Haemophilus aphrophilus is a facultatively anaerobic gram-negative bacillus and require 5 to 10 % to grow optimally. H. aphrophilus is differentiated from other members of Haemophilus species by no requirement of X or V factor. This organism is found as the normal flora in upper respiratory tract but a member of the HACEK group that cause native valve endocarditis. Since the first endocarditis of H. aphrophilus was reported at 1985 in Korea, we reported the second case. 

A 35-year-old male patient was admitted to Asan Medical Center because of fever for 15 days and altered mentality developed 2 days ago. His echocardiography revealed a mitral valve regurgitation with a hypermobile vegetation and multiple septic emboli were also found in the brain MRI. Three sets of blood cultures were taken on the day of admission, all of which grew pleomorphic, gram-negative bacilli at incubation day 1. Catalase and oxidase test was negative and Vitek NHI card (bioMerieux Vitek, Inc., Hazelwood Mo., USA) identified the organisms to H. aphrophilus 50%/H. paraphrophilus 49% (Bionumber 257310). It was finally identified to H. aphrophilus with requirement tests of X or V factors; it required neither X nor V factor. This H. aphrophilus strain was negative in lactamase and was susceptible to ampicillin, gentamicin, cefuroxime, imipenem, ciprofloxacin, aztreonam, azithromycin, rifampin, and trimethoprim/sulfamethoxazole. This patient was successfully treated with ampicillin and gentamicin after mitral valve replacement under diagnosis of H. aphrophilus endocarditis. (Korean J Clin Microbiol 2003;6(2):172-176)