Antimicrobial and antibiofilm activity of a novel bacteriophage endolysin (LysSW21) against methicillin-resistant Staphylococcus aureus

The rise of antibiotic-resistant bacteria, particularly methicillin-resistant Staphylococcus aureus (MRSA), poses a critical global health challenge. Endolysins, which enzymatically degrade bacterial cell walls, represent a promising class of antimicrobial agents. This study aimed to express and characterize the endolysin LysSW21, derived from the previously characterized Staphylococcus phage vB_SauR_SW21, and evaluate its antibacterial and antibiofilm activities against MRSA strains. In silico analysis of LysSW21 revealed a modular structure comprising an N-terminal CHAP catalytic domain (residues 20–107) and a C-terminal SH3b cell wall-binding domain (residues 164–228). The recombinant protein was successfully expressed in Escherichia coli BL21 (DE3) and purified. LysSW21 demonstrated potent bactericidal activity against reference MRSA strains (MIC/MBC: 25 μg/mL) and a clinical isolate from diabetic foot ulcer (MIC/MBC: 12.5 μg/mL). Time-kill assays revealed rapid, concentration-dependent killing, with complete eradication of planktonic MRSA within 75 min at 2 × MIC (50 μg/mL) and within 105 min at MIC (25 μg/mL). The enzyme exhibited remarkable stability across a broad temperature range (4–70°C) and pH range (4.5–10.5), maintaining 4–5 log₁₀ reductions in bacterial viability. Importantly, crystal violet assay and field emission scanning electron microscopy confirmed concentration-dependent antibiofilm activity of LysSW21, with 70–80% biofilm disruption at 4 × MIC against MRSA strains. LysSW21 exhibits potent bactericidal activity against planktonic MRSA and effectively disrupts biofilms with high stability under diverse environmental conditions. These properties position LysSW21 as a promising candidate for further preclinical development against biofilm-associated MRSA infections. Future studies should evaluate its efficacy and safety in vivo.