Contribution of Oxidative Damage to Antimicrobial Lethality

Abstract

A potential pathway linking hydroxyl radicals to antimicrobial lethality was examined by using mutational and chemical perturbations of Escherichia coli. Deficiencies of sodA or sodB had no effect on norfloxacin lethality; however, the absence of both genes together reduced lethal activity, consistent with rapid conversion of excessive superoxide to hydrogen peroxide contributing to quinolone lethality. Norfloxacin was more lethal with a mutant deficient in katG than with its isogenic parent, suggesting that detoxification of peroxide to water normally reduces quinolone lethality. An iron chelator (bipyridyl) and a hydroxyl radical scavenger (thiourea) reduced the lethal activity of norfloxacin, indicating that norfloxacin-stimulated accumulation of peroxide affects lethal activity via hydroxyl radicals generated through the Fenton reaction. Ampicillin and kanamycin, antibacterials unrelated to fluoroquinolones, displayed behavior similar to that of norfloxacin except that these two agents showed hyperlethality with an ahpC (alkyl hydroperoxide reductase) mutant rather than with a katG mutant. Collectively, these data are consistent with antimicrobial stress increasing the production of superoxide, which then undergoes dismutation to peroxide, from which a highly toxic hydroxyl radical is generated. Hydroxyl radicals then enhance antimicrobial lethality, as suggested by earlier work. Such findings indicate that oxidative stress networks may provide targets for antimicrobial potentiation.Antimicrobials that actively kill pathogens are expected to cure disease more rapidly and restrict the emergence of resistance better than agents that are largely bacteriostatic. Consequently, considerable effort has been devoted to understanding mechanisms of lethal action. In general, antimicrobials are thought to kill microbes through interaction with specific intracellular targets, followed by corruption of particular cellular processes (22). However, Collins and associates recently proposed that many bactericidal antimicrobials also share a common lethal pathway that involves the generation/accumulation of hydroxyl radicals (13,23). This hypothesis is supported by elevated hydroxyl radical levels associated with lethal antimicrobial treatment (23). How this occurs is largely unknown.Three major reactive oxygen species, superoxide, hydrogen peroxide, and hydroxyl radical, are generated as by-products of normal aerobic respiration (17,20). All three are cytotoxic, but they display different kinetics and levels of severity. For example, the effects of superoxide and hydrogen peroxide are probably less acute than those of hydroxyl radicals, since both superoxide and hydrogen peroxide can be detoxified by induced scavenging enzymes. In contrast, no enzyme can detoxify hydroxyl radicals, making them extremely toxic and acutely lethal. Hydroxyl radicals derive from hydrogen peroxide through the Fenton reaction (14), which makes the regulation of the intracellular peroxide concentration a starting point for exploring genetic pathway...

Antibiotics induce redox-related physiological alterations as part of their lethality

“…S8A) (66). These results complement independent observations by others that deficiencies in KatG or AhpC increase killing by ampicillin, kanamycin, and norfloxacin (15).…”

Role of PknB Kinase in Antibiotic Resistance and Virulence in Community-Acquired Methicillin-Resistant Staphylococcus aureus Strain USA300

“…Since the pknB mutant did not exhibit any gross morphological defects in the cell wall that may have been the underlying cause of its hypersusceptibility toward ␤-lactams, we next sought alternative explanations for the observed phenotype. Recent literature has demonstrated that bactericidal antibiotics, such as ␤-lactams, exert their killing effects through the production of reactive oxygen species (29). Thus, we investigated the potential involvement of the kinase in the stress response by examining the activity of its primary mediator, the alternative sigma factor SigB, in the pknB mutant.…”

Eradication of bacterial persisters with antibiotic-generated hydroxyl radicals

“…There is a growing body of evidence demonstrating the important role that ROS play in bactericidal antibiotic-mediated killing (24)(25)(26). This understanding of how bactericidal antibiotics result in cell death raises the hypothesis that drug tolerance may be mediated by increased abilities within a cell to detoxify ROS.…”

“…These studies demonstrate that bactericidal antibiotics with a variety of different mechanisms of action increase ROS production within cells via the Fenton reaction (24)(25)(26)(27). Numerous ROS, and in particular hydroxyl radicals, are toxic to cells and can result in cell death.…”