From May 2011 to January 2012 we performed a case control study at the Charité university hospital Berlin. Cases were defined as patients diagnosed with ESBL positive E.coli colonization within 72 h after admission. Controls were patients with ESBL negative E.coli colonization. Cases and controls with ESBL colonization within the last 12 months were excluded. In a questionnaire based interview we assessed parameters like body mass index (BMI), nutritional habits, travel habits, recent hospital admission, recent use of antibiotics and household situation. We assessed the impact of cultural background by assessing the patients’ best mastered language. ESBL positive strains were further analyzed by PCR to determine the ESBL genotype at the Robert Koch Institute, Wernigerode. Univariable and multivariable analysis were performed to identify independent risk factors for acquisition of ESBL positive E.coli strains.