INTRODUCTION
Human skin is a complex barrier organ that provides an ecological niche for a wide range of microorganisms. The majority of these microflora are harmless or beneficial, providing protection against pathogens and playing an important role in modulating the host’s cutaneous innate and adaptive immune systems.1 The symbiosis between the skin and its microbiota (microorganisms on and in the skin, typically identi ed by 16S ribosomal RNA surveys)2 depends on a complex “dialogue” and is necessary for healthy skin and an ef cient skin barrier function.1,3 The skin is constantly exposed to external and internal envi- ronmental factors (eg, ultraviolet radiation, pollution, topical medications, and skin care products) that can alter the balanced relationship between the skin and its microbiota.3 Such disruption may result in increased risk for infections, chronic inflammatory skin diseases (eg, atopic dermatitis, psoriasis, rosacea, acne), and complaints of sensitive, pruritic, and irritated skin.4 The objectives of this paper are to review recent information about the relationship between skin microbiota and barrier function, and to consider mechanisms that may aid in its preservation. THE SKIN MICROBIOTA A single square centimeter of the human skin contains up to one million microorganisms, including diverse communities of viruses, bacteria, fungi, and mites.5 While bacteria account for only 0.1% of this total (1 million/cm2), they are generally considered to be the most important living organisms in this ecosystem. Bacteria are present on the skin surface, deeper layers of the epidermis, the dermis, and dermal adipose tissue. Evolution in Understanding of Skin Bacteria Our understanding of microorganisms living on and in the skin has changed dramatically in the last several years. Culture- based studies indicated that Staphylococcus epidermidis, other coagulase-negative staphylococci, and coryneforms of the Acti- nobacteria phylum were primary bacterial colonizers of the skin.6 However, many organisms may be present that are said to be uncultivable or are outcompeted by organisms that grow more readily in culture. The development of culture-independent mo- lecular techniques for identi cation and quantitation of microbial organisms has revolutionized our view of the skin microbiome. Genomic characterization of bacterial diversity relies on ampli - cation of the 16S ribosomal RNA (16S rRNA) gene by polymerase chain reaction (PCR) directly from skin samples. The 16S rRNA gene exists in all bacteria and archaea but not in eukaryotes ex- cepted for mitochondria. The bacterial landscape obtained by 16S rRNA sequencing is a rst step in knowing skin microbiota and then skin microbiome. The main fault of this technology is that it is blind to difference between dead bacteria and living bacteria. Secondly, bacteria have inducible genes that can be expressed and others that are always running. That means that even when we get a global picture of what is there, we ignore the active genes and how this community works.The 16S rRNA contains both conserved regions that serve as binding sites for
