How to test for E. coli using the IDEXX Colilert method
What is it? E. coli (short for Escherichia coli) is a bacteria that lives in the guts of warm-blooded animals, including humans. Most strains are harmless. We test for it in water because finding it tells us that poop has gotten in recently, whether from sewage, wildlife like geese and ducks, livestock, pets, or other sources.
Why does it matter? E. coli itself usually won't make you sick, but its presence is a warning sign. Where there's fecal contamination, there can also be other germs like viruses, parasites, and some harmful bacteria, all of which can cause illness if you swallow the water or swim in it. That's why it's the bacteria most countries use to decide whether water is safe for swimming.
What does it mean? Water quality can change quickly, so beaches, rivers, lakes, and swimming holes need to be tested often. The guidelines for safe E. coli levels come from health studies that tracked how often swimmers got sick, usually with stomach issues like diarrhea, at different levels of contamination. The numbers aren't arbitrary. They're based on real-world risk.
Once you enter your results, we'll colour-code them based on the standards for your region, so you can see at a glance whether your water falls within safe levels.
How to test
The IDEXX Colilert test measures both E. coli and total coliforms in a water sample, with results in 24 hours. Paired with a Quanti-Tray, it gives you a count using a Most Probable Number (MPN) table from IDEXX.
Sample collection:
- Wearing gloves, collect a water sample in a sterile bottle or Whirl-pak bag.
- Place the sample in a cooler to keep bacteria from multiplying before you get to the lab.
Lab processing:
- Add the Colilert reagent to the water sample and mix until dissolved.
- Pour the mixture into a Quanti-Tray 2000.
- Seal the tray with the Quanti-Tray sealer.
- Incubate at 35°C ± 0.5°C for 24 hours.
- Note: dilution may be needed for samples you expect to have high bacteria counts.
Read the results:
- Yellow wells = total coliforms
- Yellow wells that also glow under UV light = E. coli
- Count the positive wells of each colour and use the IDEXX MPN table to get your result per 100 mL.
- Enter your results into the platform and we'll show you how they compare to the standards for your region.