# Stringent Response in Mycobacteria: From Biology to Therapeutic Potential
## 1 Introduction  
Bacteria encounter constantly changing environments that may threaten their survival and existence; hence, it is particularly important to study their survival strategies in different model systems [1, 2, 3]. These strategies include several sensory mechanisms and signaling pathways that are required to overcome such threats [4, 5, 6, 7, 8, 9]. These mechanisms help bacteria to sense the environmental cues and generate an appropriate adaptive response. The adaptive response is usually multilayered and may affect some or all aspects of metabolism, replication, transcription, translation, and post-translational modifications in bacteria [5, 10, 11, 12, 13]. Hence, a prompt adaptation to such abrupt changes becomes a necessity for bacterial survival. The stringent response is one such evolutionarily conserved mechanism, through which bacteria can thrive in hostile conditions [13]. It is mediated through small molecules called alarmones, which include tetrapphosphate guanosine and pentapphosphate guanosine, collectively referred to as (p)ppGpp [14] (Figure 1). Stringent response affects all the central dogma processes--replication, transcription, and translation [15]. It helps bacteria survive the stress conditions by regulating important processes, such as biofilm formation, antibiotic resistance, persistence, and virulence in bacterial pathogens [15, 16, 17]. The mediators of stringent response, (p)ppGpp, were first identified in 1969 when the nucleotide extracts of the amino-acid-starved cultures of _Escherichia coli_ K-12 were resolved by thin-layer chromatography [18]. While native culture extracts showed two spots corresponding to ppGpp and ppGpp, these so-called \"magic spots\" were absent in the extracts from a mutant that had unregulated rRNA synthesis. Since this particular strain, a methionine auxotroph, then known as \"58-161\" mutant, exhibited the unregulated or relaxed synthesis of rRNA even during the amino acid starvation, the associated locus was called \"_relA_\" [19]. Classically, the stringent response has been associated with the synthesis of alarmones to stop rRNA production during amino acid starvation in _E. coli_[20; 21; 22; 23]. However, subsequent research has shown that several bacterial phyla, including actinobacteria, produce alarmones, (p)ppGpp, upon amino acid starvation [13; 16; 24; 25; 26; 27; 28]. Additionally, in recent years, a third alarmone, pGpp, has been discovered in several bacteria, which has further expanded the repertoire of stringent response [29; 30; 31; 32; 33; 34]. Thus, the three alarmones--pGpp, ppGpp, and ppPgpp--are now collectively referred to as (pp)pGpp and are mediators of stringent response in bacteria. Myobacteria comprise various obligate human pathogens such as _M. tuberculosis_ (Mtb), _M. leprae_, non-tuberculous species (NTM) pathogens, such as _M. chelonea_, _M. avium_, _M. fortutium_, and _M. abscessus_ and soil-saprophyte such as _M. singematis_ (Msm) [35]. Among these diverse mycobacterial species, _M. tuberculosis_ is a leading cause of mortality in humans as it causes tuberculosis (TB) [36]. The tubercle bacillus can not only infect but also persist within the host for several years. To establish the long-term infection, _M. tuberculosis_ employs stringent response as one of its tools [24]. TB treatment consists of at least six months of antibiotic therapy [37]. However, the treatment may last up to two years in the case of drug-resistant TB. Drug-resistant TB also poses a serious threat to public health due to its contagious nature and spectrum of drug-resistance [8; 38; 39; 40]. Hence, there is an urgent need to shorten the duration of TB treatment and contain the threat of drug-resistant TB. These two aims can be rapidly achieved by targeting the stringent response, which regulates persistence, drug resistance, and biofilm formation in several bacterial pathogens [16; 41; 42].