The invention relates to the discovery that a putative gene of Mycobacterium tuberculosis with no previously identified function is responsible for the ability of the bacterium to activate thioamide drugs. Since M. tuberculosis has a low rate of synonymous mutations, all mutations in this gene, identified as Rv3854c and now termed "EtaA," are expected to inhibit the ability of a bacterium with the mutation to activate a thioamide or thiocarbonyl drug. Thus, detecting a bacterium with a mutation in this gene indicates that the bacterium is resistant to treatment with thioamides.