Anti-inflammatory effect of a Bacillus halotolerans DY299 strain isolated from Arshan on methicillin-resistant Staphylococcus aureus-induced atopic dermatitis in mouse

Introduction Methicillin-resistant Staphylococcus aureus (MRSA) colonization is a major contributor to atopic dermatitis (AD), and its rising resistance and virulence underscore the urgent need for novel antimicrobial strategies. Methods In this study, a halotolerant strain designated Bacillus halotolerans DY299 was isolated from Arshan water. The cell-free culture supernatant (DY299-CFS) was prepared and its anti-biofilm and antibacterial activities against MRSA were evaluated in vitro . The therapeutic efficacy was assessed in an MRSA-induced AD mouse model, with skin lesion severity, stratum corneum hydration, mast cell infiltration, and cutaneous MRSA colonization examined. Serum levels of IgE, IL-13, IFN-γ, and IL-17 were measured by ELISA. To elucidate the molecular mechanisms, whole-genome sequencing of DY299, LC-MS/MS metabolomic profiling of its supernatant, transcriptomic analysis of skin tissues, and network pharmacology-based target prediction were integrated. Results DY299-CFS demonstrated significant anti-biofilm activity, achieving an inhibition rate of 50.93% and effectively eradicating MRSA biofilm both in vitro and in vivo . Topical DY299-CFS treatment markedly alleviated inflammatory skin lesions, improved stratum corneum hydration, reduced mast cell infiltration, and suppressed cutaneous MRSA colonization in AD mice. ELISA assays revealed that DY299-CFS significantly decreased serum levels of IgE, IL-13, IFN-γ, and IL-17 compared with the model group, indicating concurrent suppression of Th2-type and Th1/Th17-type immune responses. Discussion Multi-omics analysis collectively identified the IL-17 signaling pathway as a key mediator through which DY299-CFS exerts its anti-inflammatory effects against MRSA-induced skin inflammation. Collectively, these findings establish Bacillus halotolerans DY299 as a promising anti-biofilm agent against MRSA and a valuable microbial resource for developing novel therapeutics for biofilm-driven inflammatory skin diseases such as AD, offering a multifaceted strategy that combines direct antimicrobial action with immunomodulatory effects.