Serrulatic acid diastereomers identified from an antibacterial survey of Eremophila

Abstract

In an age of growing antimicrobial resistance, new antibacterial agents are desperately needed. A rapid antibacterial and phytochemical survey was designed to screen for antibacterial leads in plants. The survey was applied to over 90 Australian native plants from the genus Eremophila, revealing Eremophila complanata and E. nivea × E. drummondii as active against Gram positive bacteria. Thin layer chromatography with bioautography, flash chromatography and nuclear magnetic resonance led to the isolation and identification of two diastereomeric serrulatic acids. A single stereoisomer of 7,8,16-trihydroxyserrulat-19-oic acid has been previously described as its methyl ester. This paper describes the NMR of both serrulatic acids epimeric at C15 and their methyl esters, and demonstrates their Gram positive antibacterial activity. It is the first time that stereoisomers of this serrulatic acid have been found together in some Eremophila species. Further characterization of E. complanata additionally found an abundance of α-selinene and β-selinene. The study validates a rapid survey approach to finding antibacterial phytochemicals.