Single-celled microorganisms in spherical, rod or screw form, which are found everywhere. Bacteria-like, even tinier microorganisms (without cell walls) are called phytoplasmas. In contrast to viruses (which have no metabolism and require a host such as a bacterium), bacteria multiply by cell division. There are more than 2,000 species, most of which are harmless. In fact, many of them make an important contribution to the biological balance in nature by degrading dead organisms to their basic substances during decay and putrefaction. Some are even able to break down environmental toxins such as mineral oil. A targeted control is carried out by special bactericides. Bacteria are involved from start to finish in winemaking, but are not always desirable. They are useful in the formation of humus by breaking down organic residues in the vineyard soil and in photosynthesis during growth. In grape must or wine most species cannot survive due to the acidity.