Introduction and aim of the study - Treatment of urinary infections caused by Escherichia coli has become increasingly problematic due to the emer- gence and spread of multidrug-resistant (MDR) strains. This retrospective study was performed to evaluate patterns of antibiotic resistance among E. coli isolated from urine samples from dogs and humans.

Materials and methods - Fifty-five canine and 85 human isolates were processed. Bacteria were identified and susceptibility tests were performed using an automated system. A panel of 23 antibiotics was tested. Strains resistant to at least three classes of antibiotics were defined as MDR.

Results and discussion - In dogs and humans the highest rates of resistance were observed for tetracycline (100% both), norfloxacin (38% and 26%, respectively), piperacillin (35% and 42%) and ampicillin (31% and 45%). Susceptibility to phosphomycin, carbapenems and nitrofurantoin was between 98% and 100%. In both species about 35% of isolates were MDR. While in humans 76% of strains were resistant to three or four classes of antibiotics and only 3% to seven classes, in dogs 37% of strains were re- sistant to seven classes. A high rate of resistance against the antimicro- bial agents more commonly prescribed in human and veterinary medicine (e.g. fluoroquinolones) was observed. Although carbapenems show high efficacy, the administration of last-line antimicrobials should be discour- aged. Nitrofurantoin still represents a valid therapeutic option, especially in empirical treatment, but its use is off-label in veterinary medicine. Our findings highlight the need for a thorough diagnostic work-up before start- ing a targeted antimicrobial therapy, especially for canine infections. The overuse of antimicrobial agents is an important risk factor for the emergence of bacterial resistance.