Antimicrobial abietane diterpenoids from Plectranthus elegans

Two novel abietane diterpenoids have been isolated from the aerial material of Plectranthus elegans and identified as 11-hydroxy-12-oxo-7,9(11),13-abietatriene and 7 alpha,11-dihydroxy-12-methoxy-8,11,13-abietatriene. Their structures were determined through rigorous use of spectroscopic methods. Both inhibited spore germination of the fungus Cladosporium cucumerinum, in direct bioautography, at a dose of 1 microgram. The new diterpenes also inhibited the growth of Gram-positive bacteria, in the concentration range 10-40 micrograms ml-1 in broth dilution assay. No effect was observed against Gram-negative bacteria. The ecological implications of these findings are discussed.