Pseudomonadota is a major phylum of gram-negative bacteria. They include both pathogenic and free-living (non-parasitic) genera. The phylum comprises six classes: Acidithiobacillia, Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Hydrogenophilia, and Zetaproteobacteria. Pseudomonadota are very diverse, with differences in morphology, metabolic processes, relevance to humans, and ecological influence.