Cefotaxime-agar medium for laboratory detection of extended-spectrum and AmpC β-lactamases in Enterobacteriaceae

Objective To develop an easy,rapid and reproducible cefotaxime-agar medium(CTX- AM)for phenotypic detection of extended-spectrum β-lactamases(ESBLs)and AmpC β-lactamases (AmpCs)in Enterobacteriaceae.Methods The surface of a cefotaxime(CTX,0.5 μg/ml)-Mueller- Hinton agar and ceftizoxime(CAZ,1 μg/ml)-Mueller-Hinton agar plate was inoculated with a lawn of E. coli ATCC 25922 according to the standard disk diffusion method,respectively.Immediately prior to use.blank and clavulanic acid(10 μg),cloxacillin(300 μg),clavulanic acid/cloxacillin(10/300 μg) disk were rehydrated with 10 μl of saline and several colonies of each test organism were applied to disks. Then the results of CTX-AM method to interpret based on a zone of growth around the periphery of disks.A total 58 of ESBL and AmpC producing and non-producing isolates of Enterobacteriaceae,as identified by the double-disk enhancement test(DDET)and the three-dimensional extract method(TDEM).were used to evaluate the CTX-AM method.Positive control(E.cloacae 029M,K.pneumoniae ATCC 700603)and negative control(E.coli ATCC 25922)strains were included.Results The results of CTX-AM method were similar to the DDET and TDEM method for detecting ESBLs and AmpC production in Enterobacteriaceae,respectively.But inhibitor-resistant β-lactamase(IR-BLs)and other β-lactamases were not detected by DDET method.Conclusions The new method described here allows for testing of ESBL and AmpCs on a single plate.It is easy to perform and interpret,and also cost-effective,clinical laboratories may use this technique routinely to detect the oresence of ESBL and AmoCs.