Chemical composition and anticandidal activity of <i>Citrus hystrix</i> peel oil: Enantiomeric profile and mechanistic insights

This study aimed to characterise the chemical composition and enantiomeric profiles of Citrus hystrix peel oil. Its antifungal activity and mode of action against Candida species were evaluated. C. hystrix oil was extracted from fresh peel and analysed by GC-MS. Enantiomeric profiling of each chemical component was also performed. The antifungal activity of essential oil and major constituents against four Candida species, namely, C. albicans, C. tropicalis, C. glabrata and C. krusei, was evaluated using broth dilution. The main components of the oil are β-pinene (22%), limonene (21%), sabinene (14%), β-citronellal (13%), β-citronellol (5%), terpinen-4-ol (4%), α-terpineol (3%), linalool (2%) and α-pinene (2%). Enantiomeric profiling with chiral GC-MS disclosed that β-citronellal and β-citronellol are present in an optically pure (-) configuration. The minimum inhibitory concentration of the oil ranges from 0.5 mg/mL to 4 mg/mL, and the minimum fungicidal concentration ranges from 1 mg/mL to 8 mg/mL. Oxygenated monoterpenes exhibit notable activity against Candida, and β-citronellol displays the highest activity, followed by β-citronellal. The time-kill curve and post-antifungal effect of C. hystrix oil were also explored. Two membrane disruption assays, namely, leakage of 260 nm intracellular absorbing materials and methylene blue dye exclusion, and scanning electron microscopy were conducted. The results demonstrated that membrane disruption is a major contributing mechanism of C. hystrix peel oil against the tested Candida strains.