Molecular approaches examining bacterial diversity have underlined the concept that the skin microbiota is dependent on the body site and that caution should be taken when selecting and comparing sites for skin microbiome studies. Our group and others have demonstrated that colonization of bacteria is dependent on the physiology of the skin site, with specific bacteria being associated with moist, dry and sebaceous microenvironments (FIG. 3). In general, bacterial diversity seems to be lowest in sebaceous sites, suggesting that there is selection for specific subsets of organisms that can tolerate conditions in these areas. Sebaceous sites that contain low phylotype richness include the forehead (six phylotypes43), the retroauricular crease (behind the ear) (15 phylotypes42), the back (17 phylotypes42) and the alar crease (side of the nostril) (18 phylotypes42). Propionibacterium spp. are the dominant organisms in these and other sebaceous areas, which confirms classical microbiological studies that describe Propionibacterium spp. as lipophilic residents of the pilosebaceous unit. Microbial transplant experiments suggest that the microenvironment of sebaceous areas (such as the forehead) is a stronger force in determining microbial colonization than the microenvironment of dry areas (such as the forearm)43.