Increased Intracellular Hydrogen Peroxide Observed in Escherichia coli in the Presence of Two Classes of Antibiotics

The modes of action of antibiotics on their target cells are well established. It has been shown previously that bactericidal classes of antibiotics stimulate hydroxyl radical production in cells as a result of the oxidative damage cellular pathway, in addition to their specific drug-target interactions; however, the production of reactive oxygen species (ROS) as a general cell death mechanism is a point of contention. This study aimed to investigate the effects of norfloxacin and tobramycin, two classes of bactericidal antibiotics, on growth rates and intracellular H O production of Escherichia coli. Three E. coli strains, BW25113 2 2 (the wild type), JW3914-1, and JW1721-1 ( ∆ katG and ∆ katE mutants respectively), were exposed to H O. Both mutants had a depressed growth rate compared to the wild type, 2 2 although growth was further reduced in ∆ katG, which is consistent with the role of KatG as the main catalase responsible for scavenging H O during log phase growth. When exposed 2 2 to sub-inhibitory levels of tobramycin and norfloxacin, results indicated that there was no difference between the growth of BW25113 or ∆ katG. A reporter system consisting of BW25113 transformed with a plasmid containing katG::luxCDABE was used to establish if norfloxacin and tobramycin indirectly induced the production of H O. Luminescence was 2 2 greater in the presence of tobramycin compared to norfloxacin, although both antibiotics did produce a signal. This experiment therefore supports the hypothesis that bactericidal antibiotics elicit the production of ROS, as indicated by the results of our lux reporter system.