Session: Poster Session: Resistance in Mycobacteria
Monday, October 27, 2008: 12:00 AM
Room: Hall C
Background: Substitutions at embB306 of M. tuberculosis (Mtb) are the most frequent and predictive mutations for ethambutol (E) resistance. However, this has been challenged. Methods: Susceptibility of 44 clinical Mtb isolates from India to 2 mg/L E, 0.1 mg/L isoniazid (H), and 40 μg/ml of rifampin (R) was performed by proportion method. Minimum inhibitory concentration (MIC) of E was determined by E-test. DNA sequencing was performed for embB, embC, and embR genes of Mtb. Translated amino acid sequences were compared to drug-susceptible Mtb H37Rv EmbB, EmbC and EmbR protein sequence NP_218312, NP_218310 and NP_215783, respectively. Mtb H37Rv was used as control in each set of experiment. RFLP-IS6110 typing was performed on all strains. Results: All 44 clinical isolates were resistant to E, while 38 and 32 were resistant to H and R, respectively, and 29/44 were multi-drug resistant (MDR). All genotypes but one were those with >15 bands, indicating low prevalence of Beijing strain. Mutations in the embB gene occurred in 35/44 isolates, but there was no difference in MIC between those with and without embB mutations (p=0.4).Of those with embB mutations, 25 (72%) had a concurrent embC mutation, while 13 (37%) had both embC and embR mutations. Only 4 (9%) isolates had embB mutations alone. The embB306 mutation was seen in only 8 (18%) isolates, and was no more likely in MDR strains than mono-resistant strains (p=0.4). However, emb306 strains had a mean MIC of 65 mg/L versus 18 mg/L (p=0.0034). EmbB mutations were evenly distributed at 306, 368, 378, 380, and 406. No strain had the emb306 ATG→CTG mutation that has been shown to have growth advantages by others. Conclusion: There were a large number of mutations in codons other than embB306 encountered in the clinical isolates. Therefore, the embB306 plays a relatively minor role in E-resistance in genotypes encounetered in India.