**2. Adaptation to growth in the phagosomal compartment of macrophages**

Macrophages are the preferred intracellular location for *M. tuberculosis in vivo.* Infected individuals cough and expel droplet nuclei which contain *M. tuberculosis* bacilli and remain suspended in the air. After inhalation and within the body, the bacilli are transported to the small alveoli in the lungs where they encounter alveolar macrophages which are relatively nonactivated (Dannenberg, 1993; Dannenberg, 1997). These nonactivated macrophages are not

© 2013 Saviola; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

efficient at killing or retarding growth of invading microbes. Initially bacilli are taken up into phagosomal compartments and may replicate. As the immune system becomes activated, macrophages are stimulated with INF-γ to increase their efficiency of mycobacterial killing, becoming more efficient at producing reactive oxygen intermediates and acidic stress. In response, *M. tuberculosis* pushes back against the macrophages and differentially regulates key genes. Within macrophages *M. tuberculosis* increases its lipid metabolism which may reflect an environment in the phagosome which lacks available carbohydrates (Table. 1). In addition the enzyme isocitrate lyase (*icl*) is strongly induced *in vivo,* and *icl* is upregulated in all macrophage models. Icl is a key enzyme in the glyoxylate shunt and utilizes fatty acids as an energy source. When *icl* and other genes in the glyoxylate shunt are mutated this results in attenuation *in vivo*. In addition within macrophages, genes involved in stress responses, cell wall component production, anaerobic respiration, siderophore production to scavange iron, diverse sigma factor production, and tranposases that may mutate the genome are all upre‐ gulated (Schnappinger et al, 2003, Beste et al, 2007, Ward et al, 2010).

tissues. Dead macrophages release lytic enzymes, and bacterial products may also result in host tissue damage and liquefaction ensues. As tissue is damaged, a cavity erodes into the lung airspace. In rabbit studies, *M. tuberculosis* can replicate to extremely high levels in this liquefied environment (Dannenberg 1993, Dannenberg et al 1997, Dannenberg 2006). For the first time *in vivo M. tuberculosis* is capable of replicating extracellularly. Liquid containing free *M.*

*Mycobacterium tuberculosis* Adaptation to Survival in a Human Host

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

5

*M. tuberculosis* within sputum contains elevated levels of lipid bodies and tends to be inhibited in its replicative process ( Table 1.) (Garton et al, 2008). In addition, sputum transcriptome analysis of *M. tuberculosis* reveals that triacylglycerol synthase, t*gs1* part of the DosR regulon, is induced and lipid bodies may be composed of increased stores of triacylglycerol (Garton et al, 2008). Lipid bodies are correlated *in vitro* with nonreplicating persistence, and may help *M. tuberculosis* survive the harsh environment *ex vivo* before it encounters another human host.

One third of the world's population is infected with *M. tuberculosis* in part because it causes a latent or dormant infection in a majority of those infected. If therapies are to be developed which can eradicate *M. tuberculosis,* a better understanding of dormancy is required. *M. tuberculosis* can persist for decades in a dormant state within hypoxic granulomas in the lung. Studies have suggested that in a dormant state *M. tuberculosis* is occupied mainly with maintaining cell wall integrity, membrane potential, and protecting its DNA structure. The mycobacterium must also resist the host's immune system. A number of *in vivo* and *in vitro* models have been used to investigate dormancy. These models include exposing mycobacteria to environments that are likely encountered within the host. In one model cultures are stirred slowly and sealed so that oxygen is gradually consumed. In another model nutrient starvation of the bacteria may induce dormancy. In addition, infection of mice, partial treatment with antibiotics, and exposure to immune suppression can lead to dormancy and reactivation

The gene encoding a transcriptional regulator, *dosR* (*devR*), part of a two component system that responds to low oxygen seems to be very important in a shift from replicating *M. tuberculosis* to a nonreplicating form (Table 1.). Carbohydrate limitation also upregulated *dosR* and there is indeed an overlap of genes upregulated in phagosomes of macrophages and low carbohydrate availability. In dormancy models aerobic respiratory metabolism was down regulated while anaerobic respiration was upregulated as were DosR controlled genes (Murphy and Brown, 2007). Amino acid and carbon starvation results in the activation of the stringent response. RelA (Rv2583c) mediates this stringent response in *M. tuberculosis* and can globally down regulate components necessary in protein translation, and thus conserve badly needed resources in the mycobacterium during times of stress. RelA may be a target to prevent *M. tuberculosis* from entering dormancy or a target to force *M. tuberculosis* out of dormancy

*tuberculosis* is expelled through cavities in the lung by coughing.

**5.** *Mycobacterium tuberculosis* **and dormancy**

(Murphy and Brown, 2007).

(Murphy and Brown, 2007).

## **3. Adaptation to granulomas and caseation**

Once infection has progressed, tubercle bacilli replicate within incompletely activated mac‐ rophages. Additional macrophages arrive to the site of infection, and engulf newly liberat‐ ed mycobacteria. The immune cells, T-cells, arrive to this location and an immune structure, the granuloma, composed of macrophages and a mantel of T-cells develops. If the host is resistant, and can robustly activate the body's macrophages, then *M. tuberculosis* infection is likely controlled. If the host immune system is weak, or is weakened, *M. tuber‐ culosis* can replicate in the incompletely activated macrophages. Genes of *M. tuberculosis* re‐ quired to resist macrophages will be important in resisting the environment of the granuloma as well. As the infection progresses in susceptible individuals, the centers of the granulomas degenerate and form a caseous, or cheesy, center. At the heart of this is an ele‐ vated lipid metabolism of the host that produces a variety of lipids including cholesterol, cholesteryl esters, triacyglycerol and others (Kim et al, 2010). Interestingly *M. tuberculosis* infection has been shown to induce elevated lipid metabolism in the host (Table 1.). The cell wall lipid of *M. tuberculosis*, trehalose dimycolate or cord factor, induces a granuloma‐ tous response in mice, and this was accompanied by foam cell formation which contains elevated lipids (Kim et al, 2010). It is intriguing to speculate that *M. tuberculosis* infection can induce elevated host lipid metabolism, and as discussed previously as part of adapta‐ tion to *in vivo* growth, *M. tuberculosis* also switches to lipid metabolism and lipids as a pre‐ ferred carbon source (Eisenreich et al, 2010). Thus *M. tuberculosis* induces the host to produce what the microbe has evolved to utilize as an energy source.

#### **4. Liquefied lesions and sputum**

Later in infection caseating granulomas continue to breakdown. At a certain point these granulomas begin to liquefy, and host lipases and proteases are present which damage host tissues. Dead macrophages release lytic enzymes, and bacterial products may also result in host tissue damage and liquefaction ensues. As tissue is damaged, a cavity erodes into the lung airspace. In rabbit studies, *M. tuberculosis* can replicate to extremely high levels in this liquefied environment (Dannenberg 1993, Dannenberg et al 1997, Dannenberg 2006). For the first time *in vivo M. tuberculosis* is capable of replicating extracellularly. Liquid containing free *M. tuberculosis* is expelled through cavities in the lung by coughing.

*M. tuberculosis* within sputum contains elevated levels of lipid bodies and tends to be inhibited in its replicative process ( Table 1.) (Garton et al, 2008). In addition, sputum transcriptome analysis of *M. tuberculosis* reveals that triacylglycerol synthase, t*gs1* part of the DosR regulon, is induced and lipid bodies may be composed of increased stores of triacylglycerol (Garton et al, 2008). Lipid bodies are correlated *in vitro* with nonreplicating persistence, and may help *M. tuberculosis* survive the harsh environment *ex vivo* before it encounters another human host.
