**2.1 Persisters**

In addition to antibiotic resistance other mechanisms allow bacterial growth in the presence of antibiotics; (i) population wide tolerance, (ii) persisters, subpopulations characterized by a transient dormant state and transient tolerance [17] and (iii) shielding that protects and enables survival in the presence of antibiotics [18].

Persisters and antimicrobial tolerance have been extensively studied in *E. coli*. One of the first and most thoroughly investigated examples of persister cell formation involving the SOS system, is activation via the toxin-antitoxin TisB/IstR module. TisB is a small membrane-acting peptide that decreases the proton motive force and ATP levels, shutting down cell metabolism and inducing dormancy [19]. The *tisB* gene is repressed by the SOS repressor LexA, while the IstR-1 antitoxin is constitutively expressed. Following DNA damage and SOS induction, *tisB* transcription strongly increases and exceeds that of the antitoxin IstR-1 [20].

Nevertheless, in *E. coli,* the SOS response in persisters also accelerates antibiotic resistance [21, 22]. Thus, from fluoroquinolone (FQ ) persisters, the SOS response promotes resistance to unrelated antibiotics following a single FQ exposure [23].

Recently, sub-inhibitory concentrations of ciprofloxacin were shown to, in *E. coli*, induce transient differentiation of a small gambler subpopulation that, generates cross-resistant mutants. Gamblers are characterized by high levels of ROS and a σ<sup>S</sup> general stress-response. In gamblers, ROS activate the σ<sup>S</sup> response, which allows mutagenic repair of antibiotic-triggered DNA double strand breaks. Further required is SOS induced inhibition of cell division, provoking the presence of multiple chromosomes. Thus, in gamblers, a highly regulated, transient differentiation process with within-cell chromosome cooperation drives evolution of resistance to new antibiotics [24].
