Bacterial Phylum Proteobacteria

Proteobacteria, a diverse phylum of Gram-negative bacteria, is divided into five classes.

Alphaproteobacteria is a diverse class containing ten orders. The order Rhizobiales is the largest containing nitrogen-fixing genera like Rhizobium and Agrobacterium.

The class Betaproteobacteria is divided into six orders. The most notable genera are Neisseria from order Neisseriales and Nitrosomonas from Nitrosomonadales.

Gammaproteobacteria is the most diverse class, with 15 orders. This class includes the order of enteric bacteria — Enterobacteriales — containing genera such as Escherichia, Salmonella, and Shigella. The other noteworthy orders are Pseudomonadales and Vibrionales, which contain genera like Pseudomonas and Vibrio.

The class Deltaproteobacteria covers eight orders of bacteria with diverse habits. Notably, the orders Bdellovibrionales and Myxococcales contain predatory bacteria such as Bdellovibrio and Myxococcus.

Species from the class Epsilonproteobacteria occur primarily in sulfur-rich environments. The order Campylobacterales contains notable genera, including Sulfurospirillum, Campylobacter, and Helicobacter.