CDC nonoxidizer group 2 (NO-2) currently consists of 15 gram-negative, rod-shaped, oxidase-negative, asaccharolytic, brown soluble pigment-producing strains isolated from blood cultures, usually from young adults. On the basis of their cellular fatty acid profiles, NO-2 strains formed a single group that was identical with the profile of Bordetella avium. 16S rRNA sequencing of one NO-2 strain and the type strains of B. pertussis, B. parapertussis, B. bronchiseptica, and B. avium showed a high degree of homology (Ն98% over 1,525 bases). The NO-2 guanine-plus-cytosine content (61.5 to 62.3 mol%) and major ubiquinone analysis (ubiquinone-8) results were both consistent with those for the genus Bordetella. DNA relatedness studies (hydroxyapatite method) confirmed a close relatedness between NO-2 and Bordetella species and demonstrated that NO-2 strains were a single new species. The name B. holmesii sp. nov. is proposed for CDC group NO-2. CDC nonoxidizer group 2 (NO-2) is the vernacular name given to gram-negative, nonoxidizing, brown soluble pigmentproducing rods sent for identification to the Special Bacteriology Reference Laboratory at the Centers for Disease Control and Prevention (CDC). The first of these strains was received in 1983. Since then, 14 additional strains have been received, all of which were isolated from blood cultures obtained, with few exceptions, from young adults. At least four of these strains were isolated from more than one blood culture per patient. Eleven NO-2 strains, including two isolates discovered outside the United States, have been received since 1989, raising the possibility that this group is an emerging pathogen. In this study we characterized the strains of NO-2 biochemically, by cellular fatty acid (CFA) and ubiquinone analysis, guanine-plus-cytosine (GϩC) content, 16S rRNA sequence analysis, and DNA-DNA hybridization. The results indicate that group NO-2 is a single, previously undescribed species whose closest relatives are in the genus Bordetella. We propose the name Bordetella holmesii for NO-2. MATERIALS AND METHODS Bacterial strains and culture conditions. The NO-2 strains and their sources are shown in Table 1. Bordetella pertussis ATCC 9797 T , Bordetella parapertussis ATCC 15311 T , Bordetella avium ATCC 35086 T , and Bordetella bronchiseptica ATCC 19395 T were obtained from the American Type Culture Collection, Rockville, Md. In addition, CFA profiles were determined for B. parapertussis F6288 and F6289, B. bronchiseptica B5821 and G1696, and B. pertussis 042, 083, and 087. The B. parapertussis and B. bronchiseptica strains were submitted to CDC as patient isolates and were identified by the Special Bacteriology Reference Laboratory. The B. pertussis strains were obtained from P. Cassiday, Pertussis Laboratory, Division of Bacterial and Mycotic Diseases, CDC. All strains were suspended in defibrinated rabbit blood and stored in a liquid nitrogen freezer until studied. Unless otherwise indicated, all strains except B. pertussis were grown on heart infusion agar with...