Review: the human cutaneous microflora and factors controlling colonisation

The human skin is an unusual habitat for microorganisms in that it is open to contamination from the environment and yet is largely unsuitable for microbial colonisation, unlike mucosal surfaces. The normal microflora of human skin consists of resident colonising species capable of maintaining a viable, reproducing population on the skin and transient contaminating species that cannot sustain growth in the cutaneous environment. The structure of the skin and physiological factors such as hydration, pH, O2 and growth substrates determine the density and diversity of colonisation. Ecological stability is maintained by interactions between the host and the microflora, and between microbial species, and the relative importance of these factors varies between individuals at equivalent sites. The distribution of skin appendages at different sites on the body determines the prevailing environmental conditions, which in turn affects the density and diversity of the microflora. Microbial colonisation is not only restricted to the surface of the skin and there are substantial populations associated with the skin appendages, in particular sebaceous follicles. The aim of this article is to review the factors which determine the composition of the skin microflora under normal conditions and assess their relative importance.