In the gut, microbial communities stabilize around 3 years of age41. Before this stabilization, strains are likely acquired from close contacts and family members42 and are predicted to be maintained throughout life29. By contrast, the relative abundance of skin microbial species is restructured during puberty, a time when increased levels of hormones stimulate the sebaceous glands to produce additional sebum.
Overall, these age-related changes in the skin microbiota are interesting, as many skin disorders are associated with age.
The prevalent teenage condition acne vulgaris is a chronic inflammatory skin condition that is associated with the bacterium P. acnes, the most abundant organism in the microbiota of healthy adults. At a functional level, gene expression profiles of P. acnes are distinct between individuals with acne and individuals without acne. The observation that almost all adults are colonized with P. acnes but only a minority have acne highlights the importance of studying diseases in the broader context of host genetics, immune or barrier defects, the microbiome and the environment. For example, increased sebum secretion is associated with the pathophysiology of acne, as secretion rates correlate with the severity of clinical symptoms.
DNA sequencing is a useful and unbiased tool for revealing the microorganisms in a sample; however, it is unable to differentiate between live colonizing and dead transient organisms. Although traditional culture techniques can distinguish between live and dead microorganisms, results are skewed by the culture conditions used. RNA sequencing may address this issue by revealing the functional activity of the microbiota, but it is technically difficult given the low biomass of organisms on skin. To measure this activity indirectly, a new analysis technique was developed that compares read distributions at the origin of replication and with those elsewhere in the genome as evidence of active bacterial replication1.