Procedure

Susceptibility testing: Micro plate alamar blue assay (MABA) was used as described by Lawal et al., (2011) [15]. Sterile distilled water (200 μl) was added to all outer perimeter wells of sterile 96-well plates to minimize evaporation. The remaining wells received 100 μl of 7H9 broth. One hundred micro liters of double concentration drug solutions were added to each of the wells in rows B to G in column 2 and serial dilutions were made through column 10 using a multi channel pipette.

Each of the wells in rows from 2–11 were inoculated with 100 μl of M. tuberculosis bringing the final volume to 200 μl per well. The wells in column 11 were drug free containing only the inoculum and broth and these acted as negative control wells. The plates were incubated at 37 °C for 24 h. The tests were prepared in triplicate for each of the strains used. Thirty micro liters of a freshly prepared alamar blue reagent was added to one of the control wells and further incubated at 37 °C for 24 h. Observation of a color change indicated that there was growth. The dye was then added to all the wells and further incubated for 24 h. The minimum inhibitory concentration (MIC) was defined as the lowest drug concentration which prevented a color change from blue to pink (Additional file 1). The bioassays were performed in a level 3 bio safety laboratory.