Delineating the evolutionary pathway to multidrug-resistant outbreaks of a Mycobacterium tuberculosis L4.1.2.1/Haarlem sublineage

Objectives: We sought to capture the evolutionary itinerary of the Mycobacterium tuberculosis L4.1.2.1/Haarlem sublineage, northern Tunisia, where it caused a major multidrug-resistant (MDR-TB) outbreak, in a context strictly negative for HIV infection.

Methods: We combined whole genome sequencing and Bayesian approaches using a representative collection of drug-susceptible and drug-resistant L4.1.2.1/Haarlem clinical strains (n =121) recovered from the outbreak region over 16 years.

Results: In the absence of drug resistance, the L4.1.2.1/Haarlem sublineage showed a propensity for rapid transmission as witnessed by the high clustering (44.6%) and recent transmission rates (25%), as well as the reduced mean distance between genome pairs. The entire pool of L4.1.2.1/Haarlem MDR strains was found to be linked to either the aforementioned major outbreak (68 individuals, 2001-2016) or to a minor, newly uncovered outbreak (6 cases, 2001-2011). Strikingly, the two outbreaks descended independently from a common ancestor that can be dated back to 1886.

Conclusion: Our data point to the intrinsic propensity for rapid transmission of the M. tuberculosis L4.1.2.1/Haarlem sublineage in northern Tunisia, linking the overall MDR-TB epidemic to a single ancestor. These findings bring out the important role of the bacillus's genetic background in the emergence of successful MDR M. tuberculosis clones.