 Trial design Multicenter prospective cohort study. Patients corresponding to the study definition will be detected by daily inspection of microbiological databases by one dedicated physician. Epidemiological variables will be detected for all patients at study admission. To identify the risk factors for infections i.e. bacteremia, pneumonia, abdomen infections, skin infections, and urinary tract infections caused by ESBL-producing Enterobacteriaceae a case-control study will be performed. A case patient will be defined as adult patient 16 years of age who has an infection caused by ESBL-producing Enterobacteriaceae according to the definitions of the Center for Diseases Control and Prevention in Atlanta CDC. In the Clinical Microbiology Laboratories, definition of ESBL production by the Clinical Microbiology Laboratories will be according to the CLSI guidelines 3, or to the BSAC guidelines for species other than Klebsiella spp., E. coli and Proteus mirabilis. One control will be selected for each case. Control will be chosen among all adult patients admitted to the same hospital during the same period within 30 days and in whom ESBL-producing Enterobacteriaceae were not isolated during their hospital stay. If more than one control will be available per case, patients with the date and time of admission closest to the case will be chosen. 