BACKGROUNDMany studies have noted an increase in the number of recognized cases of invasive infections due to Propionibacterium acnes, especially after shoulder replacement surgery. The increase in the number of recognized cases of P. acnes, a nonspore-forming, anaerobic, Gram-positive organism, appears due to both an increase in the number of shoulder operations being performed and more specimens being sent for anaerobic cultures. Nevertheless, the optimal surgical and antibiotic management of P. acnes remains controversial.METHODSWe tested the susceptibility of 106 P. acnes strains from sterile body sites collected at the Erie County Medical Center between 2012 and 2015, using Etest gradient antibiotic strips.RESULTSP. acnes is very susceptible to the penicillins and the first-generation cephalosporins. We noted an association between hemolytic phenotype on Brucella Blood Agar and clindamycin resistance.CONCLUSIONSAntimicrobial susceptibility testing of P. acnes should no longer just be confined to the research laboratory but expanded and incorporated into routine microbiological evaluation of P. acnes. This would improve patient care as well as help clarify the relationship between hemolysis and clindamycin resistance.

Introduction:The purpose of this study was to further evaluate the pathogenicity of hemolytic and nonhemolytic phenotypes of Propionibacterium acnes (P acnes) isolates from shoulders of orthopaedic patients. Methods: Thirty-one patient records were reviewed, which had a positive P acnes shoulder culture from joint aspiration fluid and/or intraoperative tissues for demographics, clinical course, culture, and laboratory data. Patients were categorized as definite infection, probable infection, or probable contaminant. Antibiotic resistance patterns and hemolysis characteristics were subsequently analyzed. Results: Hemolysis demonstrated 100% specificity with a positive predictive value of 100% and 80% sensitivity with a negative predictive value of 73% for determining definite and probable infections. Hundred percent of the patients in the hemolytic group and only 27% of patients in the nonhemolytic group were classified as infected. Presenting inflammatory markers were markedly higher in the hemolytic group. Clindamycin resistance was found in 31% of the hemolytic strains, whereas no antibiotic resistance was observed in the nonhemolytic group. Conclusion: Hemolytic strains of P acnes exhibit enhanced pathogenicity to their host by eliciting a more prominent systemic inflammatory response, increased antibiotic resistance, and a more challenging clinical course. Hemolysis may serve as a specific marker for assisting in diagnosing true infection with P acnes. Level of Evidence: Level III retrospective comparative study.I nfection after orthopaedic shoulder procedures is a devastating complication with notable diagnostic and therapeutic challenges. Treatment often requires prolonged intravenous antibiotics and the potential for multiple surgical procedures. There has been a rapid increase in the number of orthopaedic shoulder procedures performed in the United States, with total shoulder arthroplasty rates projected to increase by more than 150% by the year 2020. [1][2][3] There is a considerable need for improved diag-nostic and treatment algorithms to optimize successful patient outcomes after periprosthetic shoulder infection. The accurate diagnosis of infection and identification of the offending organism(s) continues to be a challenge. This statement is particularly true for Propionibacterium acnes (P acnes), which is one of the most common infecting organisms after orthopaedic shoulder procedures. [4][5][6][7] The prevalence of P acnes in periprosthetic shoulder infections has been shown to be equal or exceed that