Resistance to antimicrobial agents is commonly found in Enterobacteriaceae which is common pathogens isolated from clinical specimen. The most common mechanism of third generation cephalosporin resistance in Enterobacteriaceae is the production of extended spectrum beta-lactamases (ESBLs) or derepressed class C beta-lactamases (AmpC). We investigated the antimicrobial susceptibility profiles and the production of ESBLs and AmpC in 144 clinical Enterobacteriaceae strains isolated from Ramathibodhi hospital from March to April 2005 by different methods. We found that only 17 isolates (11.8%) were susceptible to all antibiotic testing while 85.0%, 48.9%, 34.6% and 33.8% were resistant to ampicillin, co-trimoxazole, ciprofloxacin, and cepodoxime, respectively. Nevertheless, multiple drug resistance was also commonly found in ESBL producing isolates. In our study, 13 ESBL producing isolates (27.1%) were resistant to beta-lactams, co-trimoxazole, ciprofloxacin, and gentamicin whereas only 9 isolates (18.8%) were resistant to beta-lactams only. Using combination of combined and double disc (amoxycillin-clavulanic acid and cefepime disc) tests, ESBLs were detected in 48 isolates. The production of ESBLs in E. coli and Kebsiella spp. accounted for 37.4% and 33.3%, respectively. Furthermore, the CTX-M type ESBLs were presented in 54.2% of ESBLs producing isolates. Moreover, all cefoxitin resistance isolates was also investigated the AmpC production. All 20 cefoxitin resistance isolates produced AmpC enzyme which could be detected by cell lysis. However, AmpC producing isolates were reported 11 isolates by Tris EDTA (AmpC) disc test and only 9 isolates presented the character of antagonism between cefoxitin and ceftriaxone or imipenem and ceftriaxone. In addition, we also tested the prevalence of plasmid mediated AmpC by PCR test. The plasmid AmpC was presented in 5 AmpC producing isolates (25%). We concluded that the sensitive and specific methods for detection of drug resistance were need. We recommended the methods for detection of ESBLs and AmpC production in Enterobacteriaceae in microbiology laboratory.

Key word: Enterobacteriaceae, -lactamase enzyme, AmpC -lactamase, Extended spectrum -lactamases

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