Kingdom: Bacteria
Family: Pasteurellaceae
Genus: Propionibacterium
Species: Acnes

The anaerobic diphtheroid bacteria responsible for acne vulgaris (common acne), and normally present in human hair follicles. P. acnes proliferates in the anaerobic growth medium of the skin's sebaceous glands' secretions (sebum), hydrolyzing triglycerides into glycerol and irritating fatty acids.

Distilled from psychiatry.medscape.com, "Approach to Acne Vulgaris", Fern A. Wirth, MD.