Chemical Composition and Synergistic Antimicrobial Effects of a Vegetatively Propagated Cameroonian Lemon, Citrus x limon (L.) Osbeck

This study analyzed the antimicrobial effects of compounds present in the roots and leaves of Citrus x limon (L.) Osbeck to help understand how this plant (1) ecologically modulates microbial defense in the rhizosphere and (2) protects against food-borne bacterial pathogens. Antimicrobial effects of C. limon collected from a farm in Foumban in Cameroon are reported against Salmonella typhi, Salmonella typhimurium, Salmonella enteritidis, Escherichia coli, Shigella flexineri, and Saccharomyces cerevisiae. The study modeled synergistic antifungal effects of an essential oil from the roots of C. limon or E-caryophyllene when combined with the isolated compounds against S. cerevisiae. Twenty-four compounds were identified from the methanol extracts of the roots and leaves: 10 prenylated coumarins (1-10), six methoxylated flavones (11-16), two limonoids (18 and 19), a phenylalanine derivative (20), lupeol, stigmasterol, sitosterol, and sitosterol-3-O-beta-D-glucoside. Major compound 16, atalantoflavone, was esterified using lauric acid to yield the undescribed 5,4'-dilaureate atalantoflavone (16a). The essential oil from the roots contained methyl esters of hexadecanoic acid (39%) and 9-octadecenoic acid (9.3%) and sesquiterpenes fi-bisabolene (10.1%) and a-santalene (8.0%). The antimicrobial effects of the root extract, leaf extract, compounds 1, 2, 4, 7-9, 11, 12, 14, 16, 16a, and 18 were evaluated against S. typhi, S. typhimurium, S. enteritidis, E. coli, S. flexineri, and S. cerevisiae. Compounds 11, 16, and 18 from the root extract of C. x limon showed significant antimicrobial effects with a minimum inhibitory concentration (MIC) of 62.5 mu g/mL against S. flexineri, whereas compounds 5, 7, 11, and 18 showed significant antimicrobial effects with a MIC of 62.5 mu g/mL against S. typhi. The essential oil from the roots synergized compounds 11-14 against S. cerevisiae. A synergistic MIC of 7.8 mu g/mL was registered by combining 12 and E-caryophyllene.