Antibiotic Resistance Patterns and Molecular Determinants of Fluoroquinolone Resistance Among Urinary Escherichia coli Isolates – A Comparison of Disc Diffusion and MIC Methods

Open Access

Abstract

Background: Urinary tract infections remain one of the major public health concerns worldwide, for which Escherichia coli (E. coli) is the predominant causative organism. The study aims to assess the resistance patterns of E. coli in urinary, identify fluoroquinolone-associated gyrA mutations, and evaluate their association with phenotypic resistance profiles. Methods & Materials: This cross-sectional study was conducted at Sir Salimullah Medical College and Mitford Hospital, Dhaka, from January to December 2020. 229 urine samples collected by purposive sampling were analyzed for this study. Identification of the bacterial isolates was performed by standard biochemical tests; antimicrobial susceptibility testing was done according to CLSI guidelines by disc diffusion and agar dilution methods to determine the MIC. Detection of mutations in the gyrA gene that confer resistance to quinolones was performed with the use of the PCR-RFLP methodology, with HinfI restriction enzyme digestion targeting codons 83 and 87. Results: Among the 138 culture-positive samples, E. coli constituted 81.8% of the total isolates. The overall fluoroquinolone resistance rate was 52.2% as determined by the disc diffusion method. The ciprofloxacin MICs were ≥4 µg/ml in 43.4% of the isolates as determined by MIC testing. Molecular analysis detected gyrA mutations in 55.7% of all E. coli isolates. 37.1% mutations were shown at both Ser83 and Asp87 codons. 92.1% of isolates with gyrA mutations showed a phenotypic ciprofloxacin resistance that established a strong genotype-phenotype correlation. Only double mutations were associated with higher MIC values (≥8 µg/ml) and a complete resistance phenotype, while single mutations were associated with lower MIC ranges (0.25-4 µg/ml). Conclusion: This study documents high fluoroquinolone resistance rates among urinary E. coli isolates, largely driven by double mutations in the gyrA gene. These findings highlight the clinical relevance of molecular surveillance in guiding empirical treatment decisions due to its close association with resistance phenotypes.