Proteobacteria include nitrogen-fixing bacteria in the root nodules of legumes, as well as enteric bacteria that live in the intestinal tract of animals (including E. Coli). The nitrogen-fixing bacteria convert inorganic nitrogen in the form of atmospheric N₂ to NH₄⁺ (ammonium) and NO₃^- (nitrates) in the soil, which can be incorporated into the amino acids and nucleic acids of plants, after being absorbed through their roots. This is the most diverse group of bacteria, and there are three main subgroups: purple bacteria, chemoautotrophic proteobacteria, and chemoheterotrophic proteobacteria.