Detection of plasmid mediated AmpC b-lactamase in clinical isolates by three different methods

Abstract number: P875

Ak S., Sener B., Hascelik G.

Objectives: AmpC mediated b-lactam resistance in Escherichia coli and Klebsiella spp. is an emerging problem all over the world. High level AmpC production is typically associated with in vitro resistance to all b-lactams except for carbapenems and cefepime. AmpC detection is important to ensure effective therapeutic intervention and optimal clinical outcome. Nevertheless, detection of AmpC mediated b-lactam resistance is a challenge for laboratories. There is no recommended CLSI guidelines for detection of this resistance mechanism. The aim of our study is to address this issue by tetsting different methods.

Methods: A total of 210 blood isolates of E. coli and Klebsiella spp. resistant to cefoxitin, amoxycillin-clavulanic acid, aztreonam, and any of cephalosporins were taken to the study. All isolates were identified by Phoenix automatised system. Susceptibility testing against imipenem, ceftazidime, cefotaxime, aztreonam, cefepim, cefotetan, cefpodoxime, cefoxitin, amikacin, tobramycin, amoxicillin-clavulanic acid and ciprofloxacin were done by disc diffusion. For the phenotypic detection of plasmid mediated AmpC b-lactamase inhibitor based method with boronic acid and modified-three dimensional test were used. The isolates found as AmpC b-lactamase positive based on phenotypic results, were confirmed by multiplex PCR for the presence of six different classes of AmpC b-lactamase coding genes.

Results: Boronic acid inhibition test revealed 46 positive isolates. When the inhibition test performed by boronic acid was coupled with clavulanic acid to rule out ESBL presence, the number of positive isolates decreased to 20. Modified-three dimensional test yielded 26 positive results. With multiplex PCR 4 isolates were confirmed as plasmid mediated AmpC b-lactamase producers. All of these four isolates had inducible type AmpC b-lactamase and none of them had ESBL.

Conclusion: Although boronic acid inhibition test can be useful for phenotypic detection of AmpC type b-lactamase, clavulanic acid must be added to the inhibition test in order to prevent the false positive results on account of presence of ESBL. For the confirmation of presence of AmpC b-lactamase, molecular methods, although expensive and labour intensive, should be used in clinical laboratories.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
Back to top