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Volume 43, Number 3, June 2010

Antimicrobial Susceptibility and Multiplex PCR Screening of AmpC Genes From Isolates of Enterobacter cloacae, Citrobacter freundii, and Serratia marcescens

Chih-Chauan Kao, Meei-Fang Liu, Chin-Fu Lin, Yi-Ching Huang, Po-Yu Liu, Ching-Wen Chang, Zhi-Yuan Shi

Received: April 30, 2009    Revised: May 14, 2009    Accepted: June 23, 2009   


Corresponding author:

 Zhi-Yuan Shi


Section of Infectious Diseases, Department of Internal Medicine, Taichung Veterans General

Hospital, 160 Section 3, Chung-Kang Road, Taichung, 40705 Taiwan.




Background and purpose: 

 The emergence of multiple drug resistance in Enterobacteriaceae is of particular concern. The aim of this study was to evaluate the antimicrobial susceptibility and screen for the ampC gene in three members of the Enterobacteriaceae family (Enterobacter cloacae,Citrobacter freundii, and Serratia marcescens) found at Taichung Veterans General Hospital during the past 5 years using multiplex polymerase chain reaction (PCR).



 The susceptibility of thirty isolates from each of the three Enterobacteriaceae family members to five antimicrobial agents (ceftazidime, flomoxef, imipenem, moxifloxacin, and colistin) was assessed. The susceptibility was analyzed by disk diffusion, screening and confirmatory tests for extended-spectrum β-lactamases (ESBL) and minimum inhibitory concentration tests according to the recommendations of the Clinical and Laboratory Standards Institute. The detection of ampC genes (3 families, including DHA, EBC and CIT) was performed by multiplex PCR. To detect the coexistence of ESBL genes, PCR was performed using five primer pairs: TEM, SHV, SHV-5, CTX-M-3, and CTX-M-14.



 Of the 90 isolates, 53 (58.9%) were positive in the screening test for ESBL. Resistance genes were detected in 12 (22.6%) of these isolates: ampC gene of DHA type in one E. cloacae isolate and EBC type in three E. cloacae isolates; ampC gene of CIT type in four C. freundii isolates; CTX-M-3-like in one C. freundii isolate and one S. marcescens isolate; TEM in three E. cloacae isolates, three C. freundiiisolates and two S. marcescens isolates; SHV in one C. freundii isolate.



 Antibiotic phenotypes cannot accurately distinguish the resistance mechanisms caused by ampC or ESBL, and especially in ESBL-ampCcombinations. However, PCR is a useful technique for the identification of the different types of resistance genes.


Key words:

 AmpC Citrobacter Enterobacter extended-spectrum β-lactamases Serratia