Soil bacteria, nitrite and the skin

Little is known about the composition of the skin microbiome and its potential significance for health and disease in the context of the ‘hygiene hypothesis’. We here propose that mammals evolved with a dermal microflora that contributed to the regulation of body physiology by providing nitrite from commensal ammonia-oxidising bacteria in response to ammonia released during sweating. We further hypothesise that modern skin hygiene practices have led to a gradual loss of these bacteria from our skin. Together with other lifestyle-related changes associated with an insufficient bodily supply with nitrite and depletion of other nitric oxide(NO)-related species, a condition we here define as ‘nitropenia’, this has led to a perturbation of cellular redox signalling which manifests as dysregulated immunity and generalised inflammation. If proven correct, this scenario would provide an additional evolutionary rationale and a mechanistic basis for the simultaneous rises in prevalence of a number of seemingly unrelated chronic illnesses over the last 3–4 decades