Factors shaping the composition of the cutaneous microbiota.

From birth, we are constantly exposed to bacteria, fungi and viruses, some of which are capable of transiently or permanently inhabiting our different body parts as our microbiota. The majority of our microbial interactions occur during and after birth, and several different factors, including age, sex, genetic constitution, environmental conditions and lifestyle, have been suggested to shape the composition of this microbial community. Propionibacterium acnes is one of the most dominant lipophilic microbes of the postadolescent, sebum-rich human skin regions. Currently, the role of this bacterium in the pathogenesis of the most common inflammatory skin disease, acne vulgaris, is a topic of intense scientific debate. Recent results suggest that Westernization strongly increases the dominance of the Propionibacterium genus in human skin compared with natural populations living more traditional lifestyles. According to the disappearing microbiota hypothesis proposed by Martin Blaser, such alterations in the composition of our microbiota are the possible consequences of socioeconomic and lifestyle changes occurring after the industrial revolution. Evanescence of species that are important elements of the human ecosystem might lead to the overgrowth and subsequent dominance of others because of the lack of ecological competition. Such changes can disturb the fine-tuned balance of the human body and, accordingly, our microbes developed through a long co-evolutionary process. These processes might lead to the transformation of a seemingly harmless species into an opportunistic pathogen through bacterial dysbiosis. This might have happened in the case of P. acnes in acne pathogenesis.