Molecular Investigation of gyrA Mutations in Clinical Isolates of Methicillin-Resistant Staphylococcus aureus Derived from Diverse Sources

Abstract

Background: Fluoroquinolones are the most effective antibiotics against Staphylococcus aureus isolates. In hospitals, excessive use of antibiotics has led to the emergence of highly resistant strains of S. aureus isolates. Objective: The aim of this study was to detect the mutations that occur in the gyrA gene encoding for DNA gyrase, which is one of the targets for fluoroquinolone resistance. Methods: Fifty clinical isolates were diagnosed as S. aureus according to molecular and bacteriological methods. The susceptibility tests were performed on all bacterial isolates by the disc diffusion method using methicillin and six fluoroquinolone antibiotics. Results: Out of fifty isolates, twelve were resistant to methicillin and all six antibiotics (nalidixic acid, lomefloxacin, ciprofloxacin, norfloxacin, ofloxacin, and levofloxacin). From the fifty isolates, 12 were resistant, 3 were intermediate, and 38 were sensitive to three or more tested antibiotics. The resistance of S. aureus isolates was also confirmed by the minimum inhibitory concentration test. The main sources of isolates were burns (10%), nose (16) wounds (8%), operation room (10%), ear (20%), urine (8%), skin (6%), and throat (22%). Twelve resistant isolates were used to examine the mutations in the gyrA gene. A direct sequence analysis found eight mutations in the gyrA gene; these mutations included 2 (25% missense mutations), 1 (12.5%) deletion mutation, and 5 (62.5%) silent mutations at various sites. Conclusion: gyrA mutations resulting from the excessive use of antibiotics may be one of the mechanisms leading to fluoroquinolone resistance.