The proteobacteria are the most metabolically diverse and largest phylum of bacteria. They constitute the majority of known bacteria of medical, industrial, and agricultural significance. All proteobacteria are gram negative and they have a wide diversity of energy generating mechanisms. The proteobacteria can be split into six classes, based on 16S rRNA gene sequences: Alpha-, Beta-, Gamma-, Delta-, Epsilon-, and Zetaproteobacteria. Only Zetaproteobacteria is composed of only one species. All the others contain many genera. Genetic traits are shared in many genomic classes of proteobacteria suggesting that horizontal gene transfer played a major role in shaping the diversity of proteobacteria.

Alphaproteobacteria:

• nearly 1,000 described species
• second largest class of proteobacteria
• most species are obligate aerobes or facultative aerobes
• many are oligotrophic
• the vast majority of species fall into the Rickettsiales,  Rhodospiralles, Sphingomonadales, Caulobacterales, Rhodobacterales, and Rhizobiales 

Betaproteobacteria

• Nearly 500 described species
• third largest class of proteobacteria
• contain species of Burkholderia, Hydrogenophilales, Methylophilates, Neisseriales, Nitrosomonadales, and Rhodocyclales

Gammaproteobacteria

• include a number of important pathogens like Salmonella and Yersinia

Deltaproteobacteria and Epsilonproteobacteria

• contain fewer species and less functional diversity than Alpha-, Beta-, and Gammaproteobacteria
• deltaproteobacteria are primarily sulfate and sulfur reducing bacteria
• Epsilonproteobacteria contain many species that oxidize the H2S  produced by the sulfate and sulfur reducers

Zetaproteobacteria

• composed of only one species- Mariprofundus ferrooxydans
• is an iron oxidizer