Total germ count
The total germ count is an unspecific quantity of the microbiological examination of drinking water according to the drinking water ordinance and when determining the microorganisms present in the air . It indicates how many colonies of microorganisms on a standardized for this purpose Agar - breeding ground form at a controlled incubation temperature after inoculation with a certain amount of sample. The duration of the incubation and the incubation temperature must be adjusted depending on the medium to be examined.
The breeding ground for the analysis of water samples is designed to cultivate the widest possible range of different types of microorganisms (active stages as well as resting stages, not just " germs ") that can use various organic substances in the environment as food. It is covered directly with the sample to be examined, with a volume of 1 ml usually being placed on or in the nutrient medium. When examining the total number of germs in the air, there is also the option of using an air germ collector to cover the nutrient media directly. More specific nutrient media are often used, one for mold , the other for bacteria . Attention is paid to the different incubation temperatures, bacteria are incubated at 30 ° C and molds at 25 ° C.
The two incubation temperatures (22 ° C and 36 ° C) are used for the water samples, because in one culture approach (at 22 ° C) preference is given to the microorganism species that live freely in the environment, in the other culture approach (at 37 ° C) C) those associated with faeces from the intestines of warm-blooded animals. From the difference between the two results, conclusions are drawn about the proportion of microorganisms brought in with faeces.
With this method - as the name of the measurand "total germ count" might suggest - all microorganisms contained in a sample are not recorded. This can be seen from the fact that different values are obtained at different incubation temperatures. This is also not possible with other cultural methods. Because there are no culture conditions (including the composition of the nutrient medium, incubation temperature, incubation atmosphere) under which all microorganisms present in a natural habitat can multiply, provided that the habitat - as is usually the case - contains a biocenosis with a broad spectrum of physiologically diverse microorganisms.
According to the German Drinking Water Ordinance, a limit of 100 CFU / ml ("colony-forming units per milliliter") applies for piped drinking water from wells and for drinking water temporarily stored in tanks of 1000 CFU / ml.
