Stéphane Corvec, PharmD, PhD, Jérémy Luchetta, MSc, and Guillaume Ghislain Aubin, PharmD
Nantes University Hospital, Microbiology Laboratory, Nantes, France
Authors’ Response
Corvec and colleagues wrote an interesting summary and make excellent points about the role of hemolysis in Propionibacterium acnes. P acnes upper extremity infection has become an increasingly recognized problem, and determining whether a P acnes culture represents a true infection or a contaminant is still a challenge. We performed this study in hopes of finding an easily usable characteristic of P acnes that would assist the clinician in identifying P acnes strains as true infections rather than contaminants.
Certain pathogenic characteristics of P acnes have been identified, but the clinical implications of this bacterium are still being evaluated. We recognize that the hemolysis phenotype is a characteristic, and may not be the main pathogenic feature, of certain phylotypes of P acnes. It is possible the hemolytic strains in our study were from the IA and IB phylotypes, but, unfortunately, we did not specifically evaluate for phylogeny in our study. This would have correlated well with the work of Sampedro and colleagues,1 which suggested most deep bone and joint infections occur with type IA and IB P acnes phylotypes. Although less common in orthopedic infections, the type II and III phylotypes of P acnes are also capable of causing deep infection, and may not cause a hemolytic reaction on blood agar, which may be why we had some patients classified as a definite infection that did not have a hemolytic strain of P acnes. It is also possible a hemolytic strain may truly be a contaminant, but we did not observe this in our small case series. A larger series may help elucidate this finding, but the majority of truly infected patients in our case series had a hemolytic P acnes phenotype.
The type of blood agar used could have also influenced our results, as noted in the Table in Corvec and colleagues’ letter. We observed the most robust hemolysis on brucella blood agar, and limited hemolysis on CDC (Centers for Disease Control and Prevention) anaerobe blood agar; however, we did not evaluate multiple different blood agar preparations, which could have identified more hemolytic strains.
In our study, the presence of hemolysis was helpful in determining whether or not a true infection existed, but the absence of the hemolytic phenotype did not offer much additional information. The hemolytic phenotype may be a potential marker for those strains that are more aggressive and possibly represent the IA and IB phylotypes, which, as previously stated, are more commonly found in deep bone and joint infections.1 Hemolysis may serve as a surrogate marker for determining these phylotypes since determining phylogeny in a hospital laboratory is burdensome and not possible in most institutions.
In summary, we agree the hemolytic phenotype is commonly observed in certain P acnes phylotypes, and that not all upper extremity orthopedic P acnes infections will have a hemolytic finding. The genetic differences in P acnes strains are complex, and finding a marker of truly pathogenic strains has yet to be established. Larger studies evaluating the clinical outcomes and laboratory findings of patients with and without hemolytic strains of P acnes and evaluating which blood agar is the best at identifying the hemolytic phenotype may be beneficial. Identifying or combining multiple clinical and microbe-specific characteristics may also help guide treatment recommendations when a positive P acnes culture is identified.
Scott R. Nodzo, MD
John K. Crane, MD, PhD
Thomas R. Duquin, MD
Department of Orthopedics
University at Buffalo
Buffalo, NY