In Russia, tuberculosis strains carry mutations that not only make them resistant to antibiotics but also help them spread more effectively, find scientists.
The latest study of TB cases in Russia indicates that rampant drug resistance may not be the only explanation for the TB rise in the region - biological factors also play a major role in it.
Researchers at Queen Mary University of London analysed 1,000 genomes from different TB isolates - the largest whole-genome study of a single bacterial species so far.
This enabled the team to identify previously unknown mutations linked to antibiotic resistance, as well as "compensatory mutations" that improve the ability of drug-resistant TB to spread.
Nearly half of the TB isolates were multi-drug resistant, which means that they were impervious to the two common first-line antibiotics that cure most TB infections.
Sixteen percent of these isolates also harboured mutations that made them impervious to "second-line" drugs.
These infections are more expensive to treat and patients who receive ineffective drugs are more likely to spread TB, said the research published in the journal Nature Genetics.
TB, which is caused by the bacterium Mycobacterium tuberculosis, exploded in Russia and other former Soviet nations in the early 1990s, after the collapse of the Soviet Union and its health system.
"It certainly adds an extra layer of worry, because one had assumed if you could solve 'programmatic' weaknesses, you would solve the problem of the drug-resistant TB," stressed Francis Drobniewski, a microbiologist at Queen Mary University.
"Although we know the general story of TB drug resistance in Russia, these new findings are still shocking," added Christopher Dye, an epidemiologist at the World Health Organisation (WHO) in Geneva.
According to Megan Murray, an epidemiologist at the Harvard School of Public Health in Boston, Massachusetts, the worst scenario is that the organisms are developing resistance, compensating for it, and evolving into something that's new and different, that's much less treatable.