**12. Two component systems**

promoters of genes involved in mycobactin synthesis it acts as a repressor, inhibiting expres‐ sion of these genes at high iron concentrations. In promoters of iron storage proteins it acts as an activator, stimulating expression of these genes at high iron concentrations and thus

*In vivo M. tuberculosis* experiences low oxygen tension that may be encountered in the cen‐ ters of granulomas as previously described. Studies have shown that tuberculous granulo‐ mas are hypoxic in a variety of animal models including rabbits, guinea pigs, and nonhuman primates (Via et al, 2008). The response to low oxygen tension is biphasic. There is an initial response that predominates and is controlled by the two component system DosS/DosT-DosR (Table 1.). This two component system upregulates genes that are known to be part of the "dormancy regulon". DosR is the transcriptional regulator, and Dos T and DosS are the sensor kinases that respond to low oxygen tension as well as nitric oxide (Park et al, 2003; Kumar et al, 2007). *hspX* ( *acr*, *Rv2031c)* is upregulated by low oxygen, is regulated by DosR, and has chaperonin activity that may aid in refolding proteins which are damaged by low oxygen tension (Vasudeva-Rao and McDonough, 2008; Florczyk et al, 2003). It is known that this protein is expressed *in vivo* as latently infected individuals pos‐ sess T-cells that are reactive to the HspX protein (Geluk et al, 2007). Interestingly one half of the genes in the DosR regulon return to their baseline level after 24 hours. After this ini‐ tial 24 hour period other regulators play a role in hypoxic responses such as sigE and sigC (Table 1.). An enduring hypoxic response begins after the initial response, and this may be important for *M. tuberculosis* to enter and stay in a dormant state (Rustad et al, 2008).

Interestingly there are many toxin-antitoxin systems within the *M. tuberculosis* genome. These systems seem to provide a mechanism by which bacteria can alter growth rate rapidly, potentially in response to environmental stressors. The toxin is not a protein secreted and targeted against the human host, but targeted against mycobacterial cellular components. The toxin is a stable protein which may be complexed with an antitoxin forming a toxin-antitoxin pair. The antitoxin is relatively unstable and environmental stressors can inactivate it causing release of a free toxin. The toxin is then available to interact with cellular components, and may function to cleave mRNA thus inhibiting subsequent translation and rapidly halting growth of the bacterium. As static bacteria are more resistant to environmental stressors and antibiotics, this system may allow *M. tuberculosis* to survive in the face of external stressors. *M. tuberculosis* possesses 88 toxin-antitoxin systems and four of these have been shown to be activated by phagocytosis of bacilli, by macrophages, or hypoxia (Table 1.). It appears that the

toxin in these systems acts by cleaving mRNA (Rapage et al, 2009).

avoiding iron stimulated oxidative damage.

10 Tuberculosis - Current Issues in Diagnosis and Management

**10. Hypoxic growth**

**11. Toxin-antitoxin systems**

Two components systems are common in many bacteria. These systems are comprised of a sensor kinase which phosphorylates the response regulator as a result of an environmental signal, which is often a stress. The sensor kinases are trans membrane proteins which are embedded into membranes. They sense external stresses and transmit these signals internally into the bacterial cell by phosphorylating a response regulator that binds to its cognate promoter DNA, and regulates transcription. The mycobacterial genome contains 11 two component systems (Hett and Rubin, 2008). The large number of these systems in the myco‐ bacterial coding regions is likely the result of evolution to accommodate bacterial responses to diverse stresses.

DosS/DosT-DosR was previously described, and responds to initial hypoxic stress (Table 1.) (Park et al, 2003). Some of the genes controlled by the transcriptional regulator DosR are upregulated by hypoxic stress, and are also part of the transcriptional regulator PhoP regulon, a member of the PhoP/R two component system. While it is unknown what environmental signal PhoP or the sensor kinase PhoR are responding to, genes controlled by PhoP either directly or indirectly are upregulated by such stresses as acidity and low oxygen (Table 1.) (Gonzalo-Asensio et al, 2008).

bacteria are inherently resistant to environmental stresses due to their thick waxy cell envelope, upregulation of genes further reinforce this defense. In addition there are proteins upregulated by environmental stressors which can detoxify the mycobacterial cell as is the case of acidic stress and upregulation of ammonia extruding pumps that neutralize acidic pH of the macrophage phagosome. Thus inducible systems allow *M. tuberculosis* to resist environmental

*Mycobacterium tuberculosis* Adaptation to Survival in a Human Host

http://dx.doi.org/10.5772/54956

13

Understanding the specific steps in infection, the stresses associated with each step, and the mycobacterial response may be of clinical relevance. The knowledge that oxidative stress and acidic stress may predominate as adaptive immunity makes the host's macrophages more activated, may lead to the development of chemotherapeutic agents that target mycobacterial components produced by these stressors during this infective stage. In addition, the knowl‐ edge that mycobacteria may utilize toxin-antitoxin systems to slow their growth and to enhance their innate antibiotic resistance may spur the development of therapies that target these systems which could be used in conjunction with traditional antibiotic treatments. Chemotherapeutic agents given to decrease activity of triacylglycerol synthase may decrease infectivity of sputum positive individuals by inhibiting lipid body production in the bacilli while antibiotic treatment lags in its sterilizing activity. Ultimately treatments may be devel‐ oped which target inducible systems upregulated by stresses, and may interfere with myco‐ bacterial responses to these stressors. By thwarting these adaptive responses potentially with chemotherapeutic agents, mycobacteria may be rendered more fragile and susceptible to the host's immune system. In addition a greater understanding of how *M. tuberculosis* enters a latent state of persistence could lead to treatments that prevent this microbe from reactivating from the dormant state, or from becoming dormant to begin with. Greater understanding of *M. tuberculosis* responses to *in vivo* growth will hopefully lead to the development of technol‐

Basic Medical Sciences, College of Osteopathic Medicine, Western University of Health

[1] Abdallah A, Gey van Pittius N, Champion P, Cox J, Luirink J, Vandenbroucke-Grauls C, Appelmelk B, and Bitter W. 2007. Type VII secretion-mycobacteria show the way.

stresses and persist in the human body to cause active or latent disease.

ogies that lessen *M. tuberculosis'* global impact on human health.

Nature Reviews in Microbiology. 5: 883-891.

**Author details**

Beatrice Saviola

**References**

Sciences, Pomona CA, USA
