Determination of Minimum Inhibitory Concentration Values of Ethambutol for Mycobacterium Tuberculosis

Tuberculosis has always been and still has the distinction of being a leading contender for one of the top places on the list of the main cause of death in India. Ethambutol is a narrowspectrum antimycobacterial agent that is used for the treatment of tuberculosis. At times the patients fail to respond to treatment with anti tubercular drugs, the reasons for this could be many, one of them being drug resistance, which is increasingly being reported from both developed and developing countries

The present study was planned to establish a method for testing of an antitubercular agent i.e. ethambutol and to determine the minimal inhibitory concentration of ethambutol.

From sputum samples of proved cases of pulmonary tuberculosis, who had not received any antitubercular drug earlier in their life time, the acid-fast bacilli were cultured and speciated by colony morphology, time taken to grow, and no growth on PNBA medium, niacin and nitrate reduction test positive were identified as M. tuberculosis. Resistance ratio method was followed. "Resistance ratio" is expressed as the ratio of the MIC of the test strain to the MIC of the standard H37Rv strain of M. tuberculosis. For each isolate, (originally from the sputum samples) L-J medium slants containing ethambutol, in serially doubling concentration ranging from 1mcg/ml to 16 mcg/ml were prepared. Two such sets were prepared: one for inoculating with test strain (obtained from sputum samples) and the other for standard H37Rv strain. Each set had a drug free L-J slope (control). All the L-J slants were labeled with appropriate drug concentrations and incubated at 37° C for 8 weeks for "growth." "Growth" was defined as presence of 20 colonies or more.

The MIC for ethambutol for both test strain as well as standard H37Rv strain was 4 mcg/ml. Resistance ratio i.e. ratio of the MIC of test strain to that of std H37Rv strain was also calculated which was found to be indicating the strains being sensitive to ethambutol. The proper determination of drug resistance by the proper method is helpful to minimize the spread of drug resistant TB. Key to success in the treatment of drug resistant tuberculosis lies with supervised therapy with appropriate modification of regimen depending on sensitivity patterns.