**1. Introduction**

192 Understanding Tuberculosis – Deciphering the Secret Life of the Bacilli

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crucial role for exopolysaccharide modification in bacterial biofilm formation,

Mycobacterium tuberculosis through two stages of nonreplicating persistence.

It is believed that persistence of small populations of *Mycobacterium tuberculosis* in hosts underlies latent tuberculosis. Very little is known about the morphological and physiological nature of tubercle bacilli in latent TB. It is under discussion whether and how tubercle bacilli adapt to latent state and remain alive in face of damaging stressful conditions such as antibiotics and host immune factors. In this respect, cell wall deficiency (existence without rigid walls) in mycobacteria and its occurrence *in vivo* suggests one of the possible pathways by which tubercle bacilli can survive, replicate and persist within the body for a long period, harboring latent tuberculosis with a risk for disease reactivation, in case of reversion to classical TB bacilli upon changes in host immune status. Essentially, cell wall deficiency, or the ability of bacteria to exist as populations of self-replicating forms with defective or entirely missing cell walls (L-forms), is considered an adaptive strategy of bacteria to survive and reproduce under unfavorable circumstances.

This chapter elaborates on some special aspects of the L-form phenomenon and its importance for discovering new fundamental aspects of TB bacillary morphology and physiology, as well as understanding the mechanisms of latent tuberculosis.
