Clinical Isolates of Escherichia coli and Klebsiella spp. with Plasmid-mediated AmpC β-lactamases at Auckland City Hospital

• AmpC β-lactamases can be chromosomal or plasmid mediated. Organisms producing plasmid-mediated AmpC β-lactamases (PMACBLs) are being increasingly reported worldwide and the dissemination of these multiply antibiotic-resistant organisms has important implications for patient management and hospital infection control. 1 • Resistance to cefoxitin is a characteristic seen with both chromosomal and plasmid-mediated AmpC βlactamases. Klebsiella spp. do not have chromosomal AmpC β-lactamases and, while E. coli do have chromosomal AmpC β-lactamases, they are usually only produced at very low levels and are not considered clinically relevant. • Cefoxitin resistance in Klebsiella spp. and E. coli can be due to changes in porin permeability or due to the acquisition of plasmids carrying the ampC gene. In the case of E. coli, hyper-production of AmpC β-lactamase following up-regulation of the chromosomal ampC gene may also occur. • Extended spectrum s-lactamase (ESBL)-producing E. coli are being increasingly isolated at our hospital. In addition, since 2005 there has been an increase in E. coli isolates that are amoxycillin-clavulanate resistant, grow on the ESBL screen plates (ceftriaxone 1 mg/L and ceftazidime 1 mg/L), but are not confirmed as ESBL producers (Figure 1). Some of these isolates are resistant or show reduced susceptibility to ceftazidime. Of the isolates available for testing, all were resistant to cefoxitin . • This study was performed to assess the local prevalence of PMACBLs and to elucidate a simple algorithm for the detection of PMACBL in a clinical microbiology laboratory.