How does the laboratory detect coliforms in a water sample?
Since visual examination of water under the microscope is unreliable, the County Laboratory uses the Colilert presence/absence method. In brief, this method combines two separate examinations. First, the 100 milliliter sample is combined with growth medium, and then incubated for 24 hours at 35.0 degrees Centigrade. After 24 hours, only coliform bacteria will use the growth medium for food, resulting in a change in the water sample’s appearance. At the same time, a second examination is performed to determine if the coliform present is from feces. Thus, the test is completed 24 hours after it has begun. A TOTAL COLIFORM ABSENT report means that there are no coliform bacteria present at the time of sampling and the water may be considered bacteriologically safe to drink. A TOTAL COLIFORM PRESENT report means that coliform bacteria is present at the time of sampling and the water may not be considered safe to drink. You will be advised to super-chlorinate the water system. A FECAL COLIFO
