Antibacterial effects of plant-derived extracts on methicillin-resistant Staphylococcus aureus

Natural chemicals have been reported to have antibacterial effects against a variety of bacteria. The present study evaluated the antibacterial effects of commercially available grape-seed extract (GSE), pomegranate polyphenols (PP), and lab-prepared cranberry proanthocyanidins (C-PAC) against two strains of methicillin-resistant Staphylococcus aureus (MRSA). GSE, PP, and C-PAC at concentrations of 2 mg/mL, 10 mg/mL, or controls were mixed with equal volumes of overnight cultures of MRSA at ~6 log₁₀ colony-forming units (CFU)/mL and incubated for 0, 1, 2, 8, and 24 h at 37°C. Treatments were neutralized/stopped using tryptic soy broth containing 3% beef extract. Serial dilutions of the treated MRSA strains and controls were spread-plated on trypticase soy agar and incubated for 24-48 h at 37°C and colonies were counted. Among the three tested agents, GSE at 1 and 5 mg/mL was found to be most effective against MRSA, resulting in a 2.9-4.0 log₁₀ CFU/mL reduction of both strains after 2 h at 37°C. PP at 1 and 5 mg/mL was found to cause 1.1-2.3 log₁₀ CFU/mL reduction, while C-PAC at 1 mg/mL caused <1 log₁₀ CFU/mL reduction of the two MRSA strains after 2 h at 37°C. All three extracts at the tested concentrations decreased the two MRSA strains to undetectable levels within 24 h, with the exception of 1 mg/mL PP for strain 33591. Scanning electron microscopy of MRSA after 2 h of treatment showed that GSE and PP caused bacterial cell wall alteration, with negligible effect observed by C-PAC treatment. However, the in vivo activity and clinical safety applications of GSE, PP, and C-PAC need to be evaluated before suggestion for use as a treatment/control measure.