COMPARATIVE IN VITRO ANTIBACTERIAL ACTIVITY OF GARLIC ESSENTIAL OIL AND CINNAMON BARK EXTRACT AGAINST PSEUDOMONAS AERUGINOSA

Abstract

The present study compared the antibacterial activity and mechanism of action of hydrodistilled essential oil of Allium sativum (garlic) versus 70% v/v ethanolic bark extract from Cinnamomum verum (cinnamon) against a multidrug-resistant Pseudomonas aeruginosa strain resistant to β lactams third generation cephalosporins, with intermediate susceptibility to imipenem and ciprofloxacin. Inhibition was observed to be dose dependent, as seen in the agar diffusion assays. At 100 mg/mL, garlic essential oil produced a zone of clearance of 32 mm, about 4 mm wider than the 28 mm at the lower concentration (25 mg/mL). The effect of the cinnamon extract (average zones of inhibition of 18 and 28 mm at 25 and 100 mg/mL, respectively) was moderate. Garlic oil was also confirmed to be more potent compared to cinnamon extract at a MIC of 0.50 mg/mL versus 1.00 mg/mL, respectively, by broth microdilution. Profiling of garlic oil by high performance liquid chromatography (HPLC) indicated the presence of allicin and other related organosulfur derivatives as major components, while cinnamon extract was mostly active due to trans cinnamaldehyde. Mechanistically, the bactericidal properties of garlic oil have been attributed to thiol disulfide exchange reactions between allicin and essential sulfhydryl (–SH) groups on bacterial enzymes, which ultimately result in enzyme inactivation, redox imbalance, and rapid cell death. Conversely, trans cinnamaldehyde induces a destabilization of the bacterial inner membrane through direct intercalation, dissipates the proton motive force, and eliminates internal ATP stores while inhibiting the cell division protein FtsZ. The marked susceptibility of this P. aeruginosa to these botanical preparations suggests potential activity against a resistant phenotype, a conventional possible antibacterial mechanism of β-lactamase and efflux mediated resistance. In particular, garlic essential oil stands out as a promising scaffold for therapeutic development against hard-to-treat P. aeruginosa infections.