Interplay in the selection of fluoroquinolone resistance and bacterial fitness

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Fluoroquinolones are critical antibacterial agents targeting DNA Gyrase and Topoisomerase IV.
• Bacterial resistance to fluoroquinolones often involves multiple mutations affecting drug targets and efflux pumps.
• Some resistance mutations can negatively impact bacterial fitness, driving the evolution of compensatory mutations.

Purpose of the Study:

• To investigate the impact of accumulating fluoroquinolone-resistance mutations on bacterial fitness.
• To identify specific mutations that confer both increased fitness and reduced drug susceptibility.
• To understand the evolutionary dynamics of fluoroquinolone resistance in the absence of the drug.

Main Methods:

• Construction and analysis of isogenic E. coli strains with up to five fluoroquinolone-resistance mutations.
• In vitro and in vivo fitness assessments of mutant strains.
• Measurement of DNA supercoiling and gyrA promoter activity.
• Selection and competition experiments in drug-free environments.

Main Results:

• Several fluoroquinolone-resistance mutations significantly reduced bacterial fitness.
• In low-fitness strains, a fourth resistance mutation restored fitness both in vitro and in vivo, concurrently decreasing drug susceptibility.
• A parC mutation in a gyrA/marR resistant strain notably enhanced fitness and reduced susceptibility, linked to altered gyrA promoter activity.
• Selection experiments demonstrated the emergence of fitter, less susceptible parC mutants without drug pressure.

Conclusions:

• Compensatory mutations can restore fitness lost due to resistance development.
• Natural selection can favor increased fluoroquinolone resistance and reduced susceptibility, even in the absence of the antibiotic.
• Understanding these evolutionary mechanisms is crucial for predicting and combating antimicrobial resistance.