Bacteriological indicators of drinking water

The numbers of individual types of bacteria are given in CFU or colony forming units. CFUs are always reported in a certain volume of water. This volume is different for different types of bacteria.

Coliform bacteria

This concerns a group of bacteria that live in the intestinal tract of warm-blooded animals, but also survive in the soil. If they are present in the water, faecal contamination of the water source can be assumed. In the case of a treated water source, they are an indicator of the effectiveness of this treatment and are used to control water disinfection. For drinking water, Decree 252/2004 Sb. requires 0 CFUs in 100 ml of tested water.

Escherichia coli

They form part of the group of coliform bacteria. They live in the tract of warm-blooded animals. Their presence in water therefore clearly proves faecal pollution. However, it is not possible to distinguish whether the source of the pollution is a person (seepage from a cesspool, sewage) or an animal. For drinking water, Decree 252/2004 Sb. requires 0 CFUs in 100 ml of tested water.

Enterococci

They live in the tract of warm-blooded animals and are another indicator of faecal pollution. Since they do not survive in water for a long time, they are an indicator of fresh pollution. For drinking water, Decree 252/2004 Sb. requires 0 CFUs in 100 ml of tested water.

Colony counts at 22°C and 36°C

This concerns non-specific groups of bacteria that represent a general indicator of possible bacterial water pollution. Their source can be, for example, compost seepage or rotting leaves.
For large, treated sources of drinking water (water supply, company well serving as a source of drinking water), Decree 252/2004 Sb. permits a maximum of 200 CFUs in 1 ml of water for bacteria cultured at 22°C and 20 CFUs in 1 ml of water for bacteria cultured at 36°C.
For water from small non-disinfected sources (e.g. privately owned wells), a higher content of these bacteria is permitted, namely 500 CFUs in 1 ml of water for bacteria cultured at 22°C and 100 CFUs in 1 ml for bacteria cultured at 36°C.