Different faces of Propionibacterium acnes infections: insights into inflammatory acne, prostate cancer and biofilm infection

Propionibacterium acnes is a Gram-positive aerotolerant anaerobe that is found predominately on the skin where it forms part of the resident microbiota. It represents the major opportunistic pathogen within the ‘cutaneous’ group of propionibacteria and has been linked, sometimes controversially, to a range of infections and conditions. Historically, the role of P. acnes in certain infections has almost certainly been underestimated due to diagnostic protocols that have poor sensitivity for the detection of the bacterium combined with dismissal as a contaminant when it is recovered. Today, data emerging from P. acnes whole genome sequencing projects, phylogenetic and genetic population analyses, and studies at the transcriptome and proteome level are challenging our understanding of this bacterium and its capacity to cause disease. In particular, isolates from the four genetic divisions (IA, IB, II and III) of P. acnes differ with regard to inflammatory potential and the production of specific virulence determinants, including proteins with phase/antigenic variation signatures, CAMP factor homologues, haemolysins, and lipase. Recent studies have found a very clear association between isolates from the type IA division, including a globally dispersed clonal lineage, and acne vulgaris. In contrast, isolates from the type IB, II and III divisions are rarely associated with this condition, although they appear to be more frequently recovered from soft tissue and medical implant-related infections. In addition to a discussion of P. acnes epidemiology in relation to acne, we will consider recent data from studies investigating a possible link between P. acnes infection of the prostate gland and prostate cancer development, and review the growing body of evidence that supports P. acnes as an important pathogen in relation to biofilm infections of prosthetic joints.