Quinolone-Mediated Bacterial Death

Abstract

2The fluoroquinolones are broad-spectrum antibacterial agents that are becoming increasingly popular as bacterial resistance erodes the effectiveness of other agents (fluoroquinolone sales accounted for 18% of the antibacterial market in 2006) (41). One of the attractive features of the quinolones is their ability to kill bacteria rapidly, an ability that differs widely among the various derivatives. For example, quinolones differ in rate and extent of killing, in the need for aerobic metabolism to kill cells, and in the effect of protein synthesis inhibitors on quinolone lethality. Understanding the mechanisms underlying these differences could lead to new ways for identifying the most bactericidal quinolone derivatives.Before describing the types of damage caused by the quinolones, it is useful to define lethal activity. Operationally, it is the ability of drug treatment to reduce the number of viable cells, usually measured as CFU on drug-free agar after treatment. This assay is distinct from measurements that detect inhibition of growth (e.g., MIC), since with the latter bacteria are exposed to drug throughout the measurement. The distinction between killing and blocking growth is important because it allows susceptibility determinations to be related to particular biological processes. For example, inhibition of growth is typically reversed by the removal of drug, while cell death is not. Thus, biochemical events associated with blocking growth should be readily reversible, while those responsible for cell death should be difficult to reverse. Reversibility can be used to distinguish among quinolone derivatives and assign functions to particular aspects of drug structure. Moreover, protective functions, such as repair and stress responses, can be distinguished by whether their absence affects inhibition of growth, killing, or both.The intracellular targets of the quinolones are two DNA topoisomerases: gyrase and topoisomerase IV. Gyrase tends to be the primary target in gram-negative bacteria, while topoisomerase IV is preferentially inhibited by most quinolones in gram-positive organisms (28). Both enzymes use a doublestrand DNA passage mechanism, and it is likely that quinolone biochemistry is similar for both. However, physiological differences between the enzymes exist, some of which may bear on quinolone lethality.In the present minireview we consider cell death through a two-part "poison" hypothesis in which the quinolones form reversible drug-topoisomerase-DNA complexes that subsequently lead to several types of irreversible (lethal) damage. Other consequences of quinolone treatment, such as depletion of gyrase and topoisomerase IV activity, are probably less immediate (42). To provide a framework for considering quinolone lethality, we begin by briefly describing the drug-topoisomerase-DNA complexes. Readers interested in a more comprehensive discussion of quinolones are referred to a previously published work (28). QUINOLONE-TOPOISOMERASE-DNA COMPLEXESAs a normal part of their reaction mechanism, g...

Organization of Ribosomes and Nucleoids in Escherichia coli Cells during Growth and in Quiescence

“…The nucleoids were disrupted into fragments and randomly scattered suggesting topological disruption and possibly double-strand breaks. We have further checked whether the nalidixic acid-mediated damages could be suppressed by blocking protein synthesis as suggested in a previous work (41). When chloramphenicol was added together with nalidixic acid, filamentation was visibly reduced, DNA break was not obvious, and both the nucleoids and the ribosomes acquired typical chloramphenicol phenotype without any empty space in the cell poles (Fig.…”

“…Eventually longer exposure (120 min) led to disruption of the cell structure as ribosomes and DNA leaked out from the cell. Effect of Exposure to Gyrase Inhibitors-Nalidixic acid belongs to a class of broad-spectrum antibacterial agents called quinolones, which blocks DNA replication by specifically targeting DNA gyrases, induces double-strand DNA breaks, leads to elongated, filamentous cells, and kills cells in a protein synthesis-dependent pathway (40,41). Exposure to nalidixic acid (Fig.…”

The epidemiology, pathogenesis, transmission, diagnosis, and management of multidrug-resistant, extensively drug-resistant, and incurable tuberculosis

“…However, their bactericidal effect is mediated through subsequent chromosomal fragmen tation and generation of reactive oxygen species, as well as at least one other poorly characterised mechanism. 338 Available fluoroquinolones differ in their MIC against M tuberculosis, with the late-generation fluoroquinolones moxifloxacin and gatifloxacin being most potent, followed by levofloxacin and then ofloxacin, but clinical trials comparing fluoroquinolones are few in number and narrow in scope. Ofloxacin has been shown to be less efficacious than the others and should be abandoned for tuberculosis therapy.…”

Antibiotic treatment enhances the genome-wide mutation rate of target cells

“…Gyrases bound with fluoroquinolone molecules result in crosslinked protein-DNA complexes containing broken DNA that induces the SOS response (18)(19)(20). Three error-prone DNA polymerases are expressed in Escherichia coli during the SOS response, substantially elevating the mutation rate (21,22).…”