Staphylococcus epidermidis DnaK alters biofilm formation and proteome in Staphylococcus aureus CIP 107093

<i>Staphylococcus aureus</i> and <i>Staphylococcus epidermidis</i>, two Gram-positive bacteria of the human skin microbiota, form biofilms that contribute to dysbiosis and inflammatory skin diseases such as psoriasis and atopic dermatitis. The human calcitonin gene-related peptide (CGRP), involved in skin inflammation, was previously shown to enhance the virulence of <i>S. epidermidis</i> MFP04. We previously observed a significant increase in the level of the molecular chaperone DnaK/Hsp70 in the secretome of CGRP-activated <i>S. epidermidis</i>. Here, we investigated the role of recombinant <i>S. epidermidis</i> DnaK in biofilm formation in both <i>S. aureus</i> and <i>S. epidermidis</i>. DnaK modulates biofilm formation in a strain-dependent manner. In commensal strains (<i>S. aureus</i> MFP03 and <i>S. epidermidis</i> MFP04), it is associated with an increase in biofilm biomass. In contrast, it significantly reduces biofilm formation in the clinical <i>S. aureus</i> strain CIP 107093. Point mutations in the substrate-binding domain (SBD) and nucleotide-binding domain (NBD) of DnaK differentially affect its modulation of biofilm formation. Specifically, only the mutation in the SBD abolishes the biofilm reduction observed in CIP 107093, while the NBD mutation results in a milder effect. Notably, these mutations have no significant impact on DnaK-induced biofilm changes in strains where DnaK promotes biofilm formation. Proteomic analyses of <i>S. aureus</i> CIP 107093 reveal that DnaK alters the <i>S. aureus</i> biofilm proteome, stabilizing protein degradation components and downregulating key biofilm regulators. These findings highlight the cross-species regulatory potential of <i>S. epidermidis</i> extracellular DnaK in the skin microbiota.