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A new evolutionary scenario for the Mycobacterium tuberculosis complex.
R Brosch1, S V Gordon, M Marmiesse
1Unité de Génétique Moléculaire Bactérienne, Laboratoire de Génomique des Microorganismes Pathogènes, and Centre National de Référence des Mycobactéries, Institut Pasteur, 25-28 Rue du Docteur Roux, 75724 Paris Cedex 15, France.
Summary
Genetic analysis of 100 tubercle bacilli strains reveals that Mycobacterium tuberculosis likely evolved from a human pathogen, not Mycobacterium bovis. Ancient deletions shaped the Mycobacterium tuberculosis complex evolution.
Area of Science:
- Genomics
- Evolutionary Biology
- Microbiology
Background:
- The evolutionary history of Mycobacterium tuberculosis complex (MTC) strains, including Mycobacterium tuberculosis, Mycobacterium africanum, Mycobacterium canettii, Mycobacterium microti, and Mycobacterium bovis, is debated.
- Previous hypotheses suggested Mycobacterium tuberculosis evolved from Mycobacterium bovis, the agent of bovine tuberculosis.
Purpose of the Study:
- To investigate the evolutionary relationships within the Mycobacterium tuberculosis complex.
- To determine the origins and evolutionary trajectory of Mycobacterium tuberculosis strains.
Main Methods:
- Genomic analysis of 20 variable regions resulting from insertion-deletion events.
- Evaluation of 100 strains from five species: M. tuberculosis, M. africanum, M. canettii, M. microti, and M. bovis.
- Identification and characterization of specific deletions, such as TbD1 and Region of Difference 9 (RD9).
Main Results:
- The majority of polymorphisms in MTC strains originated from ancient genetic events in common progenitors, not independent occurrences.
- Mycobacterium tuberculosis strains were classified into ancestral and 'modern' types based on the TbD1 deletion.
- An evolutionary lineage including M. africanum, M. microti, and M. bovis showed successive DNA loss, diverging before the TbD1 deletion event.
Conclusions:
- Findings contradict the hypothesis that Mycobacterium tuberculosis evolved from Mycobacterium bovis.
- Mycobacterium canettii and ancestral Mycobacterium tuberculosis strains appear to be direct descendants of earlier tubercle bacilli.
- The common ancestor of tubercle bacilli likely resembled Mycobacterium tuberculosis or Mycobacterium canettii and may have been a human pathogen.