Genomic sequence based scanning for drug resistance-associated mutations and evolutionary analysis of multidrug-resistant and extensively drug-resistant Mycobacterium tuberculosis

Objective: To better understand the molecular mechanisms and evolution of drug resistance in Mycobacterium tuberculosis (M. tuberculosis), we performed a genomic sequence based scanning of drug resistance-associated loci for multidrug-resistant (MDR) and extensively drug-resistant (XDR) M. tuberculosis strains.

Materials and methods: Forty-five pairs of primers covering known drug resistance-associated loci compiled in the TBDReaMDB database were designed to perform the analysis of drug resistance-associated mutations for 14 M. tuberculosis clinical isolates from TB patients in China. Genetic diversity and evolutionary analysis was done using concatenated nucleotide sequences of drug resistance-associated loci.

Results: Forty-four types of mutations were identified in 14 M. tuberculosis clinical isolates. Average nucleotide diversity for drug resistance-associated loci increased in the M. tuberculosis isolates as the drug resistance increased (π = 0, π = 0.00021, and π = 0.00028 for susceptible, MDR, and XDR isolates, respectively). The dN/dS ratios for coding regions of drug resistance-associated genes in MDR and XDR isolates were 2.73 and 1.83, respectively. MDR and XDR isolates were distributed sporadically on different branches in the phylogenetic trees.

Conclusions: Our study provides supporting evidence to demonstrate that the MDR- and XDR-M. tuberculosis strains have evolved independently driven by positive selection.