BACKGROUND:
Propionibacterium acnes (P. acnes) is frequently cultured in patients with wound infections after shoulder surgery. The purpose of this study was to characterize the colonization of various anatomic locations with P. acnes in order to explain this clinical observation.

METHODS:
Culture samples were collected from the skin overlying the shoulder, hip, and knee of 20 subjects (10 male, 10 female). Semi-quantitative cultures of P. acnes and Staphylococcus species were performed to define bacterial prevalence and burden at each site. The participants completed a questionnaire that assessed skin health, hygiene, and co-morbid medical conditions. Physical examination was performed to define local skin characteristics.

RESULTS:
Anterior and posterior acromial sites had a greater prevalence of P. acnes than the hip (anterior p=0.018; posterior p= 0.038) and knee (anterior p=0.0014; posterior p= 0.035) sites. The axilla had a greater prevalence of P. acnes than the knee (p=0.008). Males had a greater prevalence of P. acnes than females at the anterior (p=0.007) and posterior acromion sites (p=0.025). The burden of P. acnes at the anterior acromion (p=0.024), posterior acromion (p=0.035), and axilla (p=0.03) was greater than the mean burden at the hip. The burden of P. acnes at the anterior acromion (p=0.004), posterior acromion (p=0.007), and axilla (p=0.008) was greater than the mean burden at the knee. Males had a greater burden of P. acnes than females at the acromial sites (anterior p=0.0049; posterior p=0.0131).

CONCLUSIONS:
Propionibacterium acnes colonizes the shoulder at increased rates compared to the knee and hip, and men have a higher bacterial burden than females. These findings are consistent with clinical observations of postoperative shoulder infections.

LEVEL OF EVIDENCE:
Basic science study.





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