Epidemiology of infections caused by extended-spectrum beta-lactamase-producing Escherichia coli and Klebsiella spp.: a nested case-control study from a tertiary hospital in London.

Information on risk factors for acquisition of extended-spectrum ss-lactamase (ESBL)-producing organisms and their outcomes in patients with invasive infections is scant. The objectives of this study were to evaluate risk factors and all-cause mortality associated with infection due to ESBL-producing organisms using a nested case-control design, and to document transmission within a hospital employing molecular and conventional epidemiological methods. From December 2003 to April 2005, 50 patients with bloodstream infections (BSIs) due to ESBL-producing E. coli and Klebsiella spp. were recruited. Controls (N=50) were chosen, within the same period, from patients with non-ESBL-producing BSIs by simple random sampling; account was taken of potential confounding factors. Cases and controls were followed-up until November 2005, and outcomes were recorded as discharged or deceased. Molecular methods, supported by conventional epidemiology, were used to study the transmission of organisms. Logistic regression analyses showed prior ss-lactam antibiotics [odds ratio (OR) 11.57; 95% confidence intervals (CI) 2.31-51.15; P=0.003], hospital stay >15 days (OR 2.63; 95% CI 1.01-6.89; P=0.04) and prior admission to the intensive care unit (OR 13.98; 95% CI 1.88-19.15; P=0.006) to be independent risk factors for the acquisition of ESBL-producing organisms. In the first 15 days of follow-up, a significant proportion of patients with ESBL-producing organisms died; however, there was no difference in mortality between cases and controls at the end of the follow-up period. Molecular epidemiology identified five clusters amongst the ESBL-producing isolates. Conventional epidemiological analyses supported the evidence of transmission in three of these clusters.