Skin surface area
Richard Gallo, MD, a dermatologist from the University of California, San Diego, who is a leader in the microbiome field of study, says that estimates of the cutaneous microbiome’s impact on human health via skin have failed to acknowledge the inner follicular surface, thus drastically undervaluing the potential of the cutaneous microbiome to influence systemic health.5 He suggests that the surface area of skin has been miscalculated as measuring 2 m2 because it is considered a flat surface. This ignores the plethora of hair follicles and sweat ducts that significantly broaden the epithelial surface to measure closer to 25 m2 and underscores that the expansive skin microbiome is much larger than previously recognized.5 Taking the hair follicle surface area into account, the skin has vast space to harbor various organisms and microbiome environments. What our patients use on their skin certainly influences these environments. The key is trying to figure out how to manipulate the microbiome to our patient’s advantage.
Microbes have environmental preferences
Different microbial species thrive on particular regions of the diverse topography of the expansive surface area and choose their preferred environments from among sebaceous or nonsebaceous, hairy or smooth, moist or dry, and creased or noncreased areas.6,7 Other host factors that affect which microorganisms colonize the skin include hair follicle thickness, age, sex, diet (especially high fat and sugar intake), climate, occupation, and personal hygiene.7-10 Gene sequencing has revealed that these variations are partially because of factors such as ultraviolet exposure, pH, and temperature.4,6,11 For example, C. acnes has been found to be more prevalent in highly sebaceous sites on the head and upper torso.4 In general, Propionibacteriaceae (Cutibacterium) prefer sebaceous areas, whereas Corynebacteriaceae and Staphylococcaceae prevail in moist regions, such as the navel or axilla. Dry areas host the widest diversity of microbes, including Corynebacterium, Staphylococcus, and Streptococcus species.1,7,12
Impact of sebum and skin hydration on microbiome
In 2016, Mukherjee et al. measured sebum and hydration from the forehead and cheeks of 30 healthy female volunteers in a study that tested the hypothesis that differences in sebum and hydration levels in specific facial areas account for interindividual variation in facial skin microbiome. They found that the most significant predictor of microbiome composition was cheek sebum level, followed by forehead hydration level, while cheek hydration and forehead sebum levels were not predictive. The prevalence of Actinobacteria/Propionibacterium rose, while microbiome diversity diminished with an increase in cheek sebum, with such trends reversed in relation to forehead hydration. The investigators concluded that site-specific sebum and water levels impact the nature and diversity of the facial skin microbiome.13
Lability of the cutaneous microbiome
The skin microbiome changes during various times of life. For example, in puberty, more lipophilic species such as Propionibacteriaceae and Cornebacteriaceae predominate, while prior to puberty there is a preponderance of Firmicutes, Bacteroidetes, and Proteobacteria.4,14 However, in the absence of lifestyle changes, cutaneous microbial communities have been found through longitudinal studies to be relatively stable over a 2-year period.6 A person’s skin microbiome is subject to influence from an adjacent skin microbiome, such as between cohabiting couples or the influence of breastfeeding mothers.15 It is never too early to consider the role of the microbiome in health and disease. For example, infant microbiomes play a role in eczema and the atopic march.16 For this reason, those of us who treat children need to be familiar with studies that have demonstrate how the cutaneous microbiome is affected by childbirth delivery method, breastfeeding, the mother’s diet antibiotic use during pregnancy and breastfeeding.4,17