Testing for coliform bacteria in a watercourse is a fundamental and critical practice in public health and environmental science.
Coliforms as Indicator Organisms
The primary importance of testing for coliforms lies in their role as "indicator" organisms. It is expensive and time-consuming to test for every single possible disease-causing pathogen (like viruses, bacteria, and protozoa). Instead, scientists test for coliforms because:
- Easier to detect
They are more numerous and easier to culture and identify in a lab than most pathogens. - Always present in sewage.
Coliform bacteria are resident in the intestines of warm-blooded animals (mammals or birds). That may not be their source in a water sample, but their presence indicates that the sample may have been contaminated with fecal matter. - Correlate with risk
While most coliforms are not harmful themselves, their presence signals that conditions are suitable for pathogens to also be present. If fecal contamination has in fact occurred, it's possible that dangerous organisms like Salmonella, Campylobacter, Giardia, or the Hepatitis A virus are also in the water.
Escherichia coli
The reason E. coli is reported separately from the broader group of total coliforms is that it serves a much more specific and critical purpose in assessing public health risk.
Total Coliforms make up a large group of several genera of bacteria (like Klebsiella, Enterobacter, Citrobacter) that are generally harmless and widespread in the environment. They may come from soil, vegetation, decaying plant matter, and even the skin of warm-blooded animals. Their presence does not necessarily mean fecal contamination.
E. coli comes from the intestines and feces of warm-blooded animals (including humans). While once again most strains are harmless, its presence is a direct indicator that fecal matter has entered the water, and feces can contain dangerous pathogens like Salmonella, Campylobacter, Giardia, and viruses like Norovirus.
Depending on jurisdiction, regulations may allow for a small percentage of samples to test positive for total coliforms, as they can sometimes enter the system from non-fecal sources. A positive test triggers further investigation and additional sampling. The detection of E. coli is typically a "zero-tolerance" situation. A single positive sample is considered an acute health risk and mandates immediate public notification and corrective actions. A Boil Water Advisory is often triggered specifically by the detection of E. coli. What wild animals must do if they need to drink from the contaminated source is generally undisclosed by regulating organizations.
In summary:
Total Coliforms: The system is vulnerable.
E. coli: The system is contaminated with feces and is unsafe to drink, and at a certain level is unwise or unsafe for immersion (swimming).
The feces of cold-blooded animals (ectotherms, like reptiles, amphibians, and fish) contain coliform bacteria, but there is a critical distinction in their source and significance for water quality monitoring.
Coliforms, and particularly fecal coliforms like E. coli, are natural, abundant residents of the intestinal tracts of warm-blooded animals (endotherms). They are shed in their feces at high concentrations (millions per gram). The gut microbiome of cold-blooded animals is different. While they can harbor coliforms, these bacteria are not typically native, predominant residents. Instead, coliforms in their feces are usually acquired from their environment — from the water or soil they live in, or their food (e.g., insects, plants). The coliforms pass through their digestive system but don't establish the same resident population.
Cold-blooded animal feces may contain "total coliforms" (picked up from the environment), but they are not a significant source of E. coli. So E. coli detected in a a watercourse does not originate in the turtles, snakes or fish that reside there.