Infection and transmission
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Tuberculosis (TB) is primarily an airborne disease. The disease is spread from person to person in tiny microscopic droplets when a person with untreated pulmonary TB disease coughs or sneezes.
The process of catching tuberculosis involves two stages: first, a person must become infected; second, the infection must progress to disease.
To become infected, a person has to come in close contact with another person having active tuberculosis. In other words, the person has to breathe the same air in which the person with active disease coughs or sneezes. However, prolonged exposure to a person with untreated TB usually is necessary for infection to occur.
Mycobacterium tuberculosis is a well-equipped intracellular pathogen as evidenced by its natural ability to adapt and reside within the human macrophage phagosome and to persist within the human host for many years.
In most individuals, disease progression does not occur and the initial infection produces a compact, self-limited granulomatous lesion which is an effective means of containing the spread of the bacteria. Reactivation of a persistent infection occurs during immune senescence, immunosuppressive therapy, or conditions that allow the tubercle bacilli to escape immunological control and reestablish active disease.
Global and regional incidence
Tuberculosis remains a significant world health burden with approximately one-third of the world’s population estimated to be infected and a mortality rate of 2 million people per year.
Additionally, due to the rapid progression of both tuberculosis and HIV in coinfected individuals, tuberculosis is the leading cause of death in HIV-infected individuals and is the causative agent in 13% of AIDS deaths worldwide.
Role of two-component regulators in M. tuberculosis pathogenesis
The emergence and incipient spread of antibiotic resistance, including multi-drug resistance, jeopardizes our abilities to fight infectious diseases in all people within all communities, and therefore poses an incredible global threat to public health. Antimicrobial agents traditionally target pathways and/or processes in the cell that are essential for their viability or survivability within the human host, while leaving host cells unaffected. Therefore, regulatory proteins that are essential for viability or control critical virulence factors represent auspicious targets for the development of novel antimicrobial therapeutics. Two-component systems embody such regulatory models as they are ubiquitous among bacteria, but not found in humans; have conserved structural homologies, particularly surrounding catalytic sites; are essential for viability in several pathogens; are critical for bacterial virulence; and are required for bacterial adaptation in various disease processes.
PrrAB two-component system
The prrA and prrB genes constitute an operon and are cotranscribed during logarithmic growth, with transcriptional levels decreasing in stationary phase and during hypoxia. Despite the transcriptional differences, PrrA protein levels remained relatively stable throughout growth and in hypoxia. Under conditions of nitrogen limitation, prrAB transcription was induced, while acidic pH stress and carbon starvation did not significantly alter transcript levels. Deletion of the prrAB operon on the chromosome of M. tuberculosis H37Rv occurred only in the presence of an episomal copy of the prrAB genes, indicating that this two-component system is essential for viability (Haydel et al. 2012).
Ongoing goals of this research are to:
• identify the PrrAB regulon,
• discover mechanistic insights into the regulatory control of the PrrAB two-component system,
• determine the influences that the regulated targets have upon necessitating PrrAB essentiality, and
• develop a reporter system for screening and/or generating inhibitors.
Rv1626 (PdtaR) and Rv3143 orphan response regulators
The Rv1626 response regulator gene is expressed after 18, 48, and 110 h of M. tuberculosis intracellular growth, while expression of the Rv3143 orphan response regulator gene was not detected during M. tuberculosis growth in human macrophages (Haydel and Clark-Curtiss. 2006). However, in another study, the Rv3143 gene exhibits a similar expression profile in macrophages to the mprA response regulator gene which is required for maintaining a persistent M. tuberculosis infection. The Rv1626 and Rv3143 mutants will both be analyzed to determine the regulons and their roles in M. tuberculosis pathogenesis and/or physiology.
The high resolution crystal structure of Rv1626 demonstrates that the N-terminus harbors a prototypical receiver domain with a C-terminal domain exhibiting structural homology to a unique RNA binding domain involved in transcriptional antitermination (Morth et al. 2004). Demonstrated phosphorylation relay between the Rv3220c orphan histidine kinase and Rv1626 establishes these proteins as a two-component regulatory circuit (Morth et al. 2005). Due to the unique putative function, Rv1626 is designated a phosphorylation-dependent transcriptional antitermination regulator (PdtaR) and its cognate histidine kinase, Rv3220c, a phosphorylation-dependent transcriptional antitermination sensor (PdtaS). Efforts are underway to understand how this unique response regulator controls transcription via antitermination and its role in M. tuberculosis pathogenesis.
TrcRS two-component system and Rv1057 seven-bladed beta-propeller
Autoregulation of the TrcR response regulator occurs via binding of an AT-rich sequence within the trcR promoter (Haydel et al. 2002). The AT-rich characteristics of the trcR promoter were used to identify the Rv1057 gene, which is repressed by TrcR (Haydel et al. 2006).
The Rv1057 gene is expressed during M. tuberculosis growth in macrophages, and the protein appears to be a seven-bladed β-propeller of unknown function (Haydel et al. 2006). Rv1057 is activated by detergents, vancomycin, and surfactant proteins, suggesting it also has a role in maintaining envelope stability (Pang et al. 2011). Recent findings indicate that Rv0157 has an extensive and complex promoter that is directly regulated by both the MprAB and TrcRS two-component systems (Pang et al. 2011).
Goals of this research are to:
- identify the TrcRS regulon and the TrcRS-MprAB co-regulons,
- assess virulence attributes of the trcRS system and Rv1057,
- determine the high-resolution structure of Rv1057, and
- discover the biological role of the TrcRS regulatory system and the Rv1057 beta-propeller protein.