Acne, quorum sensing and danger.

Propionibacterium acnes is a ubiquitous skin commensal bacterium, which is normally well tolerated by the immune system in healthy human skin. However, there is increasing evidence to suggest a pivotal role for P. acnes in the inflammatory process underlying the acne pathogenesis. With its features of inflammation and pustulation, acne vulgaris resembles the skin's normal reaction to bacterial pathogens. P. acnes flourishes when sebum production increases in the follicles. Bacteria may undergo behavioural changes based on the surrounding bacterial population, a process called quorum sensing (QS). Evidence from in vitro studies suggests that QS enables P. acnes to upregulate its hydrolysis of sebum triglycerides by its bacterial lipases, secreting free fatty acids (FFAs) such as oleic, palmitic and lauric acids. These FFAs act as danger-associated molecular patterns (DAMPs), and activate Toll-like receptor (TLR)2 and TLR4, leading to selective T-helper (Th)-driven immunity, with subsequent expression of Th1/Th17-associated inflammatory cytokines. To our knowledge, there is currently no explanation as to what determines the shift of recognition by the immune system of P. acnes from being symbiotic to pathogenic. We present a novel hypothesis based on the essence of QS and DAMPs. P. acnes sends no or only 'safety' signals when present in 'controlled' quantities under commensal conditions, but becomes pathogenic and sends 'danger' signals via QS in the form of excess FFA production, which stimulates TLR2 and TLR4 as the bacterial population flourishes.