Occurrence of a novel SHV-type enzyme (SHV-55) among isolates of Klebsiella pneumoniae from Portuguese origin in a comparison study for extended-spectrum β-lactamase–producing evaluation

Fifty-five isolates of Klebsiella pneumoniae were evaluated for extended-spectrum β-lactamase (ESBL) detection and confirmation, using MIC testing by agar dilution, broth microdilution, and the ESBL E-Test (AB Biodisk, Solna, Sweden), according to reference laboratory criteria (RLC) and Clinical and Laboratory Standards Institute (CLSI) guidelines. The RLC classify as ESBL producers those strains for which any MIC of cephalosporins is 3-fold lower in the presence of 2 μg/mL of clavulanate. The E-Test was the only to show 100% sensitivity and specificity to detect ESBL-producer strains with either set of guidelines. MIC determination by agar dilution or broth microdilution, using NCCLS guidelines, showed sensitivity of 92.9%. Nucleotide sequencing allowed the identification of a new ESBL (SHV-55). Overall, this gold standard method confirmed the production of 18 ESBL producers, 36 non-ESBL producers, from which 9 were false ESBL producers (suggesting hyperproduction) and 1 presumptive ESBL TEM-derived. New guidelines for ESBL detection and reliable methods of ESBL identification are required.