Purple bacteria are proteobacteria that are phototroph ic, i.e. capable of produce energy through photosynthesis. They are pigmented with bacteriochlorophyll a or b, together with various carotenoids. These give them colours ranging between purple, red, brown, and orange. Photosynthesis takes place at reaction centres on the cell membrane, which is folded into the cell to form sacs, tubes, or sheets, increasing the available surface area.

Like most other photosynthetic bacteria, purple bacteria do not produce oxygen, because the reducing agent involved in photosynthesis is not water. In some, called purple sulfur bacteria, it is either sulfide or elemental sulfur. The others, called purple non-sulfur bacteria, typically use hydrogen although some may use other compounds in small amounts. At one point these were considered families, but RNA trees show the purple bacteria make up a variety of separate groups, each closer relatives of non-photosynthetic proteobacteria than one another.

Purple non-sulfur bacteria are found among the alpha and beta subgroups, including:

Rhodospirillales
Rhodospirillaceae	e.g. Rhodospirillum
Acetobacteraceae	e.g. Rhodopila
Rhizobiales
Bradyrhizobiaceae	e.g. Rhodopseudomonas
Hyphomicrobiaceae	e.g. Rhodomicrobium
Rhodobiaceae	Rhodobium
Other families
Rhodobacteraceae	e.g. Rhodobacter
Rhodocyclaceae	e.g. Rhodocyclus
Comamonadaceae	e.g. Rhodoferax

Purple sulfur bacteria are included among the gamma subgroup, and make up the order Chromatiales. The similarity between the photosynthetic machinery in these different lines indicates it had a common origin, either from some common ancestor or passed by lateral transfer.