Which organism is listed as an indicator bacterium in this context?

• A. E. Coli
• B. Salmonella
• C. Staphylococcus aureus
• D. Pseudomonas aeruginosa

Answer: (A)

Indicator bacteria serve as a quick signal of fecal contamination and potential health risk in water. E. coli is used as the standard indicator because it lives in the intestines of humans and warm-blooded animals and is typically absent from pristine water. Its presence suggests recent fecal contamination, which raises the likelihood that enteric pathogens could be present. Testing methods for E. coli are well established, reliable, and practical for routine monitoring. The other organisms listed are actual pathogens or environmental residents not specifically tied to fecal contamination: Salmonella is a pathogen, Staphylococcus aureus can be found in various environments and isn’t a fecal source indicator, and Pseudomonas aeruginosa is common in water systems but not a reliable fecal contamination indicator. So, the organism listed as the indicator bacterium is E. coli.