What's already known about this topic:-Microbes are integral components of the human ecosystem.-The cutaneous microbiota plays an important role in the regulation of skin homeostasis.-The composition of skin microbiota is influenced by many factors. What does this study add?-The dominance of P. acnes in the postadolescent sebum-rich skin regions and its role in acne pathogenesis may be explained by the disappearing microbiota hypothesis. Conflict of interest:The authors declare no conflict of interest. 2 AbstractFrom our 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 life style, have been suggested to shape the composition of this microbial community. Propionibacterium acnes (P. 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 to natural populations living more traditional lifestyles. According to the disappearing microbiota hypothesis proposed by Martin Blaser a few years ago, 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 were 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 happen in the case of P. acnes in acne pathogenesis.3

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