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.
Certain types of Acetobacter (the acetic acid bacteria) are responsible for the transformation of wine into vinegar. The French chemist Louis Pasteur (1822-1895) proved their presence and effect. The lactic acid bacteria are involved in malolactic fermentation. Most bacteria, however, are parasites, which can cause infectious diseases in humans, animals and plants. Not as numerous as viruses, certain species are also the cause of vine diseases such as bacterial necrosis, flavescence dorée, mauke and Pierce disease. The infestation of a vine by certain bacteria can be determined by indexing. Different bacteria also cause some wine faults such as acetic acid sting, mice and lactic acid sting. The bacterium Bacillus thuringiensis in turn is used in viticulture as a biological insecticide. See also under vine enemies.