HDAC9 and neuropathy: The glucose fermentation of C. acnes or its fermentation metabolite butyric acid and its derivative BA-NH-NH-BA may inhibit HDAC to activate AcH3K9 in keratinocytes, successfully ameliorating the CaP-induced pruritic inflammation and neuropathy by inhibition of IL-6 in the skin and p-ERK 1/2 in DRG.