Article ID: JJID.2017.469
Phenotypic detection of extended-spectrum β-lactamase (ESBL) is important for public health and infection control; however, plasmid-mediated AmpC β-lactamases (pAmpCs) can interfere with the ESBL-phenotyping. We focused on Enterobacteriaceae strains which were susceptible to cefepime but had mildly-elevated MIC of ceftazidime, and studied the impact of pAmpC on the ESBL-phenotyping in this population. Genotyping of ESBL and pAmpC were performed on 528 clinical isolates of Escherichia coli, Klebsiella spp., and Proteus spp. with ceftazidime MIC of ≥2 μg/mL and cefepime MIC ≤8 μg/mL, which were collected in the Nagasaki University Hospital from January 2005 to March 2011. In this population, 145 isolates (27.5%) were positive for pAmpC (pAmpC group). The concordance rates of phenotypic and genotypic detection of ESBLs were 69.2% in the pAmpC group and 88.8% in the non-pAmpC group (P = 0.04). pAmpC was more commonly detected in isolates with non-CTX-M genes (5/53, 9.4%) than in isolates with CTX-M genes (8/121, 6.6%). Our data support that the presence of pAmpC increases the false negative detection of ESBL. When ESBL-phenotyping is used, the underestimation of ESBL-producers should be taken into account.
