contaminants, respectively (P b 0.0001). Most streptococcal isolates were pathogens (12%
versus 4% contaminants; P = 0.002) with little overlap in the species encountered in the 2
categories. A similar (nonsignificant) difference was noted with Corynebacterium species
(4% versus 0.6% contaminants; P = 0.2). Micrococcus luteus and Bacillus species were
mostly associated with specimen contamination.
The pathogen and contaminant profile differed with the index joint; there were 79
isolates of P. acnes from 52 subjects. P. acnes was the identified pathogen in 45% (10/22)
of shoulder PJI, compared to 4% (6/156) of PJI in other joints (P b 0.001). When isolated
from a prosthetic shoulder joint, 87% of the P. acnes isolates were pathogens, whereas
when isolated from all other joints, P. acnes was a pathogen in 22% and a contaminant in
46% (P b 0.001).