A 71-year-old man of Italian origin was admitted to our infectious orthopedic department presenting with chronic (two-year) left knee pain, swelling and decreasing ambulation. He had received a total knee prosthesis for left knee osteoarthritis four years prior. The prosthesis had been replaced twice for radiologically confirmed loosening, with the latest replacement being two years prior to admittance. Cultures of synovial fluid collected during both replacements remained sterile after inoculation of standard solid media. Our patient underwent a dental treatment without antibiotic prophylaxis three weeks before the consultation. Upon physical examination, our patient was found to be apyretic. His left knee was swollen and radiating heat, and a moderately sized effusion was present. His white blood cell count indicated 3.82 × 10
3 polymorphonuclear cells/mL, his C-reactive protein (CRP) level was 32 mg/L and the erythrocyte sedimentation rate (ESR) was 66 mm (first hour). Our patient was referred to the orthopedic department for sample biopsy and arthroscopic lavage. The synovial fluid was directly inoculated into a set of Bactec Plus Aerobic/F and Bactec Lytic/10 Anaerobic/F bottles (BD Diagnostic Systems, Sparks, MD, USA) and collected in parallel into a sterile tube as part of an "arthritis kit", as previously described [
9]. Microscopic examination of the synovial fluid indicated 32 × 10
3 white blood cells/mL (98% neutrophil polymorphonuclear cells and 2% monocytes) and 4.77 × 10
3 red blood cells/mL without crystals. No microorganisms were observed. Inoculation of the synovial fluid onto 5% sheep-blood agar showed no growth after a five-day incubation period at 37°C under a 5% carbon dioxide (CO
2) atmosphere. However, the aerobic broth bottle produced Gram-positive cocci after a five-day incubation period at 37°C under continuous automated monitoring for bacterial growth in the medium. Subculturing these organisms on 5% sheep-blood agar and chocolate agar (BioMérieux, Marcy l'Etoile, France) at 37°C under a 5% CO
2 atmosphere produced colonies. These colonies were identified as
A. defectiva by 16S rDNA sequencing [
10] (99.9% sequence similarity with the GenBank accession number AY879308); E test testing confirmed the isolate to be broadly susceptible with minimum inhibitory concentration of 0.01 mg/L for penicillin G and rifampicin, 0.016 mg/L for amoxicillin, ceftriaxone and vancomycin, 0.02 mg/L for imipenem, 0.1 mg/L for erythromycin and 0.2 mg/L for doxycycline [
11]. PCR-based detection of bacterial DNA using universal 16S rDNA primers [
10] detected
A. defectiva DNA in the synovial fluid on the basis of sequence identity to the DNA recovered from the colonies. Three blood cultures incubated under an aerobic atmosphere and one blood culture incubated under an anaerobic atmosphere showed no growth after a five-day incubation period. A transthoracic echocardiography revealed no signs of endocarditis and a colonoscopy was unremarkable. The knee prosthesis was removed, and a temporary cement spacer containing 2 g vancomycin and 40 g cement was inserted [
12]. Re-implantation of a total knee was delayed because of an intercurrent venal thrombosis without pulmonary embolism. Our patient received 100 mg/kg/day oral amoxicillin for nine months. The knee pain and swelling subsequently resolved, and the ESR decreased to 22 mm. The one-year follow-up examination indicated complete wound healing, with no evidence of a recurrence of infection, a white blood cell count within the normal range, a CRP value of 7.1 mg/L and an ESR of 17 mm.