Indicator bacteria signal potential sewage contamination in water. Fecal coliforms have long served as this indicator because they originate in the intestines of warm-blooded animals, are typically present when sewage or fecal material is in the water, and can be detected with simple, standardized laboratory tests. Their survival in water tends to mirror that of many pathogens, so finding them suggests a possible health risk even if specific pathogens aren’t measured directly. Traditional methods focus on lactose fermentation and a thermotolerance criterion, making the testing practical for many labs and providing a consistent benchmark across water quality programs. While E. coli is a more specific member of this group and increasingly used in modern assessments, fecal coliforms remain a widely used indicator due to established methods and historical data. Enterococcus and Salmonella relate differently—Enterococcus is used in some contexts, and Salmonella is a pathogen, not a general indicator—so fecal coliforms are the classic, commonly used measure of sewage contamination.