Additive effects of ceftiofur-neomycin combination against multidrug-resistant pathogenic Escherichia coli in a murine infection model

This study aimed to evaluate the therapeutic efficacy of a ceftiofur-neomycin combination against a pathogenic multidrug-resistant Escherichia coli strain, KECS-0513, isolated from pigs, using a combination of genomic, in vitro, and in vivo approaches. The minimum inhibitory concentration, minimum bactericidal concentration, and checkerboard assays were performed. Time–kill assays were used to assess bactericidal activity over time, and a murine intraperitoneal infection model was used to evaluate survival outcomes following treatment with monotherapies or combination regimens. Whole genome sequencing indicated that KECS-0513 harboured multiple resistance genes, including bla_TEM-1B, sul3, aadA12, aad(3)-IVa, aph(3’)-Ia, and aph(4)-Ia. The resistance genes were localised within a mobile, element-rich plasmid. In vitro checkerboard assays revealed an additive interaction between ceftiofur and neomycin (fractional inhibitory concentration index = 1.0), and time–kill experiments demonstrated enhanced and sustained bacterial clearance with combination therapy. In vivo infection studies in mice demonstrated that the dual treatment resulted in a substantially higher survival rate (66.7%) compared to treatment with either agent alone (33.3% for each). These results support the practical application of ceftiofur-neomycin combination therapy for controlling swine-associated multidrug-resistant E. coli while minimising the risk of resistance emergence.