The Proteobacteria are a major group (phylum) of Gram-negative bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, Yersinia, and many other notable genera.Others are free-living (nonparasitic), and include many of the bacteria responsible for nitrogen fixation. Gammaproteobacteria are the largest subgroup which includes Acinetobacter, Pseudomonas, Escherichia, Salmonella, and Serratia species.