Mycobacterium Tuberculosis Evolutionary Pathogenesis and its Putative Impact on Drug Development

Fabien Le Chevalier; Alessandro Cascioferro; Laleh Majlessi; Jean Louis Herrmann; Roland Brosch

Disclosures

Future Microbiol. 2014;9(8):969-985. 

In This Article

Genomics & Identification of the Essential Genes of M. Tuberculosis

Within the more than 130 years since the discovery of M. tuberculosis as the causative microorganism responsible for human TB by Robert Koch, numerous scientific advances have been made that help to cope with this major pathogen. However, despite this progress, M. tuberculosis still holds many unresolved secrets and much work remains to be done in order to translate the basic findings from recent research into novel strategies against the pathogen.[12] After several anti-TB drugs were developed in the mid-20th century, there has been a long gap in which no new molecules to be used in the treatment of TB were developed. One of the reasons for that could have been that TB was believed to be a disease of the past, which would disappear in modern societies due to improved social conditions, vaccination and already-existing treatment regimens. However, it is now clear that this was not the case for developing countries and also certain population groups in industrialized countries. The synergy of TB with the HIV epidemic, the phenomenon of multidrug resistance, massive global population growth and a worsening of social conditions in many countries dramatically changed the situation, making TB a continuing global health threat even at the beginning of the 21st century.[26] This fact has also lead to a strongly renewed research interest that is likely to be continued in the future. In this context, the accomplishment and publication of the genome sequence of the widely used M. tuberculosis H37Rv strain[11] was one key step that allowed the scientific community to gain deeper insight into the overall organization of this pathogen and to design numerous follow-up strategies in a wide range of postgenomic applications.

For scientists interested in increasing the arsenal of new compounds and drugs against TB and its causative organism, the knowledge of the complete gene pool of the pathogen has helped considerably with the identification and investigation of potentially vulnerable targets of M. tuberculosis. The genome information served as a starting point for identifying which among the 4000 genes in the M. tuberculosis genome were essential for the growth and/or survival of the pathogen. The genome-wide use of high-density transposon mutagenesis in combination with the microarray-based identification of insertion sites revealed a minimal gene set of 614 genes required for optimal in vitro growth of M. tuberculosis in culture broth.[29] This technique, named transposon site hybridization (TraSH), was subsequently also adapted to other biological settings, such as infection models in mice, where a set of 194 genes implicated in growth under in vivo conditions was identified.[30] Among them, numerous genes encoding proteins involved in the pathogenicity of the tubercle bacilli can be found that are also discussed in a later section. Most recently, saturation transposon mutagenesis of M. tuberculosis grown under different in vitro conditions was combined with next-generation sequencing (NGS),[31] which allowed a set of 774 genes to be identified, of which 451 overlapped with the initial TraSH-based screen. In addition, this work permitted differences in the essential gene set to be linked to the use of cholesterol instead of glycerol as a carbon source. Identification of the bacterial genes required for cholesterol utilization in M. tuberculosis allowed the widespread metabolic changes to be predicted that are associated with the adaptation of the bacterium to this carbon source and refined the understanding of bacterial physiology.[31]

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