Mycobacterium tuberculosis: Using Quantitative Proteomics to Understand Virulence

Tuberculosis is caused by the bacterium Mycobacterium tuberculosis and is a leading cause of death worldwide. This is despite there being a vaccine for tuberculosis and antibiotics for treatment. Whole-genome analysis of the M. tuberculosis complex demonstrates significant genetic conservation and homogeneity. Despite this, it displays discrepancies in virulence, the immune response that it elicits and transmissibility. Peters et al. (2016) used qualitative and quantitative mass spectrometry tools to investigate clinically relevant strains.¹

The investigators initially obtained seven M. tuberculosis isolates: H37Rv (reference strain), Avium, Beijing, CAS, BCG, Bovis and LAM3. They performed protein extraction before separating the isolates using one-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) at 100 V. To visualize the proteins, they used Coomassie Brilliant Blue R-250 dye (Thermo Scientific). Peters et al. then analyzed the proteins on an EASY-nLC II System coupled to an LTQ Orbitrap Velos mass spectrometer (both Thermo Scientific) equipped with a nano-electrospray ion source. For selected reaction monitoring experiments, they used a TSQ Vantage Triple Quadrupole mass spectrometer (Thermo Scientific) equipped with a heated electrospray ion source.

Using bioinformatics, Peters et al. successfully obtained greater than 80% coverage at the protein level for six out of seven proteomes. Combining the four M. tuberculosis strains (H37Rv, Beijing, CAS and LAM), they identified an unprecedented 3,788 proteins out of the predicted 4,023. The investigators state that their data indicates that the total expressed complements of proteins in the different strains of M. tuberculosis are similar. Furthermore, almost the entire proteome is present in all strains. The variation in the strains stems from quantitative differences in expression levels, which the researchers suggest is responsible for the different phenotypes. In particular, the Beijing strain is able to remodel its proteome in response to a changing environment, which may be the reason behind its ability to establish a primary infection and spread effectively.

Reference
1. Peters, J.S., et al. (2016) “Identification of quantitative proteomic differences between Mycobacterium tuberculosis lineages with altered virulence,” Frontiers in Microbiology, 7(813), doi: 10.3389/fmicb.2016.00813.