Evaluation of Meropenem, Imipenem and Ertapenem Impregnated MacConkey Agar Plates for the Detection of Carbapenem Resistant Enterobacteriaceae

Background: Rapid detection of carbapenem resistant bacteria, in particular, members of the Enterobacteriaceae family (CRE), is of utmost importance for the management of infected or colonized patients. Methods: Three carbapenems ; meropenem, imipenem and ertapenem, with two different concentrations (0.5 m g/ml and 1.0 m g/ml), were impregnated in MacConkey agar . The carbapenem impregnated MacConkey agar plates; ([Mac-Mem], [Mac-Imp] and [Mac-Ert]), were then evaluated for the detection of carbapenem resistant Gram-negative bacteria in particular the bla KPC producing Enterobacteriaceae . The Limit of Detection (LOD) of the plates was determined after counting the colonies that grew on the plates after serial logarithmic dilutions of ten. Carbapenem resistant Gram-negative bacteria were prepared in normal saline, inoculated on the different plates and incubated at 35 o C for 18-24 hours. The specificity and the shelf-life of the plates were determined by challenging the plates with six ESBL positive members of the Enterobacteriaceae family ( K. pneumoniae, Salmonella species, Shigella species, E. coli , Proteus species and Citrobacter species ) and one Enterobacter species with the bla AmpC phenotype . Finally, the MacConkey agar plates impregnated with 0.5 m g/ml meropenem were further challenged by incorporating them in the routine Caritas Baby Hospital active surveillance program for the detection of carbapenem resistant bacteria. Results: Of the three carbapenems impregnated plates, Mac-Ert plates gave the lowest number of colony forming units (CFU’s) detected regardless of the concentration of the antibiotic used. This was followed by the Mac-Mem plates which showed an LOD of less than 200 CFU’s for most of the bla KPC positive bacteria tested at both antibiotic concentrations. The worst performance was noted for the Mac-Imp plate regardless of the antibiotic concentration used as a number of carbapenem resistant bacterial strains failed to grow on the plate. The Mac-Mem plates showed the best specificity as none of the ESBL and bla AmpC positive isolates grew on the plates at either antibiotic concentration tested after 18-24hours incubation in ambient air at 35 o C. On the other hand, the Mac-Ert plates failed to inhibit the growth of the Citrobacter species tested at both antibiotic concentrations and the Proteus species tested at the 0.5µg/ml antibiotic concentration. The Mac-Imp plates showed poor specificity as both concentrations failed to inhibit the growth of the Proteus , Enterobacter and Citrobacter species evaluated after 18-24 hours incubation in ambient air at 35 o C. Of all the plates tested, the 0.5 µg/ml Mac-Mem agar had the best shelf-life of up to one month at 4-8 o C. Conclusions: The high specificity and the good selectivity, in addition to the long shelf-life allowed the 0.5µg/ml Mac-Mem agar to be used as a cost effective selective medium for the isolation of carbapenem resistant Gram-negative bacteria, in particular the bla KPC producing members of the Enterobacteriaceae family.