Abiotrophia defectiva is a nutritionally variant streptococcus (NVS) and is found not infrequently in infective endocarditis patients with a negative blood culture, and thus, other methods like polymerase chain reaction are required to detect this organism [1]. A. defectiva was firstly identified by Frenkel and Hirsch [2] in 1961 in a case of sub-acute infectious endocarditis.

Because A. defectiva is primarily isolated from the oral cavity or intestinal and genitourinary tracts, it can harm normal valves in the absence of any underlying cardiac or immunosuppressive illness or previous dental manipulation. However, the bacterium affects diseased valves more frequently, by causing embolic complications and valvular destructions [34]. It has been reported infective endocarditis attributable to A. defectiva accounted for ~5% of all microbial endocarditis cases [5], but its incidence appears to be decreasing. Furthermore, the bacterium rarely involve intact valves, so physicians and laboratory workers may not familiar with this organism. Here we report a case of infective endocarditis due to A. defectiva in an otherwise healthy adult and provide a review of recent literature.

A 67-year old male with a complaint of aggravating dyspnea of three months duration was transferred to Pusan National University Yangsan Hospital under suspicion of infective endocarditis. He had not undergone any recent surgical or dental procedure. Physical examination revealed; body temperature 36.1℃, heart rate 86 beats/min, a hypotensive status (90/60 mmHg), and an oxygen saturation of 98% on an oxygen supply of 3 L/min via a nasal cannula. Cardiac auscultation revealed a regular rate and rhythm with a pansystolic murmur at the apex, and coarse crepitation in both lungs. No Janeway's lesions, Osler's nodes or Roth's spot were observed. Chest X-ray showed diffuse bilateral thoracic haziness with suspicion of pulmonary edema. Transesophageal echocardiography showed severe mitral regurgitation with resting pulmonary hypertension and vegetation (1.0×2.6 cm sized) on anterior and posterior mitral leaflets. Blood testing revealed anemia (Hb 9.6 g/dL; reference range 13.5–17.5 g/dL) and a normal leukocyte count (8,640 cells/mm³; reference range 6,510–13,320 cells/mm³). Serum C-reactive protein (7.75 mg/dL; reference range 0–0.5 mg/dL) and B-type natriuretic peptide (681 pg/mL; reference range 0–100 pg/mL) were elevated. In the absence of any neurological symptom, preoperative brain MRI (magnetic resonance imaging) showed multiple diffuse restriction foci in both cerebral hemispheres and left cerebellum, and subarachnoid hemorrhage (SAH) along both parietal and right occipital sulci. Blood cultures were requested and ceftriaxone and vancomycin were started empirically. To prevent further embolism by the cardiac vegetation, emergent mitral valve replacement was conducted through right mini-thoracotomy. Intraoperative findings showed massive destruction of anterior and posterior mitral valve leaflets with huge vegetation, which extended to posterior medial annulus of the mitral valve and to posterior left atrial endocardium. After massive debridement of all infected tissues, the mitral valve was replaced with a Carpentier-Edwards Perimount Magna mitral valve bioprosthesis (Edwards Lifesciences, Irvine, CA, USA). The preoperative blood culture revealed Gram positive cocci in three sets of culture bottles. The organism was identified as A. defectiva by MALDI-ToF/MS (matrix-assisted laser desorption/ionization time-of-flight mass spectrometry; bioMérieux, Marcy-l'Étoile, France), and E-testing showed susceptibility to penicillin and vancomycin (bioMérieux, Durham, NC, USA). Accordingly, vancomycin, ampicillin and gentamicin antibiotic treatment was continued for 6 weeks. His postoperative recovery course was uncomplicated and resulted in complete disease resolution. At the time of writing the patient had been followed uneventfully for 4 months.

A. defectiva endocarditis cases have been continuously reported since the bacterium was first identified as a cause of sub-acute infectious endocarditis in 1961 [2]. Roberts et al. [5] reported A. defectiva, which was originally called Streptococcus mitior or vitamin B6-dependent streptococcus, accounted for 5–6% of all microbial endocarditis cases during the periods 1944 to 1955 and 1970 to 1978. Subsequently the incidence of A. defectiva associated infective endocarditis seemed to decrease. For example, Brouqui and Raoult [6] reported in 2001 that 4.3% of cases of streptococcal endocarditis, that is, not all cases of microbial endocarditis, were caused by Abiotrophia spp., and Raoult et al. [7] reported in 2005 that only 2 of 348 microbiologically confirmed endocarditis were caused by A. defectiva. Recently, Doig et al. [8] reported Abiotrophia spp. was the etiology in 4 of 112 (3.6%) cases of infective endocarditis.

Since 2015, 17 cases (including our case) of A. defectiva endocarditis have been reported in the English literature (Table 1) [149101112131415161718192021], and these cases show a male predominance (11:6) and a mean age of 48.5±18.1 years. Approximately 2/3 had a pre-existing heart disease and of 15 with other medical conditions, eight had a history of some specific event like tooth extraction or pregnancy. Thus, only one case, two including the current case, did not have a pre-existing heart problem or medical condition. Among the 17 cases, the mitral valve was most frequently involved. Fortunately, all cases were successfully treated with appropriate antibiotics and/or surgery, although cardiac transplantation was needed in one case [12].

The microbiological aspects of this organism are of concern. A. defectiva requires specific growth factors, including vitamin B6, and is rarely isolated from clinical specimens, as is demonstrated by the literature [5678]. Accordingly, because it is only rarely detected laboratory workers are likely to be unfamiliar with the microorganism. However, modern automated blood culture and MALDI-ToF/MS made it easier to cultivate and identify this bacterium. Actually, our literature review showed three of six cases, in which identification methods were specified, were identified by MALDI-ToF/MS (Table 1). Antimicrobial susceptibility data for A. defectiva is also limited. Generally speaking, A. defectiva associated infective endocarditis is less susceptible to penicillin, synergistically responds to beta-lactams or vancomycin with aminoglycosides and requires long-term combination therapy (4 to 6 weeks) [22]. As noted in Table 1, a combination of beta-lactams or vancomycin and aminoglycosides were administered to 12 of 15 cases, in which treatment regimens were specified.

In conclusion, A. defectiva infective endocarditis is rarely encountered. Because of the needs for urgent surgery and long-term antibiotic therapy and likely lack of laboratory experience of the organism, physicians and laboratory workers should pay close attention to possible cases with a Gram positive cocci blood culture result.