Mycobacterium tuberculosis "Beijing" epidemics: a race against mutations?

Prevalence of Extensively Drug Resistant Tuberculosis among Archived Multidrug Resistant Tuberculosis Isolates in Zimbabwe

We conducted a cross-sectional study of second line drug resistance patterns and genetic diversity of MDR-TB isolates archived at the BRTI-TB Laboratory, Harare, between January 2007 and December 2011. DSTs were performed for second line antituberculosis drugs. XDR-TB strains were defined as MDR-TB strains with resistance to either kanamycin and ofloxacin or capreomycin and ofloxacin. Strain types were identified by spoligotyping. No resistance to any second line drugs was shown in 73% of the isolates, with 23% resistant to one or two drugs but not meeting the definition of XDR-TB. A total of 26 shared types were identified, and 18 (69%) matched preexisting shared types in the current published spoligotype databases. Of the 11 out of 18 clustered SITs, 4 predominant (>6 isolates per shared type) were identified. The most and least abundant types were SIT 1468 (LAM 11-ZWE) with 12 (18%) isolates and SIT 53 (T1) with 6 (9%) isolates, respectively. XDR-TB strains are rare in Zimbabwe, but the high proportion of “pre-XDR-TB” strains and treatment failure cases is of concern. The genetic diversity of the MDR-TB strains showed no significant association between SITs and drug resistance.

Global transcriptional profiling of longitudinal clinical isolates of Mycobacterium tuberculosis exhibiting rapid accumulation of drug resistance

The identification of multidrug resistant (MDR), extensively and totally drug resistant Mycobacterium tuberculosis (Mtb), in vulnerable sites such as Mumbai, is a grave threat to the control of tuberculosis. The current study aimed at explaining the rapid expression of MDR in Directly Observed Treatment Short Course (DOTS) compliant patients, represents the first study comparing global transcriptional profiles of 3 pairs of clinical Mtb isolates, collected longitudinally at initiation and completion of DOTS. While the isolates were drug susceptible (DS) at onset and MDR at completion of DOTS, they exhibited identical DNA fingerprints at both points of collection. The whole genome transcriptional analysis was performed using total RNA from H37Rv and 3 locally predominant spoligotypes viz. MANU1, CAS and Beijing, hybridized on MTBv3 (BuG@S) microarray, and yielded 36, 98 and 45 differentially expressed genes respectively. Genes encoding transcription factors (sig, rpoB), cell wall biosynthesis (emb genes), protein synthesis (rpl) and additional central metabolic pathways (ppdK, pknH, pfkB) were found to be down regulated in the MDR isolates as compared to the DS isolate of the same genotype. Up regulation of drug efflux pumps, ABC transporters, trans-membrane proteins and stress response transcriptional factors (whiB) in the MDR isolates was observed. The data indicated that Mtb, without specific mutations in drug target genes may persist in the host due to additional mechanisms like drug efflux pumps and lowered rate of metabolism. Furthermore this population of Mtb, which also showed reduced DNA repair activity, would result in selection and stabilization of spontaneous mutations in drug target genes, causing selection of a MDR strain in the presence of drug pressures. Efflux pump such as drrA may play a significant role in increasing fitness of low level drug resistant cells and assist in survival of Mtb till acquisition of drug resistant mutations with least fitness cost.