Proteobacteria are far and away the most diverse of all bacterial groups (Fig. 26.15). Defined largely by similarities in rRNA sequences, Proteobacteria include many of the organisms that populate our expanded carbon cycle and other biogeochemical cycles. For example, they include anoxygenic photosynthetic bacteria and chemoautotrophs that oxidize NH₃, H₂S, and Fe²⁺, as well as bacteria able to respire using SO₄^2-, NO₃^-, or Fe³⁺.

Many Proteobacteria have evolved intimate ecological relationships with eukaryotic organisms. Some of these are mutually beneficial, such as the nitrogen-fixing bacteria in soybean root nodules. Other Proteobacteria, however, are our worst pathogens: the rickettsias that cause typhus and spotted fever, vibrios that trigger cholera, salmonellas responsible for typhoid fever and food poisoning, and Pseudomonas aeruginosa strains that infect burn victims and others whose resistance is reduced. Escherichia coli, famous as a laboratory model organism (see Fig. 26.4b), is usually a harmless presence in our intestinal tract, but pathogenic strains can cause serious diarrhea and even death.