Proprionibacterium (Cutibacterium) acnes is the organism most commonly associated with deep infection after shoulder surgery, including shoulder arthroplasty. It is abundant in the sebaceous follicles of the axilla and skin around the shoulder girdle, being implicated in the pathology of acne. It is adapted to the environment of the hair follicle, where it secretes enzymes that digest sebum and provide nutrients. However in this location it is unaffected by skin preparation immediately prior to the skin incision and can therefore be carried into deeper layers Indeed, it is so commonly found in the deep tissues around the shoulder in cases with no suspected infection that the line between contaminant and pathogen can be extremely difficult to draw. To add to this, it is slow growing and culture in anaerobic conditions often does not yield a positive result until the second week of culture or later. An understanding of the organism and its impact on shoulder surgery is therefore crucial to those operating in this region.