Mutation teamwork fuels fit, doubly drug-resistant tuberculosis

Tuberculosis remains a global health problem largely because the bacterium behind it, Mycobacterium tuberculosis (Mtb), can become resistant to the drugs used to treat it. Scientists have known from work in model organisms that resistance mutations do not always act independently: they can interact in ways called epistasis, which can change how bacteria grow and survive. Until now, however, the biological and epidemiological consequences of these interactions in Mtb were not clear. In work led by corresponding author Sébastien Gagneux, researchers investigated whether mutations that confer resistance to two major classes of tuberculosis drugs might interact in ways that matter for the bacterium’s fitness and its ability to spread. The study focused on RpoB and GyrA, two genes that when mutated cause resistance to the key drugs rifampicin and fluoroquinolone, respectively. By examining patterns of mutation and their effects on bacterial biology, the team tested whether combinations of resistance-conferring mutations could produce double-resistant strains that remain robust in laboratory conditions, a property measured as in vitro fitness.

The core finding is that positive sign epistasis between RpoB and GyrA mutations can produce double-resistant Mtb strains that have high in vitro fitness. In other words, certain mutation pairs interact in a way that the combined effect is more favorable to the bacterium than expected from each mutation alone. The researchers report that two specific RpoB-GyrA mutation combinations are especially important: they account for 53% of a global collection of highly drug-resistant Mtb clinical isolates. By contrast, RpoB-GyrA combinations that show low in vitro fitness make up less than 0.7% of that collection. The study also examined effects on the cell's proteins and found that the two high-fitness RpoB-GyrA combinations are associated with a more benign and idiosyncratic proteome perturbation compared to low-fitness combinations, suggesting that some mutation pairs disrupt the bacterium’s protein makeup less and so preserve fitness.

These results matter because they show that epistasis is not just a genetic curiosity but can shape which drug-resistant tuberculosis strains emerge and spread. If some RpoB-GyrA mutation pairs produce double resistance without a large fitness cost, those combinations are more likely to circulate among people and dominate the population of highly drug-resistant isolates. The observation that high-fitness combinations perturb the proteome in a more limited and idiosyncratic way gives a biological explanation for why those combinations succeed: they achieve resistance while keeping essential cellular processes relatively intact. Taken together, the findings highlight the relevance of epistasis for the emergence and spread of antimicrobial resistance and suggest that understanding specific mutation interactions will be important for interpreting the rise of particular multidrug-resistant Mtb strains and for guiding public health responses.