Antimicrobial resistance trends of Escherichia coli bloodstream isolates: a population-based study, 1998-2007

Background: There have been contradictory results regarding temporal changes in the antimicrobial resistance of Escherichia coli from tertiary care centres. Therefore, we performed a population-based investigation to examine in vitro antimicrobial resistance trends of E. coli bloodstream isolates. Methods: In this retrospective population-based incidence study, we identified 461 unique patients with first episodes of E. coli bloodstream infection (BSI) from 1 January 1998 to 31 December 2007 through microbiology records at the two laboratories in Olmsted County, Minnesota. Logistic regression was used to examine temporal changes in antimicrobial resistance and Poisson regression for changes in incidence rates. Results: The median age of patients with E. coli BSI was 69 years; 306 (66.4%) were female. The age-adjusted incidence rate of E. coli BSI per 100000 person-years was 48.0 (95% CI: 42.5-53.4) in females and 34.0 (95% CI: 28.6-39.6) in males. The urinary tract was the most common primary source of infection (79.8%). During the study period, resistance rates of E. coli bloodstream isolates increased from 32% to 53% for ampicillin, from 23% to 45% for ampicillin/sulbactam, from 9% to 28% for trimethoprim/sulfamethoxazole and from 0% to 12% for ciprofloxacin. Resistance rates to carbapenems, cephalosporins and piperacillin/tazobactam remained low and stable. Conclusions: To our knowledge, this is the first population-based study on antimicrobial resistance trends of E. coli bloodstream isolates in the USA. We demonstrated linear trends of increasing resistance among these isolates to three different classes of antimicrobial over the past decade.