Which bacteria found in feces indicate water may be contaminated with sewage?

• A. Fecal coliform
• B. E. coli
• C. Salmonella
• D. Vibrio

Answer: (A)

The key idea is using fecal indicator bacteria to signal sewage contamination in water. Fecal coliforms are bacteria that live in the intestines of warm-blooded animals and are shed in feces, so finding them in water suggests that sewage or animal waste has entered the water system. They’re chosen as indicators because they are commonly present when fecal contamination occurs, survive in water long enough to be detected, and can be measured with relatively routine lab tests. Among the options, this group provides the broadest and most reliable cue that sewage-derived material may be present in the water. E. coli is a member of this group and is often used as a more specific indicator of recent fecal pollution, but the general presence of fecal coliforms already signals potential sewage contamination. Pathogens like Salmonella and Vibrio can appear with contamination, but their detection is more pathogen-specific and variable, making them less reliable as routine indicators of sewage in water.