The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera. Others are free-living, and include many of the bacteria responsible for nitrogen fixation. The group is defined mainly in terms of ribosomal RNA sequences, and is named for the Greek god Proteus, who could change his shape, because of the great diversity of forms found in it.

All Proteobacteria are Gram-negative, with an outer membrane mainly composed of liposaccharides. Many move about using flagella, but some are non-motile or rely on bacterial gliding. The last include the myxobacteria, a unique group of bacteria that can aggregate to form multicellular fruiting bodies. There is also a wide variety in the types of metabolism. Most members are anaerobic and heterotrophic, but there are numerous exceptions. A variety of forms, called purple bacteria are capable of producing energy through photosynthesis.

The proteobacteria are divided into five sections, referred to by the Greek letters alpha through epsilon, again based on RNA sequences. Some may be paraphyletic. These are often treated as classes. The currently recognized orders are listed at right, together with some representative genera. In addition to these, the mitochondria of eukaryotic cells, which conduct aerobic respiration, are derived from symbiotic proteobacteria, probably close relatives of rickettsias.

Caulobacter crescentus, α subdivision

Vibrio cholerae, γ subdivision

Helicobacter pylori, ε subdivision