E.coli culture and antimicrobial susceptibility testing

Prior to administering antibiotics, samples of tracheal secretion, peripheral blood, cerebrospinal fluid, pus, or urine were cultured in Columbia blood agar plate (Beiruite Biotechnology Co. Ltd., Zhengzhou, China) for bacterial identification and determination of the E. coli strains using the American PHOENIX100 bacterial identification system (Becton and Dickinson Company, Franklin Lake, NJ, USA). After identification, E. coli strains were isolated and drug sensitivity test was carried out by disk diffusion method. All disks were purchased from the Mast Co. UK. Antibiotic sensitivity testing was carried out as recommended by the Clinical and Laboratory Standards Institute (CLSI), United States. E. coli ATCC 25922 was used for quality control of the test provided by the clinical laboratory center of the Chinese Ministry of health. If the diameter of the inhibition zone around the cefotaxime/clavulanic acid and ceftazidime/clavulanic acid disks was at least 5 mm greater compared to disks without clavulanic acid, the strain was considered as ESBL-producer.