Identification of variables for aerobic bacterial density at clinically relevant skin sites.

In studies on efficacy testing of topical antimicrobial products, randomisation of test areas and a well-balanced gender ratio are not always standard. Our aim was to generate an evidence-based skin flora map using a systematic review of the literature supplemented by in vivo tests to identify variables that impact on microbial density. Ten out of 83 evaluated studies were reviewed. Microbial density was higher on sebaceous-rich and wet skin sites. In the in vivo study the forehead, upper back, lumbar area, and abdomen of 180 subjects were sampled with a standardised swab method. The highest aerobic microbial density was found on the forehead (mean log₁₀ cfu/cm² = 3.69 ± 1.00), followed by the upper back (3.00 ± 0.90), the abdomen (2.98 ± 0.74), and the lumbar area (2.35 ± 0.70). The difference between all four skin sites was significant (P < 0.001; analysis of variance). On the forehead, we found significantly more micro-organisms on the medial compared to the lateral side (P = 0.002; t-test), on the upper back we found more micro-organisms cranially than caudally (P = 0.006). Males carried significantly more micro-organisms on all sites (P < 0.001). Randomisation of the test areas is essential to obtain representative results in studies on the density of skin flora or the efficacy of skin antiseptics. A well-balanced gender ratio is also strongly recommended for any study population.