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.