**10. Hypoxic growth**

*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).

#### **11. Toxin-antitoxin systems**

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).


**Table 1.** Mycobacterial responses to in vivo stressors and conditions.
