Cutibacterium acnes is Isolated from Air Swabs: Time to Doubt the Value of Traditional Cultures in Shoulder Surgery?

Background: Given high rates of positive Cutibacterium acnes (C. acnes) cultures in cases of both primary and revision shoulder surgery, the ramifications of positive C. acnes cultures remain uncertain. Next generation sequencing (NGS) is a molecular tool that sequences the whole bacterial genome and is capable of identifying pathogens and the relative percent abundance in which they appear within a sample. The purpose of this study was to report the false positive culture rate in negative control specimens and to determine whether NGS has potential value in reducing the rate of false positive results. Methods: Between April 2017 and May 2017 swabs were taken during primary shoulder arthroplasty. After surgical time out, using sterile gloves, a sterile swab was opened and exposed to the air for 5 seconds, returned to its contained, and sealed. One swab was sent to our institution’s microbiology laboratory for aerobic and anaerobic culture and held for 13 days. The other sample was sent for NGS (MicroGen Dx, Lubbock, TX), where samples were amplified for pyrosequencing using a forward and reverse fusion primer and matched against a DNA library for species identification. Results: For 40 consecutive cases, swabs were sent for culture and NGS. C. acnes was identified by culture in 6/40 (15%) swabs and coagulase negative staphylococcus (CNS) was identified in 3/40 (7.5%). Both cases with positive NGS sequencing reported polymicrobial results with one sample (2.5%), including a relative abundance of 3% C. acnes. At 90 days after surgery, there were no cases of clinical infection in any of the 40 cases. Conclusion: We demonstrate that the two most commonly cultured organisms (C. acnes and CNS) during revision shoulder arthroplasty are also the two most commonly cultured organisms from negative control specimens. Contamination can come from air in the operating room or laboratory contamination.