Proteobacteria (purple bacteria)
•Largest group of bacteria—high diversity of metabolic phenotypes.
•Common ancestor was photoautotrophic; as they encountered new environments, the delta and epsilon groups lost ability to photosynthesize. In other 3 groups, some evolutionary lineages became chemolthotrophs or chemoheterotrophs.
•Includes some nitrogen-fixing genera such as Rhizobium; E. coli is also proteobacterium.
•Proteobacteria that are human pathogens: Yersinia pestis (plague), Vibrio cholerae (cholera), and Salmonella typhimurium (gastrointestinal disease).