Definition: A family of bacteria (order Eubacteriales) containing small, coccoid to rod-shaped, gram-negative cells which occur singly, in pairs, in short chains, or in groups. The cells may or may not show bipolar staining. Motile and nonmotile species occur; motile cells are peritrichous. V (phosphopyridine nucleotide) and/or X (hemin) factors are sometimes required for growth. Blood serum may be required or may enhance growth. Increased carbon dioxide tension may also favor growth, especially on primary isolation. These organisms are parasites and pathogens that affect warm-blooded animals, including humans, rarely cold-blooded animals; formerly called Parvobacteriaceae; type genus is Brucella.