What actually is a KBE?

In microbiology, the CFU or CFU is the most important mathematical unit. The abbreviation stands for colony forming unit. In English, it is referred to as colony forming unit, or cfu for short. This is a measure of the number of germs on a surface, in a liquid, a solid or even in a gas, usually the air. Bacteria and fungi can thus be determined in terms of quantity. Living microorganisms can thus be quantified using cultural methods.

A bacterial cell is not visible to the human eye, it is very small with 1-10 micrometers (1µm = 1000mm). Only after multiplication of the germs on an artificial nutrient medium in a laboratory can they be made visible. From one germ or several, which are very close to each other, a colony develops by division of the cells, which can be recognized by the naked eye. For this purpose, the optimal environmental conditions for the germs are set. These are the nutrient requirements, the temperature, the atmosphere and the growth time. The division of a bacterium takes different amounts of time. There are germs that divide every 20 minutes and others that take many hours. The reproduction is exponential, i.e. one cell becomes two, then four, then eight and so on. At a certain number of cells, the colony is formed. There are special nutrient media for different types of microorganisms. For example, fungi grow on special nutrient media in order to firstly count and secondly identify them. These media are called selective nutrient media. Based on the growth of the colonies, germ groups can be assigned by their characteristics. The colonies can develop specific colors, forms or also sizes, which makes a rough differentiation of the germs possible. The time needed for the formation of the colonies is also decisive. For an exact identification, further investigations have to be carried out in the laboratory.
A total count of germs of a sample can be done on so called complex media. For example, there is the aerobic total number of germs that grow in the medium temperature range under aerobic conditions, i.e. in air, for example, within 48 hours at 36°C.

But why do you need the bacterial count in a sample?
There are many reasons for this. For example, the causes of disease processes in the human body can be determined. A maximum level of germs may be contained in cosmetics, medicines, drinking water, food and other test materials. Among other things, this also affects the marketability of products and spoilage if the limit is exceeded. No germs may be detected in or on a sterile product.
The success of cleaning or disinfection can also be proven by determining the number of microorganisms on a surface. On an area of 100 cm², for example, no more than 50 CFU may be detectable on a work surface after disinfection.

Figure 1: a colony consisting of many millions of bacteria on a nutrient medium - image source Hohenstein

Figure 2: Petri dish with a solid medium on which various colonies have developed after sampling a surface - image source Hohenstein