Rapidly Correcting Frameshift Mutations in the Mycobacterium tuberculosis Gene Produce Reversible Ethambutol Resistance and Small-Colony-Variant Morphology.

We have identified a previously unknown mechanism of reversible high-level ethambutol (EMB) resistance in that is caused by a reversible frameshift mutation in the gene. A frameshift mutation in produces the small-colony-variant (SCV) phenotype, but this mutation does not change the MICs of any drug for wild-type However, the same mutation in a low-level EMB-resistant double mutant (MIC = 8 μg/ml) produces an SCV with an EMB MIC of 32 μg/ml. Reversible resistance is indistinguishable from a drug-persistent phenotype, because further culture of these SCV mutants results in rapid reversion of the frameshifts, reestablishing the correct open reading frame, returning the culture to normal colony size, and reversing the EMB MIC back to that (8 μg/ml) of the parental strain. Transcriptomic analysis of mutants compared to wild-type identified a 27-fold relative increase in the expression of , which is a cellular target for EMB. Expression of in mutants was also increased 5-fold compared to that in the parental mutant, whereas large-colony frameshift revertants of the mutant had levels of expression similar to that of the parental strain. Reversible frameshift mutants may contribute to a reversible form of microbiological drug resistance in human tuberculosis.