Risk factors for increased shoulder Cutibacterium acnes burden

BACKGROUND: Cutibacterium acnes is the primary cause of shoulder surgery infections, but the predisposition to larger skin counts and potentially higher risk for postoperative infection remains unclear. This study aimed to quantify risk factors influencing endogenous C. acnes burden and to compare counts among 4 shoulder sites. METHODS: C. acnes counts were quantified via a detergent scrub technique for 173 participants. Bivariate and multivariable stepwise linear regression statistical analyses were used to investigate the association of sex, age, ethnicity, degree of hirsutism, diabetes, smoking status, body mass index, and location with counts. A separate Wilcoxon rank-sum test was performed analyzing counts of East/Southeast Asians vs. all other ethnicities. RESULTS: Sex, age, degree of hirsutism, diabetes, smoking status, and body mass index were included in the multivariable stepwise linear regression analysis. The multiple regression analysis isolated individuals <40 years with the highest burden (P = .001). Males had a 191% increase in C. acnes counts compared with females (P = .001). Increased hirsutism was further indicated to be a risk factor for the male sex although not in a dose-dependent manner (P = .027). Wilcoxon rank-sum test results found that East/Southeast Asians had the lowest load (P = .019), although not significant in the multivariate model. CONCLUSION: Surgical site C. acnes infections occur more frequently in younger males, and males <40 years with shoulder-specific hirsutism have the highest preoperative burden. East/Southeast Asians have lower raw counts of C. acnes compared with other ethnicities that may be related to less hirsutism.