Chemical Composition and In-Vitro Antimicrobial Screening of Leaf Essential Oil of Plectranthus gerardianus Benth., and Isolation of the Major Constituents of the Oil

Background: The Lamiaceae (Labiatae) family is one of the most diverse and widespread plant families in terms of ethnomedicine and its medicinal value is based on the concentration of the volatile oil present in it. This family is important for flavor, fragrance and medicinal properties. Many plants belonging to this family have indigenous value. Methods: The essential oil of Plectranthus gerardianus Benth. (Lamiaceae) was analyzed by GC and GC-MS analyses, while the major components were isolated and confirmed by NMR spectroscopy. Results: The oil was found to be rich in oxygenated monoterpenes, which contribute around 62% of the total oil. The major components identified were fenchone (22.90%) and carvenone oxide (16.75%), besides other mono and sesquiterpenoids. The in-vitro antimicrobial activity of essential oil was tested against three gram-negative bacteria including Pasteurella multocida, Escherichia coli, and Salmonella enterica, two gram-positive bacteria namely Staphylococcus aureus and Bacillus subtilis and two fungi namely Candida albicans and Aspergillus flavus. The antimicrobial activity of the oil was also compared to the antimicrobial activity of leaf essential oil of another Himalayan plant namely Nepetacoerulescens. Conclusion: The oil showed in-vitro antimicrobial activity against all the microbial strains and can lessen the ever-growing demand for potentially hazardous antibiotics for treatment.