Comparison of performance of MacConkey agar and Mueller-Hinton agar in imipenem-Hodge test

Abstract number: P892

Hong S.G., Hong S.K., Lee H., Jeong S.H., Song W., Lee K., Chong Y.

Objectives: Imipenem-Hodge test for screening of carbapenemases is usually performed with Mueller-Hinton agar (MHA). However, some metallo-b-lactamases (MBL) producing strains show negative results. The aim of this study was to compare the performance of MacConkey agar (MCA) and MHA in imipenem-Hodge test.

Methods: The strains comprised 21 MBL-producers (14 IMP-producing Pseudomonas aeruginosa, 3 VIM-producing P. aeruginosa, 1 IMP-producing Acinetobacter baumannii, 1 VIM-producing A. baumannii, 1 SIM-producing A. baumannii and 1 VIM-producing Alcaligenes faecalis) that were not clearly positive from imipenem-Hodge test with MHA. Twenty nine isolates of non-carbapenemase-producing Enterobacteriaceae (AmpC b-lactamase producers, and/or ESBL producers) were also tested. The surfaces of MHA and MCA plates were inoculated with a lawn of indicator strain, Escherichia coli ATCC 25922, according to CLSI disk diffusion method. After drying the agar surface, a test strain was heavily streaked from the centre of the plate to the periphery and a 10 mg imipenem disk (Becton Dickinson) was placed at the centre. After overnight incubation at 35°C, presence of definite growth of indicator organism in the inhibition zone along the test strain was interpreted as positive. The two results from MHA and MCA plate were compared.

Results: With MHA, 9 of 21 MBL-producers showed negative results and 7 were weak positive, while definite positive results were obtained from only 5 strains. However, with MCA, 18 of 21 MBL-producers showed positive results and 3 were weak positive. All of the other non-carbapenemase producers were negative with MHA. However, with MCA, one AmpC-producing E. coli showed clearly positive indentation.

Conclusions: Imipenem-Hodge test with MCA were more sensitive and easier to read the results. The use of MCA should be helpful to clinical laboratories for the detection of MBL-producing bacteria.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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