From environmental bacteria to obligate human pathogen: adaptations associated with the emergence of tuberculosis bacilli

The current hypothesis is that tuberculosis (TB) bacilli, Mycobacterium tuberculosis, have emerged from an environmental ancestor by adaptation of existing functions and acquisition of specific genes. The closest relations to this ancestor are the Mycobacterium canettii strains, which rarely cause TB and are unable to transmit in humans. Here, we aim to depict the molecular events that contributed to the emergence of a highly efficient human pathogen. Genomic sequence comparison of M. canettii and M. tuberculosis strains reveals polymorphisms in the genes phoPR, which encode a two component regulatory system required for virulence. RNA-Seq analyses showed that most genes controlled by the PhoPR regulon are underexpressed in M. canettii when compared with M. tuberculosis. Consistently, most M. canettii strains display reduced capacity to produce and secrete several major virulence factors controlled by PhoPR. Genetic transfer of the phoPR allele from M. tuberculosis to a M. canettii strain deficient for phoPR resulted in a higher capacity to infect human macrophages and mice, and to induce inflammatory responses. These results shed light on the transition from opportunistic to obligatory human pathogen, and indicate that this transition selected a highly active phoPR allele that confers an advantage for colonization of mammalian hosts.