Evaluation of three methods to determine the antimicrobial susceptibility of Mycobacterium tuberculosis.

BACKGROUND & OBJECTIVES Tuberculosis continues to be a major public health problem in India, especially with the emergence of drug resistance. A study was carried out to establish a rapid and accurate method of susceptibility testing for Mycobacterium tuberculosis using three methods viz., proportion method by agar dilution on Middlebrook 7H11 agar, proportion method using the conventional Lowenstein-Jensen (L-J) medium and E test strip method. METHODS A total of seventy five clinical isolates from pulmonary and extrapulmonary sites were characterised and speciated by biochemical tests, growth and other standard parameters, and eight random isolates, also by polymerase chain reaction (PCR). Antimicrobial susceptibility of M.tuberculosis was performed by proportion method on L-J medium and Middlebrook 7H11 agar medium for isoniazide (INH), rifampicin (RIF), ethambutol (EMB), streptomycin (STM) and ciprofloxacin (CIP) using recommended critical concentrations. The two methods were compared with the E test method. RESULTS The 75 M.tuberculosis strains were isolated from sputum (47), pus (23), aspirate fluid (2), skin tissue (2) and gastric aspirate (1). Of these 49 (65.3%) isolates were sensitive and one (1.3%) was resistant to all the five drugs tested and by all the three methods. Eleven (14.7%) isolates were resistant to INH alone by the three methods. The E test method detected one isolate resistant to INH and 2 to RIF which were missed by the other two methods. The results obtained by all the three methods compared well. INTERPRETATION & CONCLUSION The three methods viz., proportion methods with L-J, Middlebrook 7H11 agar and the E test concurred fully in 57 isolates (76%). Association between L-J and Middlebrook 7H11 agar methods was 59 per cent. E test and the L-J methods did not differ significantly for all the drugs. The finding show that the E test method is superior to the other two methods in terms of simplicity of performance and the rapidity of results. Another advantage is that the MIC values can also be obtained simultaneously by this method.