Tuberculosis (TB) caused by Mycobacterium tuberculosis remains an ongoing threat to
human health. Many plant species contain antimycobacterial compounds which may serve
as template molecules for new anti-TB drugs. The Rubiaceae family is the largest family of
trees in southern Africa and preliminary evidence revealed antimycobacterial activity in
several species of the genus, motivating further studies. Leaf extracts of 15 tree species
from the Rubiaceae family were screened for antimycobacterial activity against pathogenic
M. tuberculosis and non-pathogenic M. smegmatis, M. aurum and M. bovis BCG using a two-fold serial microdilution assay. Cytotoxicity was determined using a tetrazolium-based
colorimetric assay against C3A liver cells and Vero kidney cells. MIC values as low as 0.04
mg/mL against M. smegmatis and M. tuberculosis were recorded. Activity against M.
aurum was the best predictor of activity against pathogenic M. tuberculosis (correlation
coefficient = 0.9). Bioautography indicated at least 40 different antimycobacterial
compounds in the extracts. Cytotoxicity of the extracts varied and Oxyanthus speciosus had
the most promising selectivity index values.