A genus of gram-negative, pleomorphic bacteria that are facultative anaerobes and are nonmotile and non-spore-forming. They have fastidious growth requirements, including either factor X (provided by hemin or other porphyrins) or factor V (provided by nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP), or nicotinamide nucleoside) or both, depending on the species. Erythrocytes represent an exogenous source of these factors.