Mechanistic Study of Antimicrobial Effectiveness of Cyclic Amphipathic Peptide [R 4 W 4 ] against Methicillin-Resistant Staphylococcus aureus Clinical Isolates.

Antimicrobial peptides (AMPs) are being explored as a potential strategy to combat antibiotic resistance due to their ability to reduce susceptibility to antibiotics. This study explored whether the [R 4 W 4 ] peptide mode of action is bacteriostatic or bactericidal using modified two-fold serial dilution and evaluating the synergism between gentamicin and [R 4 W 4 ] against Escherichia coli ( E. coli ) and methicillin-resistant Staphylococcus aureus (MRSA) by a checkered board assay. [R 4 W 4 ] exhibited bactericidal activity against bacterial isolates (MBC/MIC ≤ 4), with a synergistic effect with gentamicin against E. coli (FICI = 0.3) but not against MRSA (FICI = 0.75). Moreover, we investigated the mechanism of action of [R 4 W 4 ] against MRSA by applying biophysical assays to evaluate zeta potential, cytoplasmic membrane depolarization, and lipoteichoic acid (LTA) binding affinity. [R 4 W 4 ] at a 16 mg/mL concentration stabilized the zeta potential of MRSA -31 ± 0.88 mV to -8.37 mV. Also, [R 4 W 4 ] at 2 × MIC and 16 × MIC revealed a membrane perturbation process associated with concentration-dependent effects. Lastly, in the presence of BODIPY-TR-cadaverine (BC) fluorescence dyes, [R 4 W 4 ] exhibited binding affinity to LTA comparable with melittin, the positive control. In addition, the antibacterial activity of [R 4 W 4 ] against MRSA remained unchanged in the absence and presence of LTA, with an MIC of 8 µg/mL. Therefore, the [R 4 W 4 ] mechanism of action is deemed bactericidal, involving interaction with bacterial cell membranes, causing concentration-dependent membrane perturbation. Additionally, after 30 serial passages, there was a modest increment of MRSA strains resistant to [R 4 W 4 ] and a change in antibacterial effectiveness MIC [R 4 W 4 ] and vancomycin by 8 and 4 folds with a slight change in Levofloxacin MIC 1 to 2 µg/mL. These data suggest that [R 4 W 4 ] warrants further consideration as a potential AMP.