Hemolysis as a clinical marker for propionibacterium acnes orthopedic infection

Determining if a Propionibacterium acnes culture is a true infection or a contaminant remains a challenge. We conducted a study to distinguish between a true infection and a contaminated culture based on the P acnes hemolytic phenotype and clinical presentation. All P acnes strains were from orthopedic patients who had undergone arthroplasty or nonarthroplasty shoulder procedures. Hemolysis was determined according to P acnes growth on brucella blood agar plates after 48 to 72 hours. Each patient record that corresponded to the obtained P acnes strains was retrospectively reviewed for clinical data. An orthopedic surgeon involved in the care of the patients, but blinded to the hemolytic status of the bacteria, classified these infections as definite, likely, or unlikely. Of the 22 P acnes strains, 13 were hemolytic, and 9 were nonhemolytic. Of the 13 hemolytic strains, 10 were definite infections; only 3 of the 9 nonhemolytic strains were definite infections. Mean (SD) C-reactive protein level was significantly higher (P = .03) in the hemolytic group, 16 (11) mg/mL, than in the nonhemolytic group, 7.9 (10) mg/mL. A hemolytic phenotype of P acnes may represent a more pathogenic strain of bacteria, and may be more likely to be found in patients with a definite infection with P acnes rather than a contaminated culture.