Pathogenicity of Propionibacterium acnes in mixed infections with facultative bacteria

This study examined the pathogenic potential of Propionibacterium acnes, a gram-positive anaerobic bacterium normally found on skin and mucous membranes. Using a mouse subcutaneous abscess model, researchers tested 11 clinical P. acnes isolates alone and in combination with three facultative bacteria: Staphylococcus aureus, Escherichia coli, and Klebsiella pneumoniae. Six P. acnes strains induced abscesses when inoculated alone, with abscess surface areas of 39-116 mm². When these six virulent strains were mixed with any of the three facultative bacteria, abscess sizes were significantly larger in 16 of 18 combinations, reaching 128-241 mm². Bacterial counts in mixed infections also increased substantially for both P. acnes and the facultative organisms. The findings demonstrate synergistic interactions between P. acnes and common facultative bacteria, suggesting that while P. acnes is typically considered a contaminant, certain strains possess genuine pathogenic capacity and can enhance infection severity when present with other bacteria. This synergy may be mediated through mutual protection from immune mechanisms or metabolic enhancement.

Key findings

• Six of eleven P. acnes clinical isolates could independently induce subcutaneous abscesses in mice, with correlations to clinical significance
• Mixed infections of virulent P. acnes strains with S. aureus, E. coli, or K. pneumoniae produced significantly larger abscesses than single-organism infections in 16 of 18 combinations
• Bacterial counts increased significantly in mixed infections, with P. acnes numbers rising in 13 of 18 mixtures and facultative bacteria increasing in 17 of 18 combinations
• Demonstrates synergistic pathogenic capacity between P. acnes and common facultative anaerobic bacteria, challenging the perception of P. acnes as merely a clinical contaminant