Modification of the three-dimensional method for the detection of AmpC β-lactamase in Enterobacter spp. and Escherichia coli

Laboratory failure in the detection of Ambler class C (AmpC) β-lactamases in Gram negative bacteriarepresents a major cause for its spreading and may be associated with low therapeutic outcome. The aim of this study was to develop a more practical and simplified laboratory technique by modifying the three-dimensional method for the detection of AmpC β-lactamase in Enterobacter spp and Escherichia coli. Twenty eight clinical isolates including 11 isolates of Enterobacter cloacae, 2 isolates of Enterobacter aerogenes and 15 isolates of Escherichia coli tested by standard disk diffusion methods against cefoxitin 30μg disk as primary screening for detection of AmpC β-lactamase then all isolates tested for its production of AmpC β-lactamase by a modified threedimensional method (3DM) with cefoxitin. Escherichia coli ATCC 25922 was used as negative controls (non-AmpC producer). In addition, the susceptibility test of 15 Escherichia coli isolates was performed against selected betalactam antibiotics. All Enterobacter isolates and one Escherichia coli isolate showed resistance to cefoxitin and showed positive result with the modified 3DM (M3DM) by enhanced growth of the surface organism (E. coli ATCC 25922) at the point where the slit intersected the zone of inhibition, while all cefoxitin-susceptible E. coli isolates showed M3DM negative results. A high percentage of Escherichia coli isolates were non-susceptible to β-lactamantibiotics used but they showed good susceptibility to imipenem (100%).The M3DM provides a cost-effective alternative for AmpC β-lactamase testing in clinical microbiology laboratories, thus negating the need for tedious bacterial lysis procedures required in the 3DM.