A simple disc diffusion method for detecting AmpC and extended-spectrum beta-lactamases in clinical isolates of Enterobacteriaceae.
BACKGROUND
We sought to determine whether extended-spectrum beta-lactamases (ESBLs) and AmpC beta-lactamases (derepressed and inducible), alone and in combination, could be detected in unidentified members of the Enterobacteriaceae using a simple, overnight disc diffusion test.


METHODS
The genetic basis of antibiotic resistance in cephalosporin-resistant wild-type (n = 140) and culture collection (n = 140) isolates of Enterobacteriaceae was determined using PCR. A scheme for detecting these resistance mechanisms phenotypically was devised using five antibiotic discs: cefpodoxime +/- clavulanate; cefepime +/- clavulanate and cefoxitin.


CONCLUSIONS
AmpC beta-lactamases (derepressed and inducible) and ESBLs, alone and in combination, could reliably be detected using a disc diffusion method. ESBLs alone could be detected on the basis of a difference of >5 mm between cefpodoxime/clavulanate and cefpodoxime (10 microg) discs. ESBLs, in the presence of AmpC beta-lactamases, could be detected using a difference of >5 mm between cefepime/clavulanate and cefepime (30 microg) discs. AmpC beta-lactamases could be detected using a difference of >14 mm between cefepime/clavulanate and cefpodoxime/clavulanate discs. Inducible AmpC beta-lactamases could be discerned by observing the blunting of the cefpodoxime or cefpodoxime/clavulanate zones in proximity to cefoxitin (30 microg) discs.
Derbyshire
Helen
H
Department of Microbiology, Royal Hallamshire Hospital, Glossop Road, Sheffield, UK.
Kay
Gemma
G
Evans
Katie
K
Vaughan
Carmel
C
Kavuri
Umadevi
U
Winstanley
Trevor
T
D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001431:Bacteriological Techniques / Q000379:methods*; D004269:DNA, Bacterial / Q000235:genetics; D004755:Enterobacteriaceae / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics / Q000302:isolation & purification; D004756:Enterobacteriaceae Infections / Q000382:microbiology; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods; D016133:Polymerase Chain Reaction / Q000379:methods; D018440:beta-Lactam Resistance; D001618:beta-Lactamases / Q000032:analysis* / Q000235:genetics; D047090:beta-Lactams / Q000494:pharmacology
D000900:Anti-Bacterial Agents; D004269:DNA, Bacterial; D047090:beta-Lactams; D001618:beta-Lactamases