Phylum Proteobacteria (Purple Bacteria & relatives)

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Most of the familiar Gram-negative Bacteria are members of the Proteobacteria. The members of this very successful phylum predominate many environments. Most of the familiar Gram-negative Bacteria are members of the phylum. They have a very broad range of phenotypes scattered around the phylogenetic tree. This implies that these organisms can change phenotype readily, at least in terms of evolutionary history; for this reason they have been named after the shape-shifting sea god Proteus. Unlike most of the other bacterial phyla, there is no phenotypic trait that units the Proteobacteria. The purple phototrophic Bacteria are members of this group, and so this phylum is also sometimes referred to as the Purple Bacteria and relatives. In addition to purple phototrophs, members of this phylum include many heterotrophs (including some important symbionts and pathogens) and key players in the sulfur and nitrogen cycles.

Because this is such a broad phylum, with thousands of described species, the Proteobacteria are usually divided into classes, which are given the generic labels α-, β-, γ-, δ- and ε-proteobacteria. Perhaps this generic labeling has persisted because even within each of these classes there is a range of phenotypes; heterotrophs, phototrophs, and chemoautotrophs of various kinds, without any unifying themes.


Given the incredible diversity of organisms in the phylum Proteobacteria, we will only be able to touch on a few examples.