Germ content

In microbiology, the number of microorganisms in a material is referred to as the germ content or germ count , namely their number in relation to the volume or mass of the material; the unit of measurement is usually ml ⁻¹ or g ⁻¹ (reference value 1 milliliter or 1 Grams ). Microorganisms in the active or resting stage are meant, not just microorganism germs (as the name suggests).

The terms germ content and germ count are probably due to the fact that they are mostly determined by the number of colonies . This means that a certain amount of the material is distributed in a gel-like nutrient medium , if possible, so that all microorganisms lie individually, then incubated under suitable conditions and the microorganism colonies that have formed are counted. Ideally, a microorganism would become a colony and the number of colonies would be the same as the number of microorganisms in the sample (for problems with this determination method, see the article Colony-forming unit ). Since this ideal case is practically never given, one speaks of the number of colonies rather than the number of germs . With this method - in contrast to direct microscopic counting - only living individuals capable of reproduction are recorded, from which colonies emerge through their multiplication, which are therefore germs for colony formation .

If the germ content (the germ count) is determined by the colony count, a selection of the recorded microorganisms can be achieved through the choice of selective culture conditions (composition of the culture medium and the atmosphere, incubation temperature, lighting). For example, only acidophilic and acidotolerant microorganisms are detected when an acidic nutrient medium is used. In addition, the colonies of certain microorganisms can be recognized and distinguished from others on certain culture media (so-called differential culture media ), for example acid producers in culture media with suitable acid indicators. In contrast to this, the selectivity of the culture conditions can be restricted as much as possible in order to cover the broadest possible spectrum of different microorganisms. One then speaks of total germ count , often also in the case of a normal natural habitat with a biocenosis that contains a broad spectrum of physiologically diverse microorganisms, although there are no culture conditions under which all the microorganisms present can multiply.