Objectives:

Malassezia and Propionibacterium acnes co-exist in the horny plug, and overgrowth of these microorganisms can result in inflammation. Therefore, determination of the composition of the microbiome of the horny plug would enhance our understanding of the pathology of Malassezia and P. acnes. P. acnes also induces the production of proinflammatory cytokines by keratinocytes in vitro; however, the regulatory mechanisms of the P. acnes-induced inflammatory responses are unclear. In this study, the microbiome of the horny plug was analyzed comprehensively. Moreover, the effect of Malassezia species and P. acnes on cytokine production by keratinocytes (NHEK) was investigated.


Methods:
A total of 100 horny plug samples were collected from 10 healthy Japanese subjects (6 males and 4 females). The V1–V3 regions of bacterial 16S rRNA and the D1/D2 regions of fungal 26S rDNA were sequenced on the Illumina MiSeq platform. Malasseziaspecies and P. acnes were cultured with or without NHEK, and the production of IL-6, IL-1α, IL-8, and TNF-α by the keratinocytes was examined.


Results:
M. restricta predominated in the fungal microbiota, followed by M. globosa. P. acnes predominated in the bacterial microbiota, followed by Bifidobacterium and Staphylococcus. In the presence of P. acnes, M. restricta induced the production of IL-6, IL-8, and IL-1α.

Conclusion:
Both Malassezia species and P. acnes interact.

Comprehensive Analysis of the Microbiome of the Horny Plug, and Interaction of Malassezia Species and Propionibacterium acnes in Keratinocytes