 Isolates will be divided to ESBL-producing or non-producing groups by screening tests. All the available isolates were identified to the species or genus level by means of conventional methods. Isolates with same species recovered from the same an individual patient, are considered one isolate. The isolates were all stored at -70C in trypticase soy broth Difco Laboratories, Detroit, MI supplemented with 15 glycerol before being tested. 