## 4 Discussion  
FQs are some of the few antibiotics that retain activity against non-growing bacteria [10; 23; 24], and their bactericidal activity derives from their binding to type II topoisomerases (DNA gyrase and topoisomerase IV in _E. coli_), which allows DNA cleavage but prevents ligation [26; 56]. DNA gyrase is involved in replication and transcription by introducing negative supercoils in front of the replication fork and RNA polymerase [27; 57; 58]. Topoisomerase IV is involved in decatenating chromosomes at the end of replication, resolving DNA knots during recombination, and alleviating the over-winding of the double helix [26; 59]. While nutrient starvation is known to stall DNA replication, transcription still continues at reduced rates [14; 20; 60; 61], which suggests that the targets of FQs remain corruptible in nutrient-starved populations. Indeed, it has been observed that 90% or more stationary-phase _E. coli_ can be killed by treatment with FQs [23; 24; 25; 34; 35; 36; 37]. Previous studies have investigated FQ persistence in stationary-phase _E.coli_ populations [23; 24; 25; 33; 34; 36; 37; 38; 39], and though not explicitly characterized, most were likely growth inhibited due to lack of C [40]. However, many nutrients, when missing, produce growth  
Figure 4: Importance of DNA repair machinery to MOXI persistence is largely invariant in non-growing populations starved for different nutrients. Persister levels in populations of \(\Delta\)_recA_, \(\Delta\)_recB_, _lexA3_, _AuvrD_, \(\Delta\)_nruA_, _ArecF_, _ArecN_ and WT under the indicated starvation conditions were assessed with MOXI. Data points indicate the means of three biological replicates, whereas the error bars indicate the standard errors of those means. One-way ANOVA with post-hoc Tukey tests were conducted for each starvation conditions to assess significance. (**A**,**B**) Under C and N starvation \(\Delta\)_recF_ exhibited significantly higher survival compared to WT at 1, 3, and 5 h. All other strains had significantly lower survival rates than WT at these time points. (**C**) Under P starvation all strains except _ArecF_ exhibited significantly lower survival rates when compared to WT at 1, 3, and 5 h time points. (**D**) _ArecA_, _ArecB_, _lexA3_, _AuvrD_ strains had a significantly lower survival rate compared to WT for Mg\({}^{2+}\) starvation at 1, 3, and 5 h. Significance was identified as \({}^{*}p<0.05\). arrest, and importantly, the physiology of growth-arrested bacteria will depend on the deprived nutrient [15; 16; 20; 62]. [PERSON] investigated persistence to ciprofloxacin (CIP) in _E. coli_ populations having been starved of N for 20 min compared to N-replete controls [62]. Incipient N starvation increased CIP persistence compared to growing controls and roles for RelA and NtrC were identified; however, the role of DNA repair machinery was not examined [62]. [PERSON] and [PERSON] investigated tolerance to CIP in _Salmonella enterica_ populations before and after exhaustion of limited supplies of Mg\({}^{2+}\)[13]. One hour after Mg\({}^{2+}\) exhaustion, CIP tolerance was observed to increase, and while the roles of (p)ppGpp and ATP in the phenomenon were evaluated, the importance of DNA repair was not assessed [13]. Studying persistence to a panel of antibiotics, including LEVO, [PERSON] and colleagues found that 30 min after resuspension in saline, the tolerance of _Staphylococcus aureus_ cultures increased dramatically [63]. Intriguingly, supplementation of saline with Mg\({}^{2+}\) reduced the enhancement in LEVO tolerance through a pathway that involved ATP, whereas the role of DNA repair was not evaluated [63]. [PERSON] and colleagues also used saline to examine antibiotic tolerance in starved cultures, where they observed a role for proton motive force in \(\beta\)-lactam susceptibility that was absent when cultures were treated with CIP [64].