Gut bacteria and skin health

Abstract

Recently, gut commensal bacteria and their metabolites have been identified as important factors impacting host homeostasis. This chapter focuses on the relationship between phenolic metabolites produced by gut bacteria and the differentiation of host skin keratinocytes. In in vitro experiments, we have shown that phenols (phenol and p-cresol) disturb the differentiation of monolayer-cultured keratinocytes (Section 2.1). In hairless mice, a tyrosine-enriched diet caused increased levels of phenols in caecal contents, serum and flank skin, and imparted a yellowish dullness to skin, as determined by increased color meter b* values. Additional in vivo experiments were performed by challenging gnotobiotic mice with phenol-producing Morganella morganii and phenol-non-producing Escherichia coli. The skin of M. morganii-colonized gnotobiotic mice exhibited higher b* values and disturbed keratinocyte differentiation compared to E. coli-colonized gnotobiotic mice (Section 2.2). To examine the effects of gut bacteria on skin condition in humans, we conducted a prebiotic-beverage administration trial involving 19 healthy female volunteers who refrained from consuming pro- or prebiotics during a 3-week restriction period, and were then administered one daily prebiotic beverage for 3 weeks. Although serum p-cresol levels significantly increased during the restriction period, the levels markedly decreased on prebiotic administration. In addition, keratinocyte differentiation and skin conductance, an indicator of skin moisture level, significantly declined during the restriction period, but recovered following prebiotic administration (Section 2.3). Our results suggest that phenols produced by gut bacteria are absorbed, distributed by the circulatory system, and accumulate in skin, where they subsequently cause skin dullness and dryness through disruption of normal keratinocyte differentiation. Finally, we discuss approaches for decreasing phenolic metabolites through diet modification, and describe a pilot study involving the intake of a synbiotic beverage, which appears to be a promising treatment for maintaining healthy skin via decreasing p-cresol level (Section 2.4).