Effects of the expression level of mtrA gene on target genes and cell growth in M. smegmatis. (A) Drug resistance assays. The antimicrobial activity of four first-line anti-tuberculosis drugs against M. smegmatis was determined as described under "Materials and Methods". Representative growth curves for isonizid and streptomycin are shown. (B) Scanning electron microscopy assay of cell morphology. The experiment was carried out as described in the "Materials and Methods". Representative images are shown. The images were taken at 15,000× magnification. Bars, 1 μm. (C) qRT-PCR assays for the gene expression of M. smegmatis. The experiment was carried out as described in the "Materials and Methods". 16S rRNA gene, rrs, was used as control. All target genes were amplified using specific primers. Different gene expressions were normalized to the levels of 16S rRNA gene transcripts, and the folds of expression change were calculated. Representative data are shown.