Molecular characterization of Mycobacterium bovis virulence factors

Tuberculosis (TB) is a major disease of humans and animals. TB in mammals is caused by different species of mycobacteria that are clustered in the same genetically compact group named the Mycobacterium tuberculosis complex (MTBC). The members of the MTBC can be separated in two independent clades with distinct host preferences: the human-adapted strains, and the animal-adapted strains. Mycobacterium tuberculosis is responsible for the vast majority of human TB cases and shows host specificity for humans. On the other hand, Mycobacterium bovis can infect and sustain across a wide range of mammalian hosts, being primarily a pathogen of cattle but being also able to cause TB in humans trough zoonotic infections. M. tuberculosis and M. bovis have traditionally been the most studied species of the MTBC as they are the main human and animal TB pathogens, respectively. One key protein system of the MTBC is the PhoPR system, which is a two component signal transduction system that regulates many mycobacterial processes including lipid metabolism, secretion of virulence proteins and intracellular adaptation. Previous studies have shown that the phoPR locus shows variation across the members of the MTBC, with the identification of three single nucleotide polymorphisms (SNPs) in the phoPR locus of M. bovis compared to M. tuberculosis. Experiments suggested that these SNPs render the PhoPR system of M. bovis non-functional. However, later studies by other groups have provided evidence for the functionality of the M. bovis PhoPR system. The aim of this thesis was to elucidate the functional role of the PhoPR system of M. bovis. To achieve this, M. bovis mutants were constructed, transcriptomics of mutants and wild type used to define the PhoPR regulon, and in vitro experiments performed for functional characterization. The results of this thesis provide evidence that the M. bovis PhoPR system is indeed functional and that it shows differential control of genes as compared to the PhoPR system of M. tuberculosis, including many virulence genes. As such, this thesis opens new avenues for research into the molecular basis of host tropism of M. bovis, and the potential role of the PhoPR system in this process.