Protocol for Agar Dilution Checkerboard Analysis of Fosfomycin-Based Antibiotic Combinations Against...

Due to the limited treatment options for carbapenem-resistant Enterobacterales, combination therapy regimens have gained increasing importance. This study aimed to investigate fosfomycin resistance rates and evaluate the in vitro synergistic interactions of fosfomycin in combination with meropenem, gentamicin, and ceftriaxone against carbapenem-resistant Escherichia coli isolates. Sixty carbapenem-resistant E. coli strains isolated from various clinical samples (urine, blood, aspirate, wound, cerebrospinal fluid, endotracheal aspirate) obtained from patients in clinical wards and intensive care units of Dicle University Medical Faculty Hospitals were included in the study. Species identification was performed using an automated microbial identification system. Antibiotic susceptibility testing and confirmation of carbapenem resistance were performed by disc diffusion with ertapenem, imipenem, and meropenem. Fosfomycin resistance and interactions among antibiotics were assessed by agar dilution. In addition, antibiotic combinations were evaluated across serial two-fold concentration gradients using the agar dilution-based protocol, and fractional inhibitory concentration indices (FICI) were calculated to determine synergy, indifference, or antagonism. Resistance rates were 76.6% for fosfomycin, 61.6% for meropenem, 85.0% for gentamicin, and 98.3% for ceftriaxone. Synergistic interactions were observed in 35% of isolates for the fosfomycin-meropenem combination, 33.4% for the fosfomycin-gentamicin combination, and 1.7% for the fosfomycin-ceftriaxone combination. Antagonism was detected in only one isolate in the fosfomycin-meropenem combination. Combinations of fosfomycin with meropenem and gentamicin showed in vitro synergistic activity against carbapenem-resistant E. coli. Further in vitro studies are required to confirm these findings.