Drug Resistant Tuberculosis in Bulgaria: Molecular Insights

Abstract : Tuberculosis (TB) infects a significant proportion of the world population and constitutes a major public health problem, particularly, in the developing regions. A reemergence of TB accompanied by an increasing number of drug resistant Mycobacterium tuberculosis strains, including many resistant to multiple drugs, has been noted since the mid-1980s. In Bulgaria, the rate of multidrug-resistant tuberculosis (MDR-TB) was estimated to be 10.7% among newly diagnosed TB patients in 2006 that is much higher as compared to the neighboring countries. An increasing rate of MDR-TB implies importance of surveillance of resistance and its fast detection. Characterization of molecular basis of drug resistance in a survey area is a first step prior to implementation of such methods. Here, we present data on molecular basis of drug-resistance in M. tuberculosis strains currently circulating in Bulgaria. We also compared distribution of drug resistance mutations within the main genotypic clusters identified by spoligotyping and VNTR typing in our strains. The study panel consisted of 133 (including 37 drug-resistant) M. tuberculosis strains recovered from newly diagnosed, epidemiologically unlinked, pulmonary TB patients from different regions of Bulgaria in 2005-2006. Three types of the rpoB mutations were found in 20 of 27 RIF-resistant strains while rpoB S531L was the most frequent. Ten (45.5%) of 22 INH-resistant isolates had katG S315T mutation. inhA -15C>T mutation was detected in 3 INH-susceptible strains and one INH-resistant strain that also had katG315 mutation. A mutation in embB306 was found in 7 of 11 EMB-resistant strains. No mutations in rpsL43 were detected in the studied selection of STR-resistant and STR-susceptible strains. Comparison with spoligotyping and 24-VNTR locus typing data did not reveal a significant difference in the distribution of drug resistance between clustered and non-clustered isolates. To conclude, (i) rpoB and embB306 mutations may serve for rapid genotypic detection of the majority of the RIF and EMB-resistant strains in Bulgaria; (ii) the results on INH resistance are complex and further investigation of more genes is needed; (iii) emergence and spread of drug-resistant and MDR-TB in Bulgaria is not associated with any specific spoligotype or MIRU-VNTR cluster.