Detection of extended-spectrum ␤-lactamases (ESBLs) in AmpC-producing Enterobacteriaceae is problematic. A modification of the double-disk test (MDDT) has been developed for successful detection of ESBLs in gram-negative bacilli producing well-characterized ␤-lactamases as well as 212 clinical isolates of Enterobacter cloacae, Enterobacter aerogenes, Serratia marcescens, and Citrobacter freundii. MDDT accurately differentiated between ESBL producers and derepressed chromosomal AmpC mutants. MDDT provides a cost-effective alternative approach for clinical microbiology laboratories for routine susceptibility testing with simultaneous detection of ESBLs in Enterobacteriaceae.Enterobacteriaceae producing both AmpC ␤-lactamases and extended-spectrum ␤-lactamases (ESBLs) have been increasingly reported worldwide (2-6, 12, 16, 18). Since AmpC-producing organisms can act as hidden reservoirs for ESBLs, it is important for clinical microbiology laboratories to be able to detect ESBL production in these organisms on a routine basis (9). The National Committee for Clinical Laboratory Standards (NCCLS) has published guidelines for the detection of ESBLs in clinical isolates of Escherichia coli and Klebsiella spp., but there are currently none available for other genera (10). Since high-level expression of AmpC ␤-lactamases may mask recognition of ESBLs (17), a unique modification of the double-disk test (MDDT) using a combination of cefepime (FEP) and piperacillin-tazobactam (TZP) was evaluated to detect ESBLs. This evaluation was carried out with well-characterized strains producing either AmpC ␤-lactamases and ESBLs or either ␤-lactamase alone. In addition, the MDDT determined the presence of an ESBL in an E. coli isolate that showed a negative result with the NCCLS disk confirmation test.The following strains producing known ␤-lactamases were used for this study (Table 1). A total of 212 clinical isolates were also evaluated by MDDT: 94 were E. cloacae, 32 were E. aerogenes, 25 were S. marcescens, and 61 were C. freundii. The isolates were nonrepetitive (one per patient) and were obtained from clinical specimens from Universitas and Pelonomi Hospitals, Bloemfontein, South Africa, over a 9-month period during 1998 and 1999.

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