Novel mutations in emb B gene of ethambutol resistant isolates of Mycobacterium tuberculosis: a preliminary report.

BACKGROUND & OBJECTIVE Ethambutol (EMB) resistance, thought to be occurring due to mutations in embB gene of Mycobacterium tuberculosis on the rise is a cause of grave concern. The present study was planned to investigate the presence of EMB resistance in M. tuberculosis isolates and to look for prevalent mutations in embB gene. METHODS A total of 591(283 from new and 308 from previously treated cases) sputum samples from the same number of pulmonary tuberculosis cases were cultured. Isolates were tested by 1 per cent proportion method for resistance to isoniazid, rifampicin streptomycin and ethambutol. Minimum inhibitory concentration (MIC) of EMB was measured by absolute concentration method. Ten randomly selected isolates were subjected to single strand conformational polymorphism (SSCP) and direct DNA sequencing to look for mutation in 364 bp segments of embB gene. RESULTS Of 353 isolates of M. tuberculosis from 591 sputum samples, 62 (17.58%) were resistant to EMB, of which, 16 (25.8%) showed initial resistance and 46 (74.2%) acquired. Mono resistance to EMB was rare. Only two isolates showed resistance to EMB alone. From 62 EMB resistant isolates, 88.7 per cent (55) were resistant to INH, 82.2 per cent (51) to rifampicin and 61.2 per cent (38) were resistant to streptomycin. Co-resistance to isoniazid and rifampicin (multidrug resistant, MDR-TB) with EMB resistance was seen in 41(66.1%) isolates. High level of EMB resistance was seen in 16.5 per cent isolates. SSCP showed altered mobility in 8 of 10 isolates tested. Among the 8 mutants, 4 had known mutations at codon Met 306 being replaced by Val/ Leu. The second most frequent mutation encountered was at codon Phe 287 being replaced by Val, Cys or Leu (novel mutations). Sequence analysis revealed 10 novel mutations in codon 221, 225, 227, 271, 272, 281, 282, 287, 293 and 294 within embB gene. INTERPRETATION & CONCLUSION Presence of high frequency of EMB resistance, occurrence of high level EMB resistance, co-existence of MDR-TB with EMB resistance and novel mutations in emb B gene of M. tuberculosis clinical isolates reported highlight the need to work on larger samples to identify the diagnostic marker of EMB resistance in mycobacteria.