Propionibacterium acnes are a widespread opportunistic pathogen that forms part of the microbiota of human skin and mucous membranes. This anaerobic bacterium has had a long association with the common inflammatory skin condition acne vulgaris, and it is also a recognised cause of ophthalmic, dental, and indwelling medical device-related infections, and a possible cause of various chronic human conditions including sarcoidosis, SAPHO syndrome, and prostate cancer. In stark contrast to its role as a pathogen, the powerful adjuvant properties of P. acnes have long been investigated for their ability to stimulate protective host immune responses against various human cancers and more generally against Th-2-mediated diseases. Within the last decade, molecular phylogenetic and genetic population studies have identified distinct phylogroups and lineages that appear associated with disease or maintaining health. This combined with the large amount of genomic data resulting from the human microbiome project and other sequencing programs now provides a sound evidential basis for hypothesis-driven research into the exact role of this organism in human disease, and the possibility of developing vaccines and other novel therapeutic treatments for P. acnes conditions.

The aim of this special issue is to provide a description of recent developments and advances in different areas of P. acnes research, and thus to provide a framework for discussion and the identification of key issues that need to be addressed in future studies. We are especially interested in original research articles, as well as reviews, that describe host-pathogen interactions, mechanisms of virulence, whole-genome studies, and vaccine development. Manuscripts that describe the role of P. acnes in different disease processes, antibiotic susceptibility, phylogenetic and population genetic studies, molecular genetic systems, and P. acnes bacteriophages would also be welcome. Potential topics include, but are not limited to:

• Culture, cellular physiology, and phenotype
• Host-pathogen interactions and immune responses
• Putative colonisation factors and determinants of niche specificity
• Putative determinants of virulence
• Comparative genomics
• Role in infection and chronic human disease
• Antibiotic susceptibility
• Molecular phylogenetics and epidemiology
• Genetic systems
• Animal and other models
• P. acnes vaccines
• P. acnes bacteriophages: basic biology and potential therapeutic applications

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