Abstract: Objective To investigate the relationship between antibiotics therapy and producing constitutive high expression AmpC beta-lactamase (AmpC) infected by Enterobacter aerogenes, in order to provide the reference for rational antibiotics. Methods Enterobacter aerogenes were isolated from September 2011 to March 2012 from elderly nosocomial patients in the elderly ward of Huadong Hospital. The modified three dimensional test was used to detect AmpC. The homology of Enterobacter aerogenes was detected by pulsed-field gel electrophoresis(PFGE). Prospective and retrospective investigations were performed on the clinical data of the patientswith wild-type and constitutive high expession AmpC infected by Enterobacter aerogenes. Results A total of 19 isolates of Enterobacter aerogenes were isolated. The positive rates of constitutive high expression and wild-type AmpC were 31.58%(6/19) and 26.32%(5/19). The positive rate of constitutive high expression AmpC from sputum samples was 83.33%, and the positive rate from urine samples was 33.33%. The total amount of antibiotics used to treat Enterobacter aerogenes infection producing constitutive high expression AmpC was higher than that of wild-type Enterobacter aerogenes. The wild-type Enterobacter aerogenes may be changed to the constitutive high expression when cephamycin was used to inhibit for more than2 weeks. Conclusions The positive rate of producing constitutive high expression AmpC by Enterobacter aerogenes in Huadong Hospital is high, especially for isolates form respiratory tract.The 1st, 2nd and 3rd generation cephalosporins and cephamycin should be used with caution or not be used in the course of anti-infection therapy. It is necessary to strengthen the surveillance of antimicrobial resistance and reasonably use antibiotics.

Key words: Enterobacter aerogenes, AmpC beta-lactamase, Constitutive high expression, Antibiotics