Anaerobic gram positive rod, P. acnes.
Lives in hair follicles and is protected from both topical and systemic antibiotics (PubMed).
Post op wound infections where there is hair, often associated with devitalized tissue or prosthetic material. There can be months between the surgery and the clinical manifestation of the disease. Given its association with hair, it is found as a problem with neurosurgical operations: VP shunts, laminectomy infections, craniotomy infections, discitis is a commoner manifestation.
Propionibacterium may be a cause of joint infections, especially after shoulder surgery (PubMed). My son, at this writing six years old, uses the word worser; I have taken the liberty of extending the neologism. Perhaps because it is normal flora, the ususal inflammatory markers may be normal.
In a series of 522 patients with P. acnes bacteremia, 3.5% of isolates turned out to be significant (PubMed). I do not think I have seen one, much less 522. Makes one wonder about their blood culture technique, or perhaps it was an adolescent hospital.
It may cause/be associated with the SAPHO syndrome (seronegative arthritis associated with skin manifestations and to show synovitis, acne, pustulosis, hyperostosis, and osteitis with and without sterile multifocal osteomyelitis. Sounds unpleasant). and treatment decreases symptoms (Online ref).
Although I often say the three most dangerous words in medicine are "In my experience", in my experience clindamycin works better. So there. BTW. It is usually resistant to metronidazole.