In theory, drug-resistant strains of Mycobacterium tuberculosis should be less fit than their susceptible relatives. In practice, they are not.

TB remains a huge global problem, exacerbated by the emergence of bacterial strains resistant to isoniazid and rifampin, the principal anti-TB drugs. Models have been developed to try to predict the spread of such strains, but they have not considered the impact of variation in the fitness of drug-resistant bacteria.

Because they have accumulated mutations conferring drug resistance, these strains are usually less fit than susceptible strains and get out-competed in the absence of antibiotics. With funding from the Wellcome Trust and the US National Institutes of Health, Sebastien Gagneux and colleagues found that engineered strains of drug-resistant M. tuberculosis were indeed less fit – though different mutations had very different effects on fitness, as did the strain's genetic background and the presence of combinations of mutations.

In clinical samples, however, strains with little or no loss of fitness were found – and in fact were the most common strains isolated. Incorporating these findings into models of the spread of drug-resistant M. tuberculosis suggests that such strains could break out from localised outbreaks and become more widely established – a potentially highly serious development.

External links

• Gagneux S et al. The competitive cost of antibiotic resistance in Mycobacterium tuberculosis. Science 2006;312(5782):1944–6.