Phytochemical Profile and Antimicrobial Activities of Eucalyptol Oil against Klebsiella pneumoniae and Staphylococcus aureus: Time–Kill Kinetics and Comparative Study

Pneumonia remains a major global health problem, worsened by the growing resistance of pathogens such as Klebsiella pneumoniae and Staphylococcus aureus to antibiotics. This study investigated the phytochemical composition, functional groups, and antimicrobial activity of steam-distilled essential oil (EO) from Eucalyptus globulus leaves grown in Narok, Kenya. Phytochemical screening confirmed the presence of terpenoids, mainly 1,8-cineole, α-pinene, and limonene. FTIR analysis revealed characteristic absorption peaks typical of monoterpenes, including aliphatic C–H stretching at 2960–2850 cm⁻¹, CH₃ deformation at 1374 cm⁻¹, and ether C–O–C vibrations at 1214 and 1079 cm⁻¹, consistent with cineole. Additional peaks were observed at 984 cm⁻¹ (CH₂ wagging), 886 cm⁻¹ (=C–H bending), and 810 cm⁻¹ (ring vibrations), confirming the presence of bicyclic and monocyclic terpenes. Antimicrobial tests revealed concentration-dependent inhibition, with maximum zones of 19.68 ± 0.41 mm against K. pneumoniae and 18.34 ± 0.36 mm against S. aureus at 100% EO. Time–kill kinetics showed rapid bactericidal activity against S. aureus at 4×MIC within 6 hours while K. pneumoniae showed slower, mainly bacteriostatic activity at the same concentration. Comparative study showed that the Kenyan-derived EO has similar composition and activity to eucalyptus oils from other regions, but was more effective against Gram-positive bacteria than Gram-negative strains. The findings support the potential use of Eucalyptus EO as a natural antimicrobial, especially for Gram-positive respiratory pathogens, while indicating its limited action against resistant Gram-negative bacteria.