Antimicrobial resistance is a growing threat, causing upwards of 700,000 deaths globally per year. The clinical use of the widely prescribed fluoroquinolone (FQ) class of antibacterials has been undermined by this rise in resistance, making it critical to identify new agents that maintain activity against FQ-resistant strains. Mutations in gyrase and topoisomerase IV, the targets of FQs, are the most common form of resistance to this important antibacterial class. This has led to the development of the “novel bacterial topoisomerase inhibitor” (NBTI) class of antibacterials. Like FQs, NBTIs target gyrase and topoisomerase IV, but interact differently with these validated enzyme targets.
Gyrase and topoisomerase IV regulate chromosomal superhelicity and unknot and detangle the genome, respectively, by passing a segment of DNA through a transient double-stranded break that they create in a separate segment. Although FQs and NBTIs both stabilize enzyme-DNA cleavage complexes, FQs enhance primarily double-stranded DNA cleavage, while NBTIs enhance only single-stranded DNA cleavage. NBTIs also inhibit the overall catalytic activities of gyrase (supercoiling) and topoisomerase IV (decatenation). Thus, NBTIs may kill bacteria in two different ways: fragmention of the genome or alteration of chromosomal dynamics.
Despite the clinical promise of NBTIs, little is known about their mechanism of action. Our studies on the NBTI GSK126 indicate that the compound displays potent activity against gyrase from Mycobacterium tuberculosis and gyrase and topoisomerase IV from Bacillus anthracis and Escherichia coli. GSK126 enhances only single-stranded DNA scission mediated by these enzymes and suppresses the generation of double-stranded breaks. The NBTI also inhibits the catalytic activity of these enzymes. However, in the presence of ATP, GSK126 is unable to enhance DNA cleavage of E. coli gyrase and topoisomerase IV. GSK126 maintains activity against the most common clinically observed FQ resistance mutations in B. anthracis and M. tuberculosis, but not E. coli. These data suggest that GSK126 kills E. coli by robbing cells of the catalytic activities of gyrase and topoisomerase IV, whereas in other species it may act by fragmenting the genome. Thus, this study suggests that NBTIs elicit their antibacterial effects by two different mechanisms that differ in importance from species to species and cannot be generalized, an important consideration for future uses of the class.

Alexandria Oviatt

Ph.D. Candidate, Vanderbilt University

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