Bacterial water analysis

Bacterial water analysis is a routine check to make sure that the concentration of potentially pathogenic bacteria in drinking water is sufficiently low to say it is safe for human consumption at a reasonable level of confidence.

Analysis is usually performed using culture, biochemical and sometimes optical methods. Specific pathogens can be quickly detected (where suspected) using molecular biology.

In drinking water sampling the colilert culture method is common. A reagent is added to water samples of a uniform amount. These are cultivated on a nutrient tray that is sealed and incubated at 22ºC and 37ºC for 24 hours. Bacteria reproduces to form a colony and each one can be counted easily because they become fluorescent in the reagent. Prior to this method, each sample would be cultivated on a single nutrient pad such as agar and the colonies were then painstakingly counted under a microscope.

The total number of colonies is referred to as the total viable count (TVC). The unit of measurement is cfu/ml (or colony forming units per millilitre). Typically three indicator bacteria are chosen; non-specific coliforms, Escherichia coli and Pseudomonas aeruginosa. The presence of Pseudomonas may indicate that the water is not as clean as it should be, although it may still be potable if the levels are sufficiently low and prompt action is taken. Coliforms (and especially E. coli) could suggest the possibility of fecal matter contamination of a water supply.