In Vitro Antibacterial Activities of Fosfomycin against Escherichia coli Isolates from Canine Urinary Tract Infection.

Fosfomycin is a bactericidal drug recommended as an alternative treatment for canine bacterial cystitis, particularly in cases involving multidrug-resistant (MDR) infections when no other options are available. In this study, minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) of fosfomycin were determined against 79 clinical E. coli isolates using the agar dilution method. The susceptibility rate of E. coli to fosfomycin was 86.06%, with MIC 50 and MIC 90 values of 4 mg/L and 96 mg/L, respectively. MPC 50 and MPC 90 values were 64 mg/L and 192 mg/L. Using pharmacokinetic (PK) data from dogs given a single 80 mg/kg oral dose of fosfomycin, the area under the curve per MIC 50 (AUC 0-24 /MIC 50 ) was 85.79 with time above MIC 50 (T > MIC 50 ) exceeding 50%. In urine, the AUC 0-24 /MIC 50 was 10,694.78, and the AUC 0-24 /MPC 90 was 222.81, with T > MPC 90 extending beyond 24 h. Therefore, fosfomycin exhibited significant antibacterial activity against canine uropathogenic E. coli , including MDR strains, at concentrations below the susceptible MIC breakpoint. However, the high MPC values, especially the MPC 90 , indicate the critical importance of performing susceptibility testing for fosfomycin and maintaining ongoing resistance monitoring.