The Proteobacteria are a major group (phylum) of bacteria. Characteristics They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera. Others are free-living, and include many of the bacteria responsible for nitrogen fixation. The group's defined primarily in terms of ribosomal RNA (rRNA) sequences, and's named for the Greek god Proteus (also the name of a bacterial genus within the Proteobacteria), who could change his shape,… (More on Proteobacteria)