Two regulator mutations not behind spread of MDR TB clone

Tuberculosis remains a major global health problem, and multidrug resistant (MDR) strains of Mycobacterium tuberculosis are a particular obstacle because they resist key drugs such as rifampicin and isoniazid. One successful MDR lineage, the B0/W148 clone that evolved from the Beijing lineage 2, has spread widely in Russia and Europe, prompting researchers to look for genetic reasons behind its success alongside environmental and patient-related explanations. Corresponding author Régis Tournebize and colleagues set out to test whether two mutations specific to the B0/W148 clone — found in the transcriptional regulators WhiB6 and KdpDE — could help explain the clone’s large geographic diffusion or a suggested tendency toward hypervirulence. To do this, the team introduced these specific changes into a common laboratory strain background and examined how those changes affected which genes were turned on or off and how the bacteria grew in lab media and in animals. The aim was to see if these two regulators could be a driving factor in the clone’s evolutionary success, or whether other factors must be responsible.

The researchers used a laboratory Mycobacterium tuberculosis background strain to construct and study strains carrying the specific alterations: H37RvΔ whiB6, H37RvΔ kdpDE, and complemented strains to restore function. They examined transcriptional profiles and tested growth in standard mycobacterial media as well as virulence in a mouse infection model. The mutation T51P in whiB6 prevented the usual upregulation of 9 genes in the esx-1 core region and 44 genes elsewhere in the genome, showing that WhiB6 activity and downstream gene expression were altered. In the case of kdpDE, a deletion of two nucleotides in kdpD produced a fusion protein of KdpD with KdpE that inhibited the transcriptional activity of KdpE. Despite these clear changes in gene regulation, neither mutation impaired in vitro growth in standard media, and neither produced hypervirulence in mice.

These findings suggest that while the two B0/W148-specific mutations in WhiB6 and KdpDE clearly reshape the bacteria’s transcriptional landscape, they do not by themselves make the clone more virulent in the animal model used here. That outcome points toward other explanations for the widespread diffusion of the MDR B0/W148 clone: additional strain-specific mutations, or non-genetic factors such as environment and patient-related issues, may play larger roles. The work also challenges the idea that hypervirulence is the primary driver of this clone’s spread, at least for these two regulators. For researchers and public health teams, the study refines where to look next: cataloguing and testing other MDR B0/W148 specific mutations and combining genetic studies with epidemiological data will be necessary to understand and counter the spread of this MDR lineage.