Point Source Water Testing in Developing Countries

Mobile Water Testing Lab – Three Parts
Note: incubator or laboratory NOT required

Part One: The Colilert Procedure (Presence / Absence)

Step 1. – Add Colilert powder to your 10 ml. source water sample.

Step 2. – Incubate the tubes where E. Coli loves to grow and that is body temperature. And you have a body, so put tube(s) on your body and incubate it … and sleep on it all night.

Step 3. Within about 14 hours, the water in the tube will change color.

Read Results
* Colorless = safe water
* Yellow = total coliforms
* Yellow/fluorescent = E. coli

If we shine an ultra violet light on the yellow coloured water, a battery-operated ultra violet light, and the water colour turns to fluoresce blue. That indicates that E. Coli is present and unsafe to drink. If the tube turns yellow, but it does not fluoresce, you’ve got coliform bacteria but not E. Coli, among that coliform, so it is probably an environmental contaminant, i.e. plants, they don’t indicate a public health hazard.

Part Two: 3M Petrifilm – Quantification

Step 1. – You lift up the plastic film and add 1 ml. of your water sample to the 3M Petrifilm cardboard disc. You then put the film down and squash that 1 mil water over the surface so the bacteria are distributed over a very thin film of nutrients.

Step 2. – Put the disc between two pieces of cardboard. Incubate against your body. Sleep on it all night.

Step 3. – In about 12-14 hours you start to see a blue colonies starting to develop. This time you do not need a UV light but if a colony grows and it turns blue that indicates that E. Coli is present in the water. Count the colonies.

Part Three: Risk Assessment of Water Sources

Risk = number of blue colonies on petrifilm
  • Low = less 1 colonies per 10ml
  • Moderate = 1 to 9 colonies per 10ml
  • High = 10 colonies per 10ml
  • Very High = greater 10 colonies per 10ml




$1.30 US per test

Contact Information

Dr. Robert Metcalf

IDEXX Laboratories – Colilert

3M - Petrifilm