Propionibacterium acnes

One of the long-lasting open questions in understanding acne is the role of the skin bacterium Propionibacterium acnes. Opinions within the scientific community diverge regarding the importance of this Gram-positive bacterium not only in acne but also in other P. acnes-associated diseases. The recently decoded genome gives us some clues with respect to its pathogenic potential and its strategies to survive in the harsh environment of human skin.
The problem with assigning a proper role to Propionibacterium acnes in causing disease starts with its ubiquitous presence on human skin, where it usually resides as a harmless commensal, preferably within sebaceous follicles. However, evidence has increased that P. acnes has some features of an opportunistic pathogen. The bacterium has been isolated from sites of infection and inflammation ranging from acne to diverse other conditions such as late-stage prosthetic joint infections, endocarditis, endophthalmitis, osteomyelitis, and shunt-associated central nervous system infections. Under appropriate conditions, it has been shown that P. acnes has cytotoxic properties. The bacterium exhibits extensive antigenic activity; humoral, cell-mediated and innate immune responses to P. acnes have been reported. Several hypotheses have been made that address the implication of P. acnes in acne formation. The damage of skin tissue might be the result of bacterial enzymes with degradative properties, which could affect the integrity of epidermal skin cells and the barrier function of the follicular wall of sebaceous follicles. This could lead to the release of proinflammatory cytokines from adjacent keratinocytes, which will diffuse to the dermis to initiate inflammation. Other factors of P. acnes might be involved in triggering inflammation, either constitutively produced factors such as porphyrins or surface determinants like a glycocalyx polymer or proteins produced under stress conditions due to nutritional depletion, an acidic shift, or heat shock. With the decipherment of its complete genome some of P. acnes’ mysterious and contradictory traits have become or will become easier to track down. Putative virulence factors as predicted by the genome annotation can now be studied and tested for in vivo functions. The decoded genome together with postgenomic studies on its transcriptome and proteome will eventually clarify our view and pinpoint the role of this bacterium in the etiology and pathogenesis of acne and other P. acnes-associated diseases.

Circular map of the chromosome of P. acnes. The coding sequence of the chromosome is shown in yellow or green, depending on strand orientation. ORFs of P. acnes with aberrated codon usage statistics are shown in red (genomic islands). The inner blue ring shows the G+C content variation (higher values outward).