**1. Introduction**

216 Understanding Tuberculosis – Deciphering the Secret Life of the Bacilli

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Hoelscher, M. & Grange, J. (2011). Viewpoint: Scientific dogmas, paradoxes and mysteries of latent M. tuberculosis infection. *Tropical Medicine and International*  The chapter introduces and discusses the metabolic robustness and survival strategies of mycobacteria with focus on glyoxylate shunt and protein synthesis. Primarily, it is an attempt to understand the significance of glyoxylate pathway, which provides an adaptive advantage in metabolically starved situation. The other issue addressed here are the key players behind resistance mechanism in all thriving forms of the bacteria. The protein synthesis process, its challenges and advantages that can effectively be harnessed have been discussed in details. It also gives a comprehensive account of the strength, weaknesses opportunities and threats in targeting mycobacterium both in active and dormant state. Last but not the least we also highlight further perspectives to control the pathogenesis by the bacteria.

#### **Definitions:**

**Dormancy**: the reversible state of bacterial metabolic shutdown (Kell et al., 1998; Mukamolova et al., 1998; Barer & Harwood, 1999).

**Persistence**: the phenomenon whereby otherwise drug susceptible microorganisms have the capacity to survive indefinitely within mammalian tissues despite continued exposure to the appropriate drug or drugs (Mc et al., 1957).

**Latency**: *in vivo* situation where bacteria and the host have established a balanced state without causing apparent symptoms in the host, as in latent infection (Orme, 2001a)

#### **1.1 A prelude to mycobacterium: The culprit in stealth**

The enigma caused by the mycobacterium has been a challenge to the scientific community by virtue of their adaptive skills and evasion mechanism to combat immunologically educated host. It is a gram positive, acid fast pathogenic bacterium with unique cell wall. It

*Mycobacterium tuberculosis*: Dormancy, Persistence and Survival in the Light of Protein Synthesis 219

Fig. 1. Mycobacterium survival: the lipid lunch through glyoxylate shunt.

Tuberculosis is characterized by periods in which the disease may be non-obvious or even clinically inapparent but even in these regimens mycobacteria persist with the potential to reactivate the disease opportunistically. Persistence may be defined as a stage where the metabolic downshift to anaerobiosis brings about a nutritionally suspended condition. In the granulomas the bacterium does not replicate and becomes inert yet surviving in occult forms to get activated in immune-compromised situations. In persistent phase the *ala carte* is changed from glucose to delicious lipids, glycolysis is decreased and the glyoxylate shunt is upregulated allowing anaplerotic maintenance of the tricarboxylic acid (TCA) cycle (McKinney et al., 2000). The glyoxylate shunt converts isocitrate to succinate and glyoxylate, catalyzed by the enzyme isocitrate lyase (ICL), followed by the addition of Acetyl-CoA to

**2. Glyoxylate pathway: The saviour of the anarchist** 

can survive even within the hostile environment of alveolar macrophages. Statistically speaking as per the WHO fact sheet 2010/2011, Tuberculosis is reported to kill 1.7 million people in 2009 globally. It accounts for about 4700 death per day (Organization, 2010). Further complications in eradicating these 'notorious culprits of mass destruction' is the emergence of drug resistant forms as MDR-TB and XDR-TB i.e. multi drug resistance and extensive drug resistance respectively. Their smart stealth behaviour to get away from host defence together with metabolic fine-tuning in hostile environment makes it world's most successful pathogen in action.

#### **1.2** *Modus operandi* **of hide out**

It has been quite puzzling to understand the metabolic fluctuations in the changing pathophysiological microenvironment of mycobacteria. Our understanding pertaining to the process for acquisition of essential nutrients for thriving in these environments by intracellular bacterium is still in the stage of infancy. Detailed analyses have revealed a transformative process where environmental hostility brings about a lifestyle change following a reductionist agenda to minimize nutritional needs leading to dormant and/or persistent cells, collectively described as latent tuberculosis (Gomez & McKinney, 2004; Lewis, 2007). A very recent transcriptiome based analysis brought an interesting scenario to light that in *Mycobacterium tuberculosis*, low numbers of drug-tolerant persisters are present from the lag and early exponential phases, which increase sharply at late exponential and stationary phases roughly accounting for 1% of the total population. This further established a new understanding that dormancy is not an all or none phenomenon, and it is collectively governed by both deterministic and stochastic mechanisms (Keren et al., 2011).There are several models to study the phenomenon but one recent model based on multiple stress dormancy, that generates a lipid loaded drug tolerant dormant pathogen looks quite promising (Deb et al., 2009). A recent study using same model by Daniel *et.al* mimicking the microenvironment inside the human granuloma by incubating mycobacterium infected macrophages under hypoxic phase revealed that under these conditions macrophages produce lipid droplets containing Triacyl glycerol (TAG) which is smartly utilized by these bacteria too and exhibit dormancy like phenotype (Daniel et al., 2011). Pandey *et.al* from their work also demonstrated that Mycobacterium can effectively degrade cholesterol derived from host and use it for their carbon and energy source thus maintaining chronic infections in murine models and establishing persistence (Pandey & Sassetti, 2008).

#### **1.3 Scheme of the process**

Latent tuberculosis is characterized by a plethora of converging events; on one hand the immunological modulators govern the process in dynamic fashion, while on the other hand metabolic plasticity is at its best. The core strategy is to undergo a downshift in the needs in order to survive with minimal metabolic activity (Wayne & Hayes, 1996). Looking from a metabolomics perspective a variety of genes undergo fluctuations in their expression profile falling mainly under three major groups; respiratory enzymes, stress related proteins and proteins involved in fatty acid metabolism (Honer zu Bentrup & Russell, 2001). The shift to anaerobiosis leads to a metabolic shuffle triggering alternative pathway of glyoxylate shunt to meet the challenges ahead.

can survive even within the hostile environment of alveolar macrophages. Statistically speaking as per the WHO fact sheet 2010/2011, Tuberculosis is reported to kill 1.7 million people in 2009 globally. It accounts for about 4700 death per day (Organization, 2010). Further complications in eradicating these 'notorious culprits of mass destruction' is the emergence of drug resistant forms as MDR-TB and XDR-TB i.e. multi drug resistance and extensive drug resistance respectively. Their smart stealth behaviour to get away from host defence together with metabolic fine-tuning in hostile environment makes it world's most

It has been quite puzzling to understand the metabolic fluctuations in the changing pathophysiological microenvironment of mycobacteria. Our understanding pertaining to the process for acquisition of essential nutrients for thriving in these environments by intracellular bacterium is still in the stage of infancy. Detailed analyses have revealed a transformative process where environmental hostility brings about a lifestyle change following a reductionist agenda to minimize nutritional needs leading to dormant and/or persistent cells, collectively described as latent tuberculosis (Gomez & McKinney, 2004; Lewis, 2007). A very recent transcriptiome based analysis brought an interesting scenario to light that in *Mycobacterium tuberculosis*, low numbers of drug-tolerant persisters are present from the lag and early exponential phases, which increase sharply at late exponential and stationary phases roughly accounting for 1% of the total population. This further established a new understanding that dormancy is not an all or none phenomenon, and it is collectively governed by both deterministic and stochastic mechanisms (Keren et al., 2011).There are several models to study the phenomenon but one recent model based on multiple stress dormancy, that generates a lipid loaded drug tolerant dormant pathogen looks quite promising (Deb et al., 2009). A recent study using same model by Daniel *et.al* mimicking the microenvironment inside the human granuloma by incubating mycobacterium infected macrophages under hypoxic phase revealed that under these conditions macrophages produce lipid droplets containing Triacyl glycerol (TAG) which is smartly utilized by these bacteria too and exhibit dormancy like phenotype (Daniel et al., 2011). Pandey *et.al* from their work also demonstrated that Mycobacterium can effectively degrade cholesterol derived from host and use it for their carbon and energy source thus maintaining chronic

infections in murine models and establishing persistence (Pandey & Sassetti, 2008).

Latent tuberculosis is characterized by a plethora of converging events; on one hand the immunological modulators govern the process in dynamic fashion, while on the other hand metabolic plasticity is at its best. The core strategy is to undergo a downshift in the needs in order to survive with minimal metabolic activity (Wayne & Hayes, 1996). Looking from a metabolomics perspective a variety of genes undergo fluctuations in their expression profile falling mainly under three major groups; respiratory enzymes, stress related proteins and proteins involved in fatty acid metabolism (Honer zu Bentrup & Russell, 2001). The shift to anaerobiosis leads to a metabolic shuffle triggering alternative pathway of glyoxylate shunt

successful pathogen in action.

**1.3 Scheme of the process** 

to meet the challenges ahead.

**1.2** *Modus operandi* **of hide out** 

Fig. 1. Mycobacterium survival: the lipid lunch through glyoxylate shunt.
