2. The different repair times after exposure of TG1 E. coli to three doses of CIP (a: 10 μg/ml, b: 1 μg/ml, and c: 0.1 μg/ml) for 40 min are presented. Viability (%) is indicated

next to each repair time. Each dose is shown with its respective #AG-881 randurls[1|1|,|CHEM1|]# culture (above) in which the antibiotic was present during the incubation time. After exposure to the highest dose (10 μg/ml), all nucleoids were extremely fragmented, i.e., class IV. The DSB repair was limited and clearly noticeable only after 4 h; 82.5% of nucleoids were of class III after 5 h. Remarkably, all the nucleoids from the bacteria observed after 24 h showed massive fragmentation (class IV). Viability was very low after 0, 1.5, 3, and 4 h, and zero after 5 and 24 h (Fig. 5a). Immediately after incubating with the 1 μg/ml dose, all nucleoids were class IV. A higher EPZ015666 repair level was observed than after the highest dose, predominantly class III (58.7%) after 4 h, class I (41.0%) after 5 h, and class I (47.1%) after 24 h. Apparently repaired nucleoids without diffusing DNA fragments (10.2%) were visualized after 5 h, and this increased to 22.2% after 24 h. However, the viability was very low, as in the experiment with the highest dose (Fig. 5b). In contrast to the results at the higher doses,

repair activity was evident in the cultures exposed continuously to 0.1 μg/ml of CIP for the various times (Fig. 5c); 53.0% of nucleoids were class III after 4 h, and 31% were Amisulpride class I and 31% class 0 after 6 h. This latter time was assessed further in this experiment. The frequency of class 0 increased from 2.3% after 4 h to 67.3% after 24 h. In all cases, viability was very low or zero. Removing the drug resulted in faster repair kinetics, predominantly of class II (76.2%) after 1.5 h and class

0 (81.0%) after 5 h (Fig. 5c). The nucleoid pattern was similar to that of the untreated control cells after 24 h. Viability was initially very low, 2–4% after 4–6 h, and increased to 56.8% after 24 h (Fig. 5c). Thus, we found no clear relationship between the extent of repair of CIP-induced DNA breakage and cell viability. Evaluation of strains with known mechanisms of low sensitivity to CIP The other E. coli strains used have been described previously [16]. They include strains with one amino acid substitution mutation in GyrA (C-15), two substitution mutations in GyrA (1273), and two substitution mutations in GyrA and another two in ParC (1383). The more mutations, the greater the resistance level, as reflected in the MIC values (Table 2). We also evaluated a strain with a qnrA1 plasmid (J53 qnrA1) [17] (Table 2). Doses lower than the MIC never resulted in visible DNA fragments. Thus, in strains with a MIC of 0.