## 6 Stringent Response and Toxin/Antitoxin Systems
### Sporulation and Quiescence  
The abovementioned mechanisms led to the development of dormant bacteria (Figure 3). Sporulation and quiescence are conceptually related phenomena leading to long-term inactive or almost inactive microorganisms. Two notes: bacteria may remain viable for years in spore or quiescent states [149], and it should be considered that antibiotic susceptibility is restored when they are reactivated.  
Sporulation is a bacterial mechanism of bacterial survival in extreme conditions in which the bacteria suffer a series of morphological alterations and enter into a truly dormant state, in which no metabolic process takes place until environmental conditions are more favorable [149]. Meanwhile, quiescence is another bacterial response to environmental stress. Quiescent microorganisms are in a viable non-replicating state, but in contrast to spores, quiescent bacteria display a basal metabolic process with no morphological alterations [149]. As mentioned previously, the quiescent state is also present within biofilm communities in bacteria living in the most internal biofilm layers [114].  
DNA Gyrase and Topoisomerase IV, both classified as Type II Topoisomerases, are actively involved in different DNA processes, with the introduction of negative supercoiling and DNA decatenation of replication products as the most relevant and well established [55; 150]. The quinolones need Topoisomerase activity to interact with its targets in order to block the DNA replication forks, generating cleaved DNA-Type II Topoisomerase complexes, and subsequently inducing rapid bacterial death through different protein synthesis-dependent or -independent pathways, with the SOS system possibly being involved in slow bacterial death [136; 137].  
Figure 3: Models of development of persister cells. Note that while all pathways are represented as independent routes, different interactions among them may be present. Reproduced from reference [142], published under a Creative Commons license.  
In agreement, a series of studies have reported negative relations between bacteriostatic ribosomal-targeting antibacterial agents and different quinolones, ranging from indifference tending to antagonism, to declared antagonism leading to suppressive effects [131; 136; 151]. A related and paradoxical effect has been described in which bacterial survival at high nalidixic acid concentrations was higher than when the concentrations did not reach these values. This was explained by the bactericidal action of nalidixic acid being modified to bacteriostatic action, inducing a metabolic block of bacterial activity leading to a bacterial quiescent state [131]. [PERSON] et al. [131] also showed that after exposure to >200 mg/L of nalidixic acid, the surviving _E. coli_ K16 did not show higher nalidixic acid MIC than those at baseline. In accordance with this scenario, full bacterial dormancy in the sporulate and the almost inactive quiescent states allow bacteria to remain unaffected by the presence of quinolones, displaying higher resistance levels and increasing bacterial survival in the presence of these antimicrobial agents and, in summary, escaping the action of quinolones irrespective of their MIC to quinolones when reactivated.