# Stringent Response in Mycobacteria: From Biology to Therapeutic Potential
## 5 Role of Stringent Response in Mycobacterial Drug Resistance  
Bacterial infections are treated through antibiotic therapy. However, the indiscriminate use of antibiotics has led to the rise of several drug-resistant bacteria, and mycobacteria are no exception. The rise of multidrug-resistant (MDR-TB) and extensively drug-resistant (XDR-TB) bacteria is now posing a serious threat to public health worldwide [84; 85; 86]. Stringent response has also been shown to regulate antibiotic resistance in different bacteria. _E. coli_ subjected to amino acid starvation show resistance to \(\beta\)-lactam antibiotics [87; 88; 89]. This was later corroborated when various strains of _E. coli_ defective in (p)ppGpp signaling were found to be sensitive to a wide range of antimicrobial compounds [90]. Furthermore, activation of the stringent response--and thus, an increase in (p)ppGpp levels--are associated with antibiotic tolerance in _Pseudomonas aeruginosa_[91]. We will discuss a few studies which show the relationship between stringent response, regulators of stringent response and antibiotic susceptibility in mycobacteria.  
Recently, it has been shown that the _M. tuberculosis_ strain deficient of Rel has altered metabolism and loses the ability to become quiescent [92]. Further, inhibitors targeting Rel not only kill _M. tuberculosis_ but also enhance the potency of isoniaid. These results show the importance of the stringent response in persistence, and its therapeutic importance to develop new drugs. The relationship between stringent response and antibiotic susceptibility in _M. snegmatis_ has been studied using high-throughput phenotype microarray technology [93; 41]. The phenotype microarray analyses showed that the \(\Delta\)_relMsm_ strain was resistant to multiple antibiotics compared to the parental mc\({}^{2}\)155 _M. snegmatis_. The results of phenotype microarray were subsequently verified by determining the minimum inhibitory concentration (MIC) of representative antibiotics using the broth microlithium assay. The \(\Delta\)_relMsm_ strain showed increased resistance to rifampicin in both MIC-based assay and phenotype microarray [94; 41]. However, the exact cause of increased resistance to rifampicin by \(\Delta\)_relMsm_ could not be deciphered. It was proposed that changes in cell wall lipid compositions of \(\Delta\)_relMsm_ might have hindered the uptake of rifampicin, thus contributing to increased rifampicin resistance [41]. Increased expression of genes encoding several multidrug resistance-associated proteins that encode catalase and superoxide dismutase was also proposed to be a possible cause of resistance shown by \(\Delta\)_relMsm_ to rifampicin and other antibiotics [65; 42] (Figure 2). Additionally, the qRT-PCR analysis of \(\Delta\)_relMsm_ showed down-regulation of porins, which might contribute to its multidrug resistance [65].  
Interestingly, the \(\Delta\)_relMsm_ strain is not a (p)ppGpp null mutant strain, as _M. snegmatis_ also encodes RelZ, a small alarmone synthase [47; 52]. To elucidate the role of RelZ, the SAS in _M. snegmatis_, the antibiotic sensitivity profile _relZ_ knockout was studied. Unlike the \(\Delta\)_relMsm_ strain, the \(\Delta\)_relZ_ strain was sensitive to several antibiotics such as bleomycin, offloxacin, and rifampicin. The double knockout strain \(\Delta\)_relMsm_\(\Delta\)_relZ_ showed an antibiotic sensitivity profile similar to that of the \(\Delta\)_relZ_ strain [30]. Thus, it appears that in _M. snegmatis_, the relationship between (pp)pGpp levels and antibiotic resistance is more complex than other bacteria. Moreover, the \(\Delta\)_relMsm_\(\Delta\)_relZ_ strain is also not a (p)ppGpp null mutant strain, as it is predicted to possess another (pp)pGpp synthase. Hence, to completely decipher the role of (pp)pGpp in antibiotic resistance in _M. smegmatis_, a (pp)pGpp null strain would be helpful.