The increasing levels of multi drug resistant tuberculosis (MDRTB) in endemic regions are a cause of concern. When MDR strains exhibit higher fitness they threaten global tuberculosis control. Evaluation of factors other than the particular non-synonymous substitutions that contribute to the fitness of strains may have an important bearing on understanding the spread of the disease. The aim of the study was to determine the fitness of MDRTB strains from TB endemic region, acquiring drug resistance during the course of treatment.

Paired isolates of different lineages (one that was drug resistant and one that was drug susceptible) from 5 patients, which acquired resistance during antibiotic treatment and bearing drug resistance mutations in the rpoβ and katG/inhA regions were mixed axenically. Fitness of Mycobacterium tuberculosis (M.tb) strains was evaluated using the difference in generation times of drug resistant and susceptible population obtained by enumeration of CFU.

The drug resistant strains bore no significant fitness cost. The selection of specific allelic combinations in multiple drug resistance governing loci may attribute the enhanced/retained fitness of the drug resistant strains.