Drug susceptibility testing of rapidly growing mycobacteria in extrapulmonary tuberculosis

There has been an increasing awareness of the rapidly growing mycobacteria (RGM), of which numerous species and phylogenetic groups are clearly established human pathogens. It is important to appropriately distinguish RGM from other mycobacteria, as first-line antituberculous drugs are ineffective for their treatment. Variability in susceptibility of RGM is seen in relation to species, different geographical areas, and time. Therefore, we conducted a study to speciate the isolates of RGM and perform antimicrobial susceptibility testing. The study was carried out in the department of microbiology of a tertiary care hospital. This study included 40 isolates of RGM obtained from clinical specimens from suspected cases of extrapulmonary tuberculosis. Forty isolates of RGM were speciated by phenotypic methods and drug susceptibility testing was done by broth microdilution method. Of the 40 isolates of RGM, 55% belonged to Mycobacterium fortuitum group, 35% were M. smegmatis group, and 10% were M. chelonae-abscessus group. In M. fortuitum group, sensitivity was seen to amikacin (13.63%), cefoxitin (18.18%), imipenem (31.81%), ceftriaxone (22.72%), and cotrimoxazole (31.81%). Only 14.28% and 7.14% of M. smegmatis were sensitive to cotrimoxazole and amikacin, respectively. M. chelonae-abscessus group was resistant to all the antibiotics tested and showed only intermediate sensitivity to amoxicillin-clavulanic acid (50%) and gatifloxacin (25%). A variability in sensitivity to different antimicrobials exists in all groups. Hence, it is advisable to perform antimicrobial susceptibility test before commencement of therapy.