A large clade of Gram-negative bacteria that includes numerous free-living and pathogenic species. Proteobacteria share similarity in the sequence of their genes encoding ribosomal RNA, regarded as indicating evolutionary relatedness, and can be divided into five subgroups labelled alpha, beta, gamma, delta, and epsilon. Among the members of the clade are well-known human commensals and pathogens such as Salmonella typhimurium (responsible for gastrointestinal disease), Helicobacter sp. (causing gastric ulcers), Vibrio cholera (causing cholera), and Escherichia coli. The forms and lifestyles of proteobacteria are diverse, and include photoautotrophs, chemoautotrophs, and heterotrophs. See also sulphur bacteria.