Reply to “Anaerobic Thioglycolate Broth Culture for Recovery of Propionibacterium acnes from Shoulder Tissue and Fluid Specimens”

REPLY

It is with great interest that we read the letter by Shannon et al. (1) describing improved time to detection of Propionibacterium acnes from orthopedic periprosthetic tissue and fluid specimens using anaerobic thioglycolate broth. Previous studies (2, 3), including our own recent study (4), observed that extended incubation beyond 7 days was required for optimal recovery of P. acnes. The ability to recover this organism consistently within 7 days using anaerobic thioglycolate broth is a welcome development. As noted by the authors, parameters beyond the duration of culture incubation are certainly likely to influence the recovery of P. acnes. In support of this assertion, we observed that a diagnosis of P. acnes prosthetic joint infection would have been missed in 29.4% of patients had extended incubation of aerobic culture media also not been performed (4). The letter by Shannon et al. did not include any information on aerobic culture media, so it is unclear whether the overall recovery of P. acnes using thioglycolate broth is equivalent to that attained through extended incubation of both aerobic and anaerobic culture media.

We do not believe that specimen volume accounts for the differences in time to positivity observed between both studies: all culture media in our study were inoculated with disposable Pasteur pipettes using specimen volumes identical to those listed in the Shannon et al. letter. In our study, we examined the culture characteristics of both P. acnes-infected and nondiagnostic events (19 and 23 patients, respectively). In doing so, we observed that P. acnes-infected events were 9 times more likely to have ≥2 culture media positive for growth. Although Shannon et al. did not include information on their overall incidence of nondiagnostic P. acnes events, the proportion of P. acnes-infected events lacking growth on anaerobic plate media was identical to that in our study (21% in both studies). No formal comparison of culture media for the recovery of P. acnes from prosthetic joint specimens has been published (5). However, these data suggest that culture medium may indeed have an influence on recovery of this organism.

Although we did not employ specific anaerobic transport devices in our study, the majority of specimens received for culture were tissues over 1 mm³ in size. Importantly, the same group who wrote the Shannon et al. letter published a seminal work showing superior sensitivity of implant sonication cultures over traditional periprosthetic tissue and fluid cultures (6). Interestingly, in the case of the prosthetic shoulders, these explants were not transported under anaerobic conditions and anaerobic culture media were incubated for 14 days (7). Although the superior performance of these specimens is believed to result from improved recovery of biofilm-associated organisms on the surface of the explant, the aerobic conditions used for specimen transport suggest that recovery of P. acnes does not necessarily require that specimens be transported under anaerobic conditions. Further study is therefore required to formally determine the optimal culture media and transport conditions for the recovery of P. acnes from periprosthetic specimens.