# Stringent Response in Mycobacteria: From Biology to Therapeutic Potential
## 4 Stringent Response Regulates Mycobacterial Virulence  
Most of the TB infections remain asymptomatic as _M. tuberculosis_ can successfully establish a latent chronic infection. It has been shown that the virulence of _M. tuberculosis_ is also regulated by the stringent response [65; 77]. The H37 Rv\(\Delta rel_{Mtb}\) mutant can establish an infection in the mice model and during the first few weeks post-infection, its growth is indistinguishable from the parental H37 Rv strain. However, after five weeks of infection, the viability of the mutant starts dropping, and four months post-infection, the bacterial load in both lungs and spleen is 500-fold lower than the parental _M. tuberculosis_ H37 Rv [65]. The lungs of mice infected with the parental strain showed the presence of several granulomas, which covered almost one-third of lung tissue. In contrast, lungs infected with H37 Rv\(\Delta rel_{Mtb}\) strain showed significantly fewer granulomas, and almost normal lung architecture [65]. Thus, stringent response is necessary for the maintenance of chronic _M. tuberculosis_ infection. Transcriptomic analysis through microarray showed that several genes associated with mycobacterial virulence and antigens were differentially expressed in H37 Rv\(\Delta rel_{Mtb}\) mutant compared to the parental H37 Rv strain [65]. These included _groEL2_ and _groES_, the 19-kDa antigen LpqH, and members of the _PE_PGRS_ family. Secreted antigens such as _esst6_, the antigen 85 complex, _mpt83_, and _cfp7_ were also found to be differentially expressed in H37 Rv\(\Delta rel_{Mtb}\) mutant. The expression of lipoprotein LpqH decreases in H37 Rv\(\Delta rel_{Mtb}\) mutant. LpqH is known to inhibit cytokine secretion, decrease the antigen presentation to macrophage and promote macrophage apoptosis [78] (Figure 2). The PE_PGRS proteins modulate antigenic variation in clinical isolates of _M. tuberculosis_[79; 80]. Moreover, these proteins also contribute to the survival of _M. tuberculosis_ in granuloma [81]. Recently, it was shown that PE_PGRS3, localized on the mycobacterial cell surface, is expressed during phosphate limitation, a condition which also triggers the stringent response in _M. tuberculosis_ and _M. smegmatis_[82]. The C-terminal of PE_PGRS3 is arginine-rich and, hence, positively charged. This helps _M. tuberculosis_ to establish contact with negatively charged phospholipids on the host cell membrane. The expression of PE_PGRS3, which is dependent on (p)ppGpp, is needed for interaction between _M. tuberculosis_ and the host cell, and to eventually establish the infection [83]. Thus, at the molecular level, stringent  
Figure 2: Pathways and genes affected by alarmones (pp)pGpp in mycobacteria: Alarmones (pp)pGpp (orange square) and associated Rel proteins regulate several processes in mycobacteria. Regulation of these processes results in the alteration of specific genes and their cognate pathways. The figure shows four major schemes—(i) virulence and pathogenicity, (ii) antibiotic resistance, (iii) stress, and (iv) cell envelope-related processes. The specific pathways and genes under these schemes have been depicted. response regulates mycobacterial persistence inside the host by modulating the expression of important genes involved in virulence and antigen presentation. The effect of stringent response on mycobacterial survival has also been assessed in guinea pigs, as the granulomas in guinea pigs resemble those found in humans in terms of architecture, composition, and caseation necrosis [77]. There was a reduced burden in the lungs of guinea pigs infected with H37 Rv\(\Delta\)_relMth_ strain compared with the parental H37 Rv strain, which indicated that impairment of the stringent response reduces _M. tuberculosis_ survival during infection in lungs. Moreover, the guinea pig lungs infected with the H37 Rv\(\Delta\)_relMth_ strain showed considerably fewer tubercle lesions and caseous granulomas. Thus, in the absence of the stringent response, _M.