Antimicrobial and anti-virulence activities of Mastoparan C and its derivatives against methicillin-resistant Staphylococcus aureus

AIMS: We aimed to quantify the antibacterial potency (MIC, MBC) of a wasp venom-derived antimicrobial peptide, Mastoparan C (MPC), and its alanine to lysine analogues against methicillin-resistant Staphylococcus aureus (MRSA); elucidate their mechanisms of action via circular dichroism (CD), molecular dynamics (MD) and membrane-integrity assays; assess their impact on virulence traits (staphyloxanthin, biofilm) at phenotypic and transcriptional levels; and evaluate antibiotic sensitisation in a sequential sub-MIC co-exposure model. METHODS AND RESULTS: CD and MD revealed α-helical contents of ∼75% for MPC, ∼70-73% for MPC-A8K and MPC-A5K, A8K, and ∼10% for MPC-A5K. All peptides exhibited MICs of 12.5-25 µg ml-1 and MBC/MIC ≤ 4, classifying them as bactericidal, and achieved increased nucleic acid accessibility, consistent with early membrane perturbation at ½×MIC and comparable to 1% Triton X-100. At ½×MIC, staphyloxanthin production fell by 40-60% and biofilm biomass by 50-70%, relative to the untreated MRSA ATCC 33592. RT-qPCR showed 3-5-fold downregulation of crtM and the icaABCD operon, ∼4-fold repression of abcA and ∼2-fold upregulation of norA versus untreated. An 8 h pre-treatment with peptides lowered ampicillin MIC toward MRSA ATCC 33592 by up to 40-fold and MBCs by up to 80-fold, with similar sensitisation for erythromycin, tetracycline and ciprofloxacin. CONCLUSIONS: MPC and its analogues exhibit bactericidal activity against MRSA through rapid membrane disruption and modulation of virulence gene expression, influenced by peptide helicity and cationic charge.