Lethal action of quinolones against a temperature-sensitive dnaB replication mutant of Escherichia coli

Abstract

Inhibition of DNA replication in an Escherichia coli dnaB-22 mutant failed to block quinolone-mediated lethality. Inhibition of protein synthesis by chloramphenicol inhibited nalidixic acid lethality and, to a lesser extent, ciprofloxacin lethality in both dnaB-22 and wild-type cells. Thus, major features of quinolone-mediated lethality do not depend on ongoing replication.

FIG. 1.

Bactericidal action of nalidixic acid with a dnaB-22 replication mutant. (A) Effect of nalidixic acid treatment time. E. coli strains K12SH-28 (wild type [wt]) and FA22 (dnaB-22) were grown at 28°C, and part of the culture was shifted to 42°C (wild type, triangles; dnaB mutant, inverted triangles). After 15 min, aliquots of K12SH-28 (circles) and FA22 (squares) at 28°C (open symbols) and 42°C (filled symbols) were treated for the indicated times with 60 mg/liter nalidixic acid, after which cells were plated on drug-free agar to determine percent survival. Similar results were obtained in a replicate experiment. (B) Effect of nalidixic acid concentration. The E. coli strains used in panel A were treated with the indicated concentrations of nalidixic acid for 105 min at 28°C (open symbols) or following a shift to 42°C (filled symbols) for strain K12SH-28 (wild type; circles), FA22 (dnaB-22; squares), and KD2672 (dnaB-22 gyrA S83L D87Y; diamonds). Similar results were obtained in a replicate experiment.

FIG. 2.

Effect of chloramphenicol on quinolone-mediated lethality with a dnaB-22 mutant. (A) Effect of chloramphenicol on survival in the presence of nalidixic acid. Cells were grown as in Fig. 1 at 28°C and shifted to 42°C for 5 min, after which zero (filled symbols) or 10 mg/liter chloramphenicol (open symbols) was added for an additional 10 min followed by the indicated concentration of nalidixic acid for 105 min. The strains were DM4100 (wild type [wt]; circles) and KD2773 (dnaB-22; squares). Similar results were obtained in a replicate experiment. (B) Effect of chloramphenicol on survival in the presence of ciprofloxacin. Cells were grown as in Fig. 1 at 28°C and shifted to 42°C for 5 min, after which zero (filled symbols) or 10 mg/liter chloramphenicol (open symbols) was added for an additional 10 min followed by the indicated concentration of ciprofloxacin for 30 min. The strains were DM4100 (wild type; circles) and KD2773 (dnaB-22; squares). Similar results were obtained in a replicate experiment.