viruses
• C. Protozoa
• D. Coliform bacteria

Answer: (D)

Indicator organisms are used to assess biological contamination in water because they’re easy to detect and their presence suggests fecal input, which raises the risk that harmful pathogens could be present. Coliform bacteria fit this role well: they are commonly found in the intestines of warm-blooded animals and can be recovered from water samples using simple tests. If coliforms appear, it signals potential fecal contamination and warrants further investigation for specific pathogens. While viruses and protozoa can indicate contamination, they’re harder and more expensive to detect routinely, so coliforms serve as a practical, reliable screening tool. E. coli is a member of the coliform group and provides more specific confirmation of recent fecal contamination, but the initial, widely used indicator is the broader coliform bacteria group.