Evaluation of a new screen agar plate for detection and presumptive identification of Enterobacteriaceae producing extended-spectrum β-lactamases

A new agar screen plate for extended-spectrum β-lactamase (ESBL) detection was evaluated with 50 clinical isolates of ESBL-producing Enterobacteriaceae species: Enterobacter cloacae (n = 10), Escherichia coli (n = 10), Klebsiella oxytoca (n = 3), Klebsiella pneumoniae (n = 25), and Proteus mirabilis (n = 2). Fecal samples were artificially inoculated with 2 concentrations (25 and 250 colony forming units [CFU]/plate) of the test strains and then applied to the new agar screen plates. By this approach, the new agar formula detected growth that was suggestive of ESBL activity in 44 of 50 (88%) and 50 of 50 (100%) of ESBL strains with 25 and 250 CFU/plate, respectively. A limitation of the agar screen plates was a lack of some specificity. Among 15 strains with resistant phenotypes other than ESBL (K1 producers of K. oxytoca, 6 strains; 9 strains with AmpC phenotype), growth was recorded in 7 (25 CFU/plate) and 11 (250 CFU/plate) of 15 strains. In conclusion, the new agar screen plate is a sensitive and convenient method to directly screen for ESBL organisms in rectal swabs or stool samples, with the potential for incorporation into routine clinical laboratory service.