Experimental confirmation that an uncommon, yet clinically relevant mutation (G878A) in the rrs gene of Mycobacterium tuberculosis confers resistance to streptomycin.

Abstract

The effective treatment of patients diagnosed with drug resistant tuberculosis (TB) is highly dependent upon the ability to rapidly and accurately determine the antibiotic resistance/ susceptibility profile of the Mycobacterium tuberculosis isolate(s) involved. Thus, as more and more clinical microbiology laboratories advance towards the routine use of DNA sequence-based diagnostics, it is imperative that their predictive functions extend beyond the well-known resistance-conferring mutations, in order to also encompass as many of the lower-frequency mutations as possible. However, in most cases, the fundamental experimental proof that links these uncommon mutations with phenotypic resistance is still lacking. One such example is the G878A polymorphism within the rrs gene encoding the 16s rRNA. We, and others, have identified this mutation within a small number of drug-resistant M. tuberculosis isolates, although prior to this study a consensus regarding exactly which aminoglycoside antibiotic(s) it conferred resistance toward seems not to have been reached. Here we have employed oligo-mediated recombineering to specifically introduce the G878A polymorphism into the rrs gene of M. bovis BCG - a species very closely related to M. tuberculosis - and demonstrate that it confers low-level resistance to streptomycin alone. In our hands, it does not confer cross-resistance towards amikacin, capreomycin, nor kanamycin. We also demonstrate that the rrsG878A mutation exerts a substantial fitness defect in vitro, that may at least in part explain why clinical M. tuberculosis isolates bearing this mutation appear to be quite rare. Overall, this study provides clarity to the resistance phenotype attributable to the rrsG878A mutation and is relevant to the future implementation of genomics-based diagnostics, as well as the clinical management of patients in situations where this particular polymorphism is encountered.