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 <i>Escherichia coli</i> strain, KECS-0513, isolated from pigs, using a combination of genomic, <i>in vitro</i>, and <i>in vivo</i> 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 <i>bla</i> _TEM-1B, <i>sul3</i>, <i>aadA12</i>, <i>aad(3)-IVa</i>, <i>aph(3')-Ia</i>, and <i>aph(4)-Ia</i>. The resistance genes were localised within a mobile, element-rich plasmid. <i>In vitro</i> 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. <i>In vivo</i> 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 <i>E. coli</i> while minimising the risk of resistance emergence.