Pathogenicity of Propionibacterium acnes in mixed infections with facultative bacteria.

Single and mixed infections with 11 clinical isolates of Propionibacterium acnes and three facultative bacteria (Staphylococcus aureus, Escherichia coli and Klebsiella pneumoniae) were studied in a subcutaneous abscess model in mice. Abscesses were induced by pure cultures of six of 11 strains of P. acnes and by the three facultative bacteria. The abscesses produced by each of the six "virulent" P. acnes isolates mixed with S. aureus, E. coli or K. pneumoniae were larger than those induced by the single organisms in 16 of the 18 combinations. There was a significant increase in the numbers of the six P. acnes strains in 13 of the 18 bacterial mixtures and in the numbers of the facultative bacteria in 17 of the 18 combinations. These data illustrate the potential virulence of some P. acnes strains and their synergic capacity with facultative bacteria.