Detection of extended-spectrum b-lactamase in Enterobacteriaceae with AmpC b-lactamase type of inducible resistance

Abstract number: p515

Fokas  Sp., Fokas  St, Lauranou  E., Sarris  I., Kalkani  M., Dionysopoulou  M.


ESBL production in Enterobacteriaceae with inducible AmpC chromosomal enzymes is rare, while its detection is problematic. In this study, the prevalence of the concurrent production of these enzymes in clinical Enterobacteriaceae strains was determined and three phenotypic confirmatory methods were also compared.


Fifty-five nonrepetitive clinical strains of Enterobacter spp. (n = 35), Citrobacter freundii (n = 10), Morganella morganii (n = 7), and Providencia stuartii (n = 3) exhibited an inducible AmpC b-lactamase type of resistance were isolated from urine, blood and pus over a 15-month period. Cefoxitin resistance, identification to species level and the cefoxitin/cefotaxime (CTX) disk antagonism test were used to screen for likely inducible AmpC producers. Three confirmatory phenotypic methodologies to address the detection of ESBLs were used: (a) combination disk tests CTX ± clavulanic acid (CA), ceftazidime (CAZ) ± CA, and cefepime (CEF) ± CA, (b) double-disk tests CTX/CA, CAZ/CA, and CEF/CA, and (c) modified double-disk test with CEF/piperacillin + tazobactam (TZP). AmpC production was confirmed by the three-dimensional test.


Of the 55 isolates that were tested, 53 were found to be ESBL non-producers; only two (3.6%) strains (E. aerogenes and E. cloacae) were resistant to cefotaxime and appeared to be co-producers of ESBL and AmpC b-lactamases enzymes. Combination disk test with CEF ± CA, double-disk test with CEF/CA and the modified double-disk test with CEF/TZP were capable of detecting the ESBL positive strains while the others tests failed to confirm the concomitant ESBL production.


In Enterobacteriaceae, production of inducible AmpC-beta lactamases can mask ESBL activity making its detection a challenge. The use of cefepime instead of cefotaxime or ceftazidime in inhibitor-based confirmatory tests seems to be preferable. In our institution, the occurrence of co-production of extended-spectrum and inducible chromosomal AmpC beta-lactamases, in the selected isolates we studied, was too low (3.6%) for justifying the routine use of detection methods. These procedures can be carried out in strains isolated from serious infections and to survey for complex resistance mechanisms.

Session Details

Date: 01/08/2007
Time: 00:00-00:00
Session name: XXIst ISTH Congress
Location: Oxford, UK
Presentation type:
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