Single-celled microorganisms in spherical, rod or screw form that occur everywhere. Bacteria-like, even tinier microorganisms (without cell walls) are called phytoplasmas. Bacteria, unlike viruses (which have no metabolism and need a host such as a bacterium), reproduce by cell division. There are more than 2,000 species, most of which are harmless. In fact, many make an important contribution to the biological balance in nature by breaking down dead living organisms into their basic substances during decay and putrefaction. Some are even able to break down environmental toxins such as mineral oil. Specific bactericides are used to combat them. In winemaking, bacteria are involved from start to finish, but they 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 because of the acidity.