From environmental bacteria to obligate pathogen: the study of adaptations enhancing the persistence of tuberculosis bacilli

The highly contagious human pathogen Mycobacterium tuberculosis is responsible for an estimated 1.5 million deaths each year. A major feature of M. tuberculosis is its unique capacity to survive within the host for decades. In contrast, the evolutionary early-branching rare tubercle bacilli Mycobacterium canettii, which is thought to have originated from the same pool of ancestors as M. tuberculosis, rarely causes tuberculosis (TB) in humans and has a reduced capacity to persist and cause disease in the mouse model. Our objectives are to identify the genetic adaptations which allow persistence of TB bacilli within the host, and to characterise their functional consequences. Using an experimental evolution strategy, we have shown that recurrent in vivo passaging of a M. canettii strain (for more than 2 years) allows clones to emerge as “persistent mutants” that have a greater ability to survive in the host, and gain the characteristics of classical M. tuberculosis strains. Several “persistent mutants” were selected and the persistence phenotype was confirmed in several mouse genetic backgrounds. Next, these mutants were subjected to extensive phenotypic characterisation and were found to have enhanced resistance to stress encountered during infection, and increased virulence in human monocyte-derived macrophages. Currently, the mutations responsible for these phenotypes are being characterised. Comparing the genetic differences between the parental M. canettii strain, the “persistent” M. canettii clones and M. tuberculosis will help identify the molecular mechanisms driving evolution from putative generalist mycobacteria, towards professional human pathogen.