Our result suggests that the DNA fragments liberated from the nucleoid are of fairly regular size and that more fragments are released as the CIP dose increases. It also supports the possibility of clusters of preferential DNA gyrase Screening Library cleavage sites [19]. It is possible that doses smaller than the MIC could induce a small amount of DSBs, which

could be spaced widely in the different domains but not cause spreading of the fragments. In our previous report, a CIP dose of 0.012 μg/ml produced slightly more damage than in the present study [15]. This is probably because of the harsher lysing conditions in our previous study, which may have caused additional DNA damage. This was corrected in the conditions used in our current study. Adding 1 μg/ml of CIP to TG1 in LB broth and instantaneous processing using our technique produced just barely detectable DNA fragmentation. Taking TG1 from LB agar reduces the extent of damage. DNA damage increases progressively with incubation time, and a 30 min incubation is needed to achieve the maximum level of DNA fragmentation. Remarkably, when the bacteria came from exponentially growing cultures in LB broth, the highest DNA fragmentation level was observed immediately (0 min). These results suggest that the CIP effect

on DNA is cumulative with time and that its veloCity is dependent on the growing conditions. We confirmed this hypothesis by analyzing aliquots removed periodically from a batch culture incubated with 1 μg/ml CIP for 0 and 5 min. The DNA fragmentation level declined progressively as the bacteria proceed into the stationary phase. Most E. coli cells divide uniformly in BGB324 purchase exponentially growing cultures but stop dividing when they achieve the stationary phase

[21]. Bacteria grown in LB agar should be heterogeneous with regard to the growing phase, both exponential and stationary. The MIC is an average of the bacterial sensitivity to the antibiotic, which reflects the different effect of CIP on DNA. The DNA fragmentation yield is homogeneous among the nucleoids in exponentially growing TG1 but is slower and tends to be more heterogeneous in the stationary phase. This greater heterogeneity was evident after short incubation with 1 μg/ml CIP but tended to be homogeneous after 40 min of treatment. Pulse field gel electrophoresis shows that the norfloxacin-induced Rho fragmentation in E. coli nucleoids is low in the stationary phase of growth [20]. This phenomenon could reflect decreased drug uptake, increased drug efflux, downregulation of topoisomerases, or a more tightly packed nucleoid structure as demonstrated by atomic force microscopy [22]. Using our procedure, we have also observed more compacted nucleoids in the stationary phase. The most probable explanation is the activation of multidrug transporters that exclude fluoroquinolones, which is mediated by quorum-sensing signals. In fact, the quorum-sensing transcription factor SdiA from E.