The major differences in chemical composition and antibacterial activity of two closely related Leonotis species (Lamiaceae) may have taxonomic value

Abstract

Several Leonotis species are used widely for medicinal purposes in Africa. There have been drastic changes in the taxonomic treatment of Leonotis species during the past decade. Two species, L. dysophylla and L. microphylla occurring in Pretoria have been considered as varieties of the same species and as different species by different authors. Because Leonotis species are used widely as medicinal plants inter alia against bacterial infections, we decided to compare the chemical composition and antibacterial activity of four plants from each of two populations of the species. The chemical composition of acetone extracts of finely ground leaves was determined by thin layer chromatography followed by spraying with vanillin-sulphuric acid. There were hardly any differences between plants from the same population. There were major differences between the two species in the composition of pigments separated by thin layer chromatography and for compounds visualized with the vanillin-sulphuric acid spray reagent. This supported the viewpoint that the two species should not be considered as varieties. The major differences found in chemical composition indicate that chemical parameters may play an important role in resolving taxonomic differences. Because such a small quantity of material is needed, it may be feasible to analyze one or two leaves obtained from herbarium sheets as an additional taxonomic parameter. The antibacterial activity of the acetone extracts was determined using a two-fold serial dilution microplate method with tetrazolium violet as indicator of growth. The specific strains of the four most important nosocomial bacterial pathogens suggested by the United States National Committee for Clinical Laboratory Standards were used: Staphylococcus aureus (American Type Culture Collection 29213), Pseudomonas aeruginosa (ATCC 27853), Escherichia coli (ATCC 25922) and Enterococcus faecalis (ATCC 21212). The minimum inhibitory activity of the crude extract of L. microphylla was 58, 33, 113 and 15 µg/ml against the four pathogens respectively. The MIC of the L. dysophylla extracts were 110, 95, 113 and 63 µg/ml respectively. If the total activity was calculated by dividing the quantity in mg extracted from one gram of each species with the MIC, the following values were obtained against the respective bacteria using the L. microphylla extract: 700, 1400, 381 and 2100 ml/g. The values for the L. dysophylla extract were 381, 420, 420 and 700 ml/g. This means that if one gram of dried leaves of L. microphylla were extracted with acetone it could be diluted to 2100 ml and it would still kill E. faecalis. These results not only prove the possible use of chemical and biological activity as taxonomic markers, but also the potential value of L. microphylla acetone extracts in treating infections with P. aeruginosa and E. faecalis. The activity of the crude extract against P. aeruginosa was as good as or better than the activity of ampicillin, gentamicin, nitrofurantoin, trimethroptin or sulfi soxazole. The activity against E. faecalis was as good as or better than ampicillin, gentamicin, nitrofurantoin or sulfi soxazole. If the acetone leaf extract of L. microphylla is stable and not toxic there is a good possibility of developing a commercially useful antibacterial product from it. The results indicate the taxonomic value of chemical parameters and biological activity and support the view that the two taxa should be considered as different species.