Boronic acid disk tests for identification of extended-spectrum β-lactamase production in clinical isolates of Enterobacteriaceae producing chromosomal AmpC β-lactamases

A study using boronic acid (BA) was designed to detect the extended-spectrum β-lactamases (ESBLs) in Enterobacteriaceae producing chromosomal AmpC β-lactamases. A total of 197 clinical isolates of Enterobacter spp. (n = 100), Serratia marcescens (n = 62) and Citrobacter freundii (n = 35) were analysed. Genes encoding ESBLs were detected by polymerase chain reaction (PCR) amplification followed by direct sequencing of PCR products. The Clinical and Laboratory Standards Institute confirmatory test detected only 72.1% of the ESBL-producing isolates. When a ≥5 mm increase in the zone diameter of either the cefotaxime/clavulanic acid and/or the ceftazidime/clavulanic acid disks tested in combination with BA versus cefotaxime and/or ceftazidime containing BA was considered to be a positive for ESBL, the method detected 60 (98.4%) of the 61 isolates that harboured ESBLs and showed no false-positive results for ESBL-non-producing isolates. In conclusion, the BA disk test is a highly sensitive and specific method for the detection of ESBLs in Enterobacteriaceae producing chromosomal AmpC β-lactamases.