Total Coliform and E. coli Testing: What the Results Can and Cannot Tell You

Coliform and E. coli testing can provide useful evidence of possible contamination, but the results must be interpreted with the right context.

Total coliforms, fecal coliforms, and Escherichia coli are commonly used as indicator organisms in water quality testing and sewage contamination investigations. They are not tested because they identify every possible pathogen. They are used because their presence may indicate that conditions allowed contamination to enter the sample or system.

For building investigations, drinking water screening, wells, environmental water, and sewage-contaminated surfaces, the key question is not simply whether bacteria were detected. The more important questions are: what was sampled, why was it sampled, how was it collected, and what decision will the result help support?

This page explains the practical meaning of total coliform and E. coli testing and why careful sampling and interpretation matter.

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What Are Total Coliforms?

Total coliforms are a broad group of bacteria that may be found in soil, vegetation, water, animal waste, sewage, and other environmental sources. Because some members of this group are associated with fecal contamination, total coliform testing is commonly used as a general screening tool.

A total coliform positive result does not automatically prove recent fecal contamination. However, in drinking water or other controlled systems, their presence may suggest that contamination has entered the system or that sanitary integrity has been compromised.

This is why total coliforms are often described as indicator organisms. They are useful warning signs, but they do not identify every possible microbial hazard or source of contamination.

What Is E. coli and Why Is It Important?

Escherichia coli, commonly called E. coli, is more specifically associated with fecal contamination from humans and other warm-blooded animals. For this reason, E. coli is often considered a stronger indicator of fecal contamination than total coliforms alone.

Not all E. coli strains cause disease, but some strains can be pathogenic. The detection of E. coli in drinking water, well water, recreational water, or a surface sample can be significant because it may indicate fecal contamination and the possible presence of other microorganisms of concern.

Because E. coli results may have important practical implications, sample collection, transport, laboratory method, and interpretation must be handled carefully.

Why Test for Total Coliform and E. coli?

Testing may be useful when there is concern about water quality, sewage contamination, fecal contamination, or the effectiveness of cleanup after a contamination event.

Common reasons for testing include:

• Screening private wells or drinking water sources
• Checking water after plumbing work, flooding, or suspected contamination
• Investigating possible sewage or wastewater impact
• Assessing surfaces after sewage backup cleanup
• Supporting post-cleanup verification after water damage or sewage contamination
• Understanding whether additional investigation or corrective action may be needed

The value of the result depends heavily on whether the sample was collected from the right location, at the right time, using the right method.

What a Positive Result May Mean

A positive total coliform result may indicate that bacteria entered the water or surface sample from an environmental or sanitary source. In a drinking water system, it may suggest a problem with the well, plumbing, storage, treatment, or distribution system.

A positive E. coli result is generally more concerning because it is more closely associated with fecal contamination. In water testing, E. coli detection may indicate that the water has been affected by sewage, animal waste, surface runoff, or another fecal source.

However, interpretation should always consider the sample type, collection procedure, environmental conditions, recent work on the system, disinfection history, and whether repeat or confirmatory testing is needed.

What a Negative Result Does Not Prove

A result reported as absent, not detected, or negative does not prove that a site, surface, or water system is free of all microbial hazards. It means that the target organism was not detected in the specific sample analyzed by the specific method used.

False reassurance can occur if the sample was collected from the wrong location, collected after disinfection without follow-up, transported too slowly, held at the wrong temperature, or collected in a way that does not represent the area of concern.

Good interpretation requires both laboratory data and knowledge of the sampling situation.

Sampling Water for Total Coliform and E. coli

Water sampling for microbiological testing must be done carefully to avoid contaminating the sample during collection. The sample bottle should not be rinsed, and the inside of the cap or bottle should not be touched.

For faucets, spigots, hydrants, or pumps, water is commonly allowed to run at a moderate rate before sampling so that stagnant water is flushed from the outlet. The sample container should be opened only when ready to collect the sample and closed immediately after collection.

General water sampling reminders include:

• Use the correct sterile sampling bottle supplied or approved by the laboratory
• Do not rinse the bottle
• Avoid touching the inside of the cap or bottle
• Label the sample clearly with location, date, and time
• Keep the sample cool if required by the laboratory
• Deliver the sample to the laboratory as soon as possible

Always follow the sampling instructions provided by the laboratory or applicable regulatory authority for the specific test and sample type.

Sampling Rivers, Lakes, Reservoirs, or Other Surface Waters

Surface water samples require care because the sample can be affected by shoreline debris, sediment, surface scum, flow direction, and the sampler’s handling technique.

Samples should generally be collected away from the immediate bank or edge, where possible, and collected below the surface rather than from floating surface material. In flowing water, the bottle opening is typically directed into the current. In still water, the bottle may be tilted and allowed to fill slowly.

Because surface water conditions can change quickly, the sampling location, weather conditions, recent rainfall, and nearby contamination sources should be recorded when they may affect interpretation.

Sampling Sewage-Contaminated Surfaces

Surface sampling may be useful after a sewage backup, wastewater release, or cleanup event. The goal is usually to determine whether indicator organisms are still detectable on selected surfaces after cleaning and disinfection.

Swabs can be used on flat or irregular surfaces, including walls, floors, corners, equipment, or materials that were likely affected by contamination. Sampling should focus on areas most likely to have been contaminated or areas where cleanup effectiveness needs to be verified.

Practical considerations include:

• Select sample locations based on the contamination history and cleanup objective
• Use sterile swabs and appropriate transport media
• Sample a defined area where possible so results can be compared more meaningfully
• Keep the swab sterile before and during collection
• Label each sample with the location, date, and time
• Send samples to the laboratory as soon as possible after collection

Surface results should be interpreted with caution because recovery can be affected by surface type, dryness, residue, cleaning agents, disinfectants, and how the swab was collected.

How Total Coliform Testing Is Commonly Performed

Total coliform testing is often performed using methods that detect bacterial activity through a colour change, fluorescence, growth response, or other laboratory endpoint. If total coliforms are detected, additional testing may be performed to determine whether E. coli is present.

The exact method may vary depending on whether the sample is drinking water, environmental water, surface swab, wastewater-related material, or another sample type. Different methods have different detection limits, reporting formats, and interpretation limits.

For that reason, the laboratory report should be read together with the method used, sample description, sample volume or area, and the purpose of the investigation.

Common Interpretation Mistakes

Coliform and E. coli results are sometimes misinterpreted when the result is treated as a complete answer rather than one piece of evidence.

Common mistakes include:

• Assuming total coliform always means recent fecal contamination
• Assuming a negative result means there is no microbial concern
• Ignoring how and where the sample was collected
• Comparing results from different sample types as if they mean the same thing
• Overlooking the effect of disinfectants, drying, surface type, or holding time
• Using indicator organism results as if they identified all possible pathogens

A stronger interpretation considers the sampling objective, organism tested, matrix, sample condition, method, site history, and applicable guideline or regulatory context.

When Training in Coliform and E. coli Testing Is Useful

Training can help professionals understand when coliform and E. coli testing is appropriate, how samples should be collected, and how results should be explained without overclaiming.

This knowledge may be useful for:

• Restoration contractors dealing with sewage backups
• Environmental consultants and indoor air quality professionals
• Industrial hygienists
• Property managers and facility managers
• Water damage restoration professionals
• Environmental health and safety personnel
• People responsible for reviewing water or surface bacterial test reports

Our online coliform testing course provides a practical introduction to sewage contamination, indicator organisms, sampling requirements, sample handling, and result interpretation.

Course access: $45.93 CAD Learn Coliform Testing

Build Better Understanding Before You Interpret the Result

Total coliform and E. coli testing can be very useful, but only when the sample is collected properly and the result is interpreted in context. A good result is not just a number or a presence/absence statement. It is evidence that must be connected to the site conditions and the question being asked.

If you work with sewage cleanup, water testing, environmental investigations, or bacterial reports, this course can help you better understand what the results mean and where the limitations are.

Learn coliform and E. coli testing: Learn Coliform Testing

Questions before enrolling? Contact us by phone or email before purchasing the course.