Conventional and molecular epidemiology of trimethoprim-sulfamethoxazole resistance among urinary Escherichia coli isolates

BACKGROUND: Antibiotic resistance is increasing in Escherichia coli, the most common cause of urinary tract infections, but its epidemiology has not been well described. We evaluated the epidemiology of trimethoprim- sulfamethoxazole-resistant E. coli in a large, public health care system in Denver, Colorado. METHODS: Outpatients with E. coli urinary tract infections during the first 6 months of 1998 were evaluated retrospectively. A prospective study was then performed to confirm the rate of trimethoprim-sulfamethoxazole resistance. We used several strain-typing methods (pulsed-field gel electrophoresis, ribotyping, serotyping) to evaluate the molecular epidemiology of the resistance. RESULTS: The rate of trimethoprim-sulfamethoxazole resistance was similar in the retrospective (24% [161/681]) and prospective (23% [30/130]) phases of the study (P = 0.89). Almost all trimethoprim- sulfamethoxazole-resistant strains (98%) were resistant to at least one other antibiotic. Risk factors for infection with a resistant strain included age ≤3 years, Hispanic ethnicity, recent travel outside the United States, and a prior urinary tract infection. However, rates of resistance were >15% among nearly all of the subgroups. Most strains had high-level resistance (>1000 μg/mL) to trimethoprim-sulfamethoxazole. Of the 23 resistant isolates evaluated, 10 (43%) belonged to the clone A group. There was no correlation between conventional epidemiologic characteristics and the molecular mechanism of resistance or strain type. CONCLUSION: Resistance to trimethoprim- sulfamethoxazole among E. coli isolates among patients in a Denver public health care system is common, with high rates of resistance even among patients without risk factors.