Simultaneous ethambutol & isoniazid resistance in clinical isolates of Mycobacterium tuberculosis

There is a need to understand the nature of drug resistance patterns and predictors of emergence of drug resistance in Mycobacterium tuberculosis. There could be common factors/mechanisms for resistance to the drugs, isoniazid and ethambutol, both acting on cell wall. The present study was conducted to analyze the antimycobacterial susceptibility patterns of M. tuberculosis isolates to determine the minimum inhibitory concentrations (MICs) of ethambutol for M. tuberculosis; and to find out possible association of ethambutol resistance with isoniazid resistance.

A total of 380 M. tuberculosis isolates were tested for their susceptibilities to ethambutol at 2, 4, 6 microg/ml, isoniazid at 1 microg/ml and rifampicin at 64 microg/ml using MIC method.

44.21, 24.73 and 14.21 per cent isolates were resistant to ethambutol at concentrations of 2, 4 and 6 microg/ml respectively. At 6 microg/ml of ethambutol concentration, 85.18 per cent ethambutol resistant isolates were resistant to isoniazid also. At the same ethambutol concentration a fraction of 28.75 per cent isoniazid resistant isolates were ethambutol resistant.

Ethambutol resistance was accompanied with isoniazid resistance in a large percentage of isolates whereas ethambutol resistance was weakly linked with multidrug resistance. On the other hand, association between isoniazid and ethambutol resistance was weak showing one way linkage.