Effectiveness of fluoroquinolones against gram-positive bacteria.

Fluoroquinolones are broad-spectrum and therapeutically effective antibacterial agents that have retained high activity against methicillin-susceptible Staphylococcus aureus (MSSA) and streptococci. Primary targets of these agents in Gram-positive bacteria are DNA topoisomerase and DNA gyrase. High-level resistance is associated with at least two mutations in either target, or combinations of at least two mutations in multiple targets, particularly affecting the ParC subunit of topoisomerase IV and the GyrA subunit of DNA gyrase. This resistance may be enhanced when combined with mutations that increase fluoroquinolone efflux. Data from fluoroquinolone-resistant clinical isolates suggest that as yet unidentified mutations may also be involved in clinical resistance.