Objective: To compare bioi lm formation in trimethoprim/sulfamethoxazole(SXT)-susceptible Escherichia coli(E. coli)(SSEC) and SXT-resistant E. coli(SREC) isolated from patients with urinary tract infections, and study the motile ability and physical characteristics of the isolates.Methods: A total of 74 E. coli isolates were tested for antimicrobial susceptibility with the disc diffusion assay. Based on the SXT-susceptibility test, the E. coli isolates were divided into SSEC(N = 30) and SREC(N = 44) groups. All E. coli isolates were examined for motile ability by using a motility test medium, and for checking bioi lm formation a scanning electron microscope was used. Bacterial colony size was measured with a vernier caliper and bacterial cell length was measured under a light microscope. The bacterial growth rate was studied by plotting the cell growth(absorbance) versus the incubation time. Results: The frequencies of non-motility and biofilm formation in the SREC group were signii cantly higher than that in the SSEC group(P < 0.01). The SREC bacterial cell length was shorter than that in the SSEC group [(1.35 ± 0.05) vs.(1.53 ± 0.05) μm, P < 0.05)], whereas the bacterial colony size and mid-log phase of the growth curve were not signii cantly dif erent. Conclusions: The present study indicated that bioi lm formation and phenotypic change of uropathogenic E. coli can be attributed to the mechanism of E. coli SXT resistance. Objective: To compare bioi lm formation in trimethoprim / sulfamethoxazole (SXT)-susceptible Escherichia coli (SSEC) and SXT-resistant E. coli (SREC) isolated from patients with urinary tract infections, and study the motile ability and physical characteristics of the isolates. Methods: A total of 74 E. coli isolates were tested for antimicrobial susceptibility with the disc diffusion assay. Based on the SXT-susceptibility test, the E. coli isolates were divided into SSEC (N = 30) and SREC (N = 44) groups. All E. coli isolates were examined for motile ability by using a motility test medium, and for checking bioi lm formation a scanning electron microscope was used. Bacterial colony size was measured with a vernier caliper and bacterial cell The bacterial growth rate was calculated by plotting the cell growth (absorbance) versus the incubation time. Results: The frequencies of non-motility and biofilm formation in the SREC group were signii ca The SREC bacterial cell length was shorter than that in the SSEC group [(1.35 ± 0.05) vs. (1.53 ± 0.05) μm, P <0.05)], whereas the bacterial colony size and mid-log phase of the growth curve were not signii cantly dif erent. Conclusions: The present study shows that bioi lm formation and phenotypic change of uropathogenic E. coli can be attributed to the mechanism of E. coli SXT resistance.