Rhizobiales is an order of alpha proteobacteria. They are Gram-negative. The rhizobia, which fix nitrogen and are symbiotic with plant roots, appear in several different families. The four families Bradyrhizobiaceae, Hyphomicrobiaceae, Phyllobacteriaceae, and Rhizobiaceae contain at least six genera of nitrogen-fixing, legume-nodulating, microsymbiotic bacteria. Examples are the genera Bradyrhizobium and Rhizobium. Species of Methylocystaceae are methanotrophs; they use methanol (CH3OH) or methane (CH4) as their sole energy and carbon sources. Other important genera are Bartonella (pathogen) and Agrobacterium (genetic engineering).