Euclea natalensis A.DC., Knowltonia vesicatoria (L.f) Sims, and Pelargonium sidoides DC. are South African plants traditionally
used to treat tuberculosis. Extracts from these plants were used in combination with isoniazid (INH) to investigate the possibility
of synergy with respect to antimycobacterial activity. The ethanol extract of K. vesicatoria was subjected to fractionation to identify
the active compounds. The activity of the Knowltonia extract remained superior to the fractions with a minimum inhibitory
concentration (MIC) of 625.0 μg/mL against Mycobacterium smegmatis and an MIC of 50.00 μg/mL against M. tuberculosis.
The K. vesicatoria extract was tested against two different drug-resistant strains of M. tuberculosis, which resulted in an MIC of
50.00 μg/mL on both strains. The combination of K. vesicatoria with INH exhibited the best synergistic antimycobacterial activity
with a fractional inhibitory concentration index of 0.25 (a combined concentration of 6.28 μg/mL). A fifty percent inhibitory
concentration of this combination against U937 cells was 121.0 μg/mL. Two compounds, stigmasta-5,23-dien-3-ol (1) and 5-
(hydroxymethyl)furan-2(5H)-one (2), were isolated from K. vesicatoria as the first report of isolation for both compounds from
this plant and the first report of antimycobacterial activity. Compound (1) was active against drug-sensitive M. tuberculosis with
an MIC of 50.00 μg/mL.
This paper is part
of the thesis submitted by the first author to the Department
of Plant Science of University of Pretoria.