There are several different types of bacteria that are found on the skin surface and in the pilosebaceous ducts of acne patients. The one that appears to have the most impact on acne is Propionibacterium acnes (P. acnes).²

When P. acnes are trapped by the comedone, they multiply and infect the pilosebaceous duct. As the bacteria multiply, metabolize their own waste products, and die within this enclosed environment, they stimulate production of inflammatory cytokines. There is also strong evidence that at least some of the inflammation associated with acne may be from a triggered immune response to P. acnes.²

It has been suggested that some people are more sensitive to P. acnes, which may explain some of the differences in acne severity from one person to another. P. acnes can also trigger a serious systemic condition called acne fulminans characterized by severe inflammatory acne ulcers, fever, and arthritis.¹