Bacterial and fungal co-colonization alters Malassezia restricta metabolism and host proteolytic activity in reconstructed human epidermis.

INTRODUCTION: On the skin, bacteria and fungi form mixed communities whose interactions can reshape both microbial physiology and host tissue, yet the microbial mechanisms remain poorly defined. Although polymicrobial colonization severity often exceeds the effects of individual pathogens in these conditions, the mechanism driving this synergy is unclear. METHODS: We used a three-dimensional reconstructed human epidermis to characterize how co-colonization by Staphylococcus aureus and Malassezia restricta alters barrier structure. RESULTS: Co-colonized tissue thickened in the viable layers rather than the stratum corneum, a pattern absent from both monocultures whereas M. restricta alone thickened the stratum corneum. Filaggrin redistributed toward the stratum corneum, and cytokine release and loss of cell viability tracked with the presence of S. aureus. M. restricta shifted its transcriptional program toward reduced oxidative phosphorylation and elevated stress response, yielding 453 differentially expressed genes. Screening these genes for secreted hydrolases returned no candidate that would account for host protease activity. Trypsin-like activity nonetheless rose more than tenfold under co-colonization while remaining at baseline under either species alone. DISCUSSION: Barrier disruption in this model therefore emerges from the two-species condition rather than from either organism which offers one explanation for why polymicrobial colonization tracks with severity in inflammatory skin disease.