A PILOT STUDY OF THE IN VITRO INTERACTION OF FOSFOMYCIN AND MEROPENEM AGAINST METALL0-B-LACTAMASE PRODUCING PSEUDOMONAS AERUGINOSA USING ETEST AND TIME-KILL ASSAY

Pseudomonas aeruginosa is a nosocomial pathogen containing various resistance mechanisms. Among them, metallo-β-lactamase (MBL)-producing Pseudomonas are difficult to treat. Fosfomycin is an older antibiotic that has recently seen increased usage due to its activity against a broad spectrum of multidrug-resistant organisms. Our aim was to evaluate the combination of fosfomycin and meropenem against 20 MBL-producing P. aeruginosa (100% meropenem-resistant and 20% fosfomycin-resistant) using both an Etest minimal inhibitory concentration (MIC): MIC method and time-kill assay. MICs for fosfomycin and meropenem were determined by Etest and by broth microdilution method for the latter. The combination demonstrated synergy by Etest in 3/20 (15%) isolates and 5/20 (25%) isolates by time-kill assay. Results from the Etest method and time-kill assay were in agreement for 14/20 (70%) of isolates. No antagonism was found. Comparing both methods, Etest MIC: MIC method may be useful to rapidly evaluate other antimicrobial combinations.