bacteria A diverse group of ubiquitous microorganisms all of which consist of only a single cell that lacks a distinct nuclear membrane and has a cell wall of a unique composition (see illustration). Bacteria constitute the prokaryotic organisms of the living world. However, their classification is a controversial issue. It is now recognized, on the basis of differences in ribosomal RNA structure and nucleotide sequences (see molecular systematics), that prokaryotes form two evolutionarily distinct groups. Traditionally these were placed in a single kingdom, variously named Bacteria or Prokaryotae, which was divided into two subkingdoms: Archaea (archaebacteria), including the descendants of ancient bacterial groups; and Eubacteria, representing the vast majority of present-day bacteria. However, it is now recognized that these groups of prokaryotes are so distinct that they should each be raised to the status of domain: Archaea (the archaebacteria, containing a variable number of kingdoms) and Bacteria (containing a single kingdom, Eubacteria). Generally speaking, the term ‘bacteria’ includes both archaebacteria and eubacteria.

Bacteria can be characterized in a number of ways, for example by their reaction with Gram's stain or on the basis of their metabolic requirements (e.g. whether or not they require oxygen: see aerobic respiration; anaerobic respiration) and shape. A bacterial cell may be spherical (see coccus), rodlike (see bacillus), spiral (see spirillum), comma-shaped (see vibrio), corkscrew-shaped (see spirochaete), or filamentous, resembling a fungal cell. The majority of bacteria range in size from 0.5 to 5 μm. Many are motile, bearing flagella, possess an outer slimy capsule, and produce resistant spores (see endospore). In general bacteria reproduce only asexually, by simple division of cells, but a few groups undergo a form of sexual reproduction (see conjugation). Bacteria are largely responsible for decay and decomposition of organic matter, producing a cycling of such chemicals as carbon (see carbon cycle), oxygen, nitrogen (see nitrogen cycle), and sulphur (see sulphur cycle). A few bacteria obtain their food by means of photosynthesis, including the Cyanobacteria; some are saprotrophs; and others are parasites, causing disease. The symptoms of bacterial infections are produced by toxins.