Characterisation & drug susceptibility patterns of extrapulmonary mycobacterial isolates

There is a paucity of information on extrapulmonary tuberculosis as much of the attention is focussed on pulmonary tuberculosis. This prospective study aimed at identification and characterisation of mycobacterial isolates from extra pulmonary sites and the evaluation of the drug susceptibility patterns of Mycobacterium tuberculosis isolates from extrapulmonary sites using the conventional method and the E-test.

A total of 350 specimens from patients of extrapulmonary tuberculosis with varied presentation, were studied. Speciation and characterisation of isolates were done on the basis of growth and biochemical characteristics. Drug susceptibility testing for M. tuberculosis isolates was done by proportion method for isoniazid, rifampicin, ethambutol and pyrazinamide, whereas resistance ratio method was used for streptomycin. E-test (AB Biodisk, Sweden) was carried out to compare susceptibility patterns of the M. tuberculosis isolates for isoniazid and rifampicin with the conventional method.

Thirty two of 350 (9.14%) patients clinically suspected to have extrapulmonary tuberculosis were culture positive for mycobacteria. On characterisation, 20 of the 32 isolates were identified as M. tuberculosis and 12 as non-tubercular mycobacteria (NTM) with 5 of the 12 being Mycobacterium avium complex. Among M. tuberculosis isolates both initial and acquired resistance was highest for streptomycin followed by isoniazid, rifampicin and ethambutol. No strain showed resistance to pyrazinamide. Two strains were found to be multidrug resistant. Drug susceptibility patterns by conventional method corroborate with the E-test results.

This study shows that the characterisation and species identification of mycobacterial isolates along with drug susceptibility testing help in better understanding of extrapulmonary tuberculosis. E-test had the advantage of being rapid and simple without need for additional equipment.