Proteomic profiling of Mycobacterium tuberculosis (MTB) at the Statens Serum Institute in Copenhagen has revealed interesting new toxin-antitoxin (TA) systems that may contribute to the parasite’s ability to persist within the human host for years without causing disease.1
The World Health Organization act sheet from 2010/2011 reports that tuberculosis killed 1.7 million people in 2009 globally, accounting for about 4,700 deaths per day. MTB’s exceptional ability to avoid host defenses together with metabolic fine-tuning in hostile environments make it the world’s most successful pathogen in action.2 While MTB is a necessary component for the disease, it is also insufficient to produce clinical symptoms in every human host. Many people host the bacteria but do not exhibit symptoms. So far, researchers have not established the mechanisms through which MTB establishes this latent metabolic state, though many theories have been suggested. Because the dormant bacilli are resistant to antimycobacterial agents, an evaluation of the dormancy mechanism may provide strategies for preventing MTB persistence and spread.
The Copenhagen team used label-free LC-MS/MS analysis and two-dimensional DIGE to compare the proteome of MTB cultured under nutrient starvation with the proteome of MTB in its active phase. They identified 230 proteins that were increased in nutrient-starved culture filtrates, while 208 were decreased. Gene ontology clustering analysis pointed to significant metabolic differences in nutrient-starved MTB. Most significantly, proteins from several TA systems were present in larger quantities in the nutrient-starved bacterial cultures, whereas several T-cell antigens, including the ESAT-6 family proteins, were reduced.1
TA systems usually comprise a two-gene operon encoding a toxic protein that targets an essential cellular function and a corresponding antitoxin that inhibits the toxin. TA systems self-regulate by differential production of the stable toxin and the unstable antitoxin.4 Although the function of TA systems is hotly debated, one common hypothesis suggests that TA systems arrest cell growth rather than promote cell death, halting protein synthesis until more favorable conditions occur.1
A previous study described TA systems in MTB and discovered that there were far more than in any other intracelullar pathogen studied so far.4 This suggests that TA systems are involved not only in normal bacterial physiology, but more so in the nonreplicating persistent cells.3 With so high a number of TA systems, scientists have posited and showed that a different TA system would activate given varying stresses, including hypoxia, macrophage activation, and heat shock, thus providing MTB with a “super” survival mechanism no matter what stress condition it encounters.4
Recent studies have presented the unexpected finding that human T-cell epitopes of MTB are hyperconserved, possibly reflecting that T-cell recognition is beneficial for the bacterium in its virulent form.1 The reduction in T-cell antigens can help to explain why, in the dormant phase, MTB is less easily discovered by and aided by the immune system.
The Cophenhagen study confirms the findings of genomic-based studies that TA systems may present important areas for drug discovery for dormant MTB. By hampering the bacteria’s ability to move into dormancy by altering TA systems, scientists may potentially eliminate the dormant, resistant form of the bacteria. On the other hand, by interfering with the T-cell epitope manufacture, scientists may be able to slow the logarithmic growth of the virulent form.
1. Albrethsen, J., et al., (2013) ‘Proteomic profiling of the Mycobacterium tuberculosis identifies nutrient starvation responsive toxin-antitoxin systems‘, Molecular and Cellular Proteomics, published online January 23, 2013. doi: /10.1074/mcp.M112.018846
2. Cardona, P.-J. (2012) ‘Understanding tuberculosis – deciphering the secret life of the bacilli‘, InTech
3. Yamaguchi, Y., et al. (2011) ‘Toxin-antitoxin systems in bacteria and archaea‘, Annual Review of Genetics, 45, (pp. 61–79)
4. Ramage, H.R., et al. (2009) ‘Comprehensive functional analysis of Mycobacterium tuberculosis toxin-antitoxin systems: implications for pathogenesis, stress responses, and evolution‘, Public Library of Science Genetics, 5 (12), (p. e1000767)