|* or kingdom, see text|
A bacterium (plural form: bacteria) is a certain type of single-celled or colonial organism without a nucleus (the other type being the archaea). Bacteria are among the oldest and most numerous living things and are found in the soil, water, and inside and outside most multicellular organisms. They are very small, typically on the order of a few micrometres. Their study, bacteriology, is part of microbiology.
A few kinds of bacteria are pathogens responsible for disease. These may be treated by antibiotics, which can be classified as bacteriocidal and bacteriostatic, which at concentrations that can be reached in bodily fluids either kill bacteria or hamper their growth, respectively. Many other bacteria are found as symbionts in humans and other organisms. Further, two organelles, mitochondria and chloroplasts, are generally believed to have been derived from endosymbiotic bacteria.
Bacteria show a wide variety of different metabolisms. A number conduct photosynthesis. These include the cyanobacteria, which are some of the oldest known organisms and probably played an important role in creating the Earth's oxygen atmosphere. Other photosynthetic bacteria undergo different processes which do not produce oxygen. These comprise the green sulfur, green non-sulfur[?], purple sulfur[?], purple non-sulfur bacteria and heliobacteria[?].
Most bacteria have a cell wall. This usually includes a second membrane surrounding the cell, but in a few groups this is absent, and instead the cell wall is composed mostly of glycoproteins. These two sorts are called gram-negative and gram-positive bacteria. A few bacteria without the second membrane, however, lack the glycoproteins and show up as gram-negative despite belonging to gram-positive groups.
Many bacteria can move about, either using flagella, bacterial gliding, or changes of buoyancy. Spirochaetes have a distinctive helical body which twists about, but this movement has its ultimate cause in flagella located between their two membranes. Motile bacteria are attracted or repelled by certain stimuli, behaviors called taxes - for instance, chemotaxis, phototaxis[?], and mechanotaxis[?]. In one peculiar group, the myxobacteria, individual bacteria attract to form swarms and may differentiate to form fruiting bodies.
Under favourable conditions, bacteria may form aggregates visible to the naked eye, such as bacterial mats. Some bacteria are used in industry, for processes as varied as producing cheese and cleaning up oil.
The classification of bacteria has changed radically to reflect thoughts about phylogeny, and many groups and even species undergo frequent alteration or renaming. However, this places bacteriology in an ideal position to exploit recent advances in gene sequencing, genomics, bioinformatics and computational biology.
Originally the bacteria were considered a group of fungi, except the cyanobacteria, which were not considered bacteria at all but rather blue-green algae. The discovery of their common prokaryotic cell structure, as distinct from all other organisms (all of them eukaryotes), led to their treatment as a single and separate group, variously called "Monera", "Bacteria", and "Prokaryota". It was generally believed that this was a grade[?], in that the eukaryotes arose from prokaryotes.
Looking at RNA, Woese found that the prokaryotes comprised two separate groups. These he called the Eubacteria and Archaebacteria, but they have since become renamed the "Bacteria" and "Archaea", which is the usage followed here. Woese argued that these two groups, together with the eukaryotes, comprised separate domains which had originated separately from a primordial organism. Researchers have abandoned this model, but the three-domain system has gained general acceptance. In this case the Bacteria, so restricted, may be divided into several kingdoms, though in other systems they are treated as a single kingdom. They are generally considered a monophyletic group, though this has been disputed.
Until very recently it was believed that Bacteria were one of the earliest forms of life to develop, and that more developed organisms developed from them. In 1998, however, three New Zealand scientists proposed that the simplicity of bacteria was not a product of their primitivness, but rather indicated the sophistication of their development. Bacteria lack much of the duplication and unecessary genetic structure found in other organisms, however, it is now known that this duplicated structure was almost certainly essential for early life to evolve. Bacteria rather than being primitve are, in fact, a stremlined creature that has dropped the unecessary genetic baggage of the past and are probably more recent developments that bulkier protazoa. This simplicty allowed bacteria to survive in a much wider array of locations than other more complex organisms that could not properly duplicate themselves in adverse surroundings. Most important was the ability of bacteria to survive and replicate in much higher temperatures than other organisms which has lead to vast bacteria populations below the Earth's surface.
Some representative bacteria are Escherichia coli, Salmonella, Helicobacter, Bacillus, Staphylococcus, and Streptococcus. The first three belong among the Proteobacteria, a large group of Gram-negative forms, and the latter three among the Firmicutes, a large group of Gram-positive forms.