Identification of a virulence gene cluster of Mycobacterium tuberculosis by signature-tagged transposon mutagenesis

Tuberculosis remains the greatest cause of death worldwide due to a single pathogen, In order to identify the genes required for the pathogenicity of Mycobacterium tuberculosis, a functional genomic approach was developed. A library of signature-tagged transposon mutants of this bacterium was constr ucted and screened for those affected in their multiplication within the lu ngs of mice. From 1927 mutants tested, 16 were attenuated for their virulen ce. The insertions harboured by the selected mutants were mapped on the M. tuberculosis genome and most of the mutated loci appeared to be involved in lipid metabolism or transport across the membrane. Four independent mutati ons identified a cluster of virulence genes located on a 50 kb chromosomal region. These genes might be involved in the production of phthiocerol and phenolphthiocerol derivatives, a group of molecules restricted to eight myc obacterial species, seven of them being either strict or opportunistic path ogens. The interaction of five mutant strains with mouse bone marrow macrop hages was investigated. These five mutants were still able to multiply in t his cell type, However, in three cases, there was a growth defect in compar ison with the wild-type strain. The other two strains exhibited no clear di fference from the virulent strain, MT103, in this model. This study, which is the first global research of virulence factors of M. tuberculosis, opens the way to a better understanding of the molecules that are key players in the interaction of this pathogen with its host.