Some South African Rubiaceae tree leaf extracts have antimycobacterial activity against pathogenic and non-pathogenic Mycobacterium species

Abstract

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.