Double Synergy Differential Test for detection of plasmid-mediated AmpC-type beta-lactamases in Proteus mirabilis.

Proteus mirabilis resistant to expanded-spectrum cephalsoporins (ESC) is an emerging problem. In addition to extended-sectrum -lactamases (ESBLs), acquired AmpC-type -lactamases represent an important mechanism of resistance to ESC in P. mirabilis, at least in some countries. Distinction of clinical isolates producing an ESBL or an AmpC-type enzyme is a matter of interest for the implications in surveillance, infection control, and treatment issues. This work describes a simple and practical Double Synergy Differential Test (DSDT) that couples the detection of ESBLs and AmpC-type enzymes by means of a combo-disk approach using cefotaxime and ceftazidime as indicator substrates, and clavulanate and boronic acid as enzyme inhibitors. The DSDT was tested with a collection of 104 P. mirabilis with different beta-lactamase profiles, and proved to be highly sensitive and specific for the detection of ESBL- and AmpC-producing isolates.