Bacteria – A diverse group of ubiquitous microorganisms all of which consist of only a single cell.

Bacteria can be characterized in a number of ways, for example their reaction with Gram’s stain or on the basis of their metabolic requirements (e.g. whether they require oxygen) or shape. A bacterial cell may be spherical (coccus), rodlike (bacillus), spiral (spirillum), comma shaped (vibrio), corkscrew-shaped (spirochaete), or filamentous. The majority of bacteria range in size from 0.5 to 5 um. Many are motile, bearing flagella. Some can produce endospores.

In general bacteria reproduce only asexually, by simple cell division, but a few groups undergo a form of sexual reproduction in the form of conjugation.

Bacteria are largely responsible for decay and decomposition of organic matter, creating by-products such as carbon, oxygen, nitrogen and sulphur when using organic matter as a fuel. A few bacteria obtain their energy by means of photosynthesis (such as cyanobacteria), some are saprotrophs and others are parasites, causing disease.

The symtoms of bacterial infections are produced by toxins released by the bacteria.