## 6 Stringent Response and Toxin/Antitoxin Systems  
Stringent response and the so-called toxin/antitoxin systems are bacterial systems involved in bacterial stress response [138; 139].  
Stringent response is a bacterial mechanism involved in a series of adaptations in response to different stress situations, such as nutrient or iron deprivation, oxidative stress, or others [140; 141]. Meanwhile, toxin/antitoxin systems encode both a toxin able to interfere with different metabolic pathways leading to cell death or arresting cell growth and an antitoxin that inhibits the action of the toxin component [139]. To date, up to 8 T/A systems have been described, classified based on the mechanism of action of antitoxin component [141].  
Different proposals about how these metabolic routes drive towards dormant and quiescent bacteria have been made, including interdependent and independent routes of stringent response and T/A systems (Figure 2). Thus, while several authors propose that stringent response and T/A systems are two independent routes, it has also been proposed that the action of stringent response results in increased production of the Lon protein which, in turn, proceeds with the degradation of the antitoxin, allowing the toxin to exert its action, or the inverse scenario in which the inactivation of the antitoxin starts a series of metabolic processes leading to the overproduction of RelA, resulting in the activation of stringent response [138; 142].  
Stringent response is related to the action of a series of genes, including the _relA_, _spoT_ and _dksA_ genes [140; 141]. The induction of stringent response results in the synthesis of the alarmones ppGpp or ppPGpp (collectively referred to as (p)ppGpp) [143]. The impact of this mechanism on survival has been highlighted in studies in which mutants with impaired (p)ppGpp synthesis showed decreased survival in the presence of quinolones and lower MBC/MIC compared with parental isolates, while those with impaired hydrolysis of (p)ppGpp showed an opposite scenario [144].  
It has been confirmed that a series of toxins from T/A systems, such as ParE toxin (not to be confounded with ParE, a subunit of Topoisomerase IV) or TiSb from ParDE or TiSb-IstR-1, are released during ciprofloxacin exposure, favoring bacterial survival [145; 146]. In the case of TiSb, this finding is directly related to the presence of a Lex-box in its promoter region, which is activated by the SOS system that, in turn, is activated by DNA lesions related to the action of the quinolone [145]. On the other hand, increased levels of Lon have been claimed as the reason for the release of ParE [147]. Of note, there is currently controversy about the real role of T/A systems in the development of dormant cells [148].  
Figure 2: Proposed model of independent action of stringent and toxin/antitoxin systems. TA: Toxin/antitoxin system; QS: Quorum sensing; SS: Secretion systems; ROS. Reactive Oxygen Species; T3 SS: Type 3 secretion systems. (**A**) Molecular mechanisms underlying bacterial persistence. (**B**) Different models explaining the involvement of (p)ppGpp in persistence and representative publications (see reference [135]). (**B**) red: T/A systems are activated by (p)ppGpp. grey: T/A systems induce quiescence independently; light green: (p)ppGpp protects against oxidative stress, favoring the development of persister cells; dark green: (p)ppGpp induces dimerization of ribosomes, which subsequently lead to persistence. References presents in the figure may be found at [138]. Reproduced from reference [138], with the permission of Elsevier.