Carbapenem-resistant Acinetobacter baumannii and Pseudomonas aeruginosa: characterization of carbapenemase genes and E-test evaluation of colistin-based combinations Raghdaa A Ramadan,1 Manar G Gebriel,1 Heba M kadry,1 Ahmed Mosallem2 1Medical Microbiology and Immunology Department, Faculty of Medicine, Zagazig University, Zagazig, Sharkia, Egypt; 2Anesthesia and Surgical Intensive Care Department, Faculty of Medicine, Zagazig University, Zagazig, Sharkia, Egypt Background: Carbapenamase producing Acinetobacter baumannii and Pseudomonas aeruginosa are emerging worldwide limiting the use of carbapenems as effective and safe drugs. Purpose: To characterize different carbapenemase genes carried by carbapenem-resistant (CR) A. baumannii and P. aeruginosa isolates and to evaluate the in vitro effect of some colistin-based combinations by E-test method in Zagazig University Hospitals ICU isolates. Methods: CR A. baumannii and P. aeruginosa isolated from the surgical intensive care unit (ICU) were tested for carbapenemase genes by polymerase chain reaction and the effect of colistin/meropenem and colistin/tigecycline combinations was evaluated by E-test. Results: Genes coding for OXA-23, NDM and GES were detected in 90, 66.7 and 50% of CR A. baumannii, respectively, while genes coding for VIM, GES, NDM and IMP were detected in 50, 40.9, 27.3 and 18.2% of CR P. aeruginosa, respectively. Colistin/tigecycline combination showed synergistic and additive effect in 20% and 60% of A. baumannii isolates, respectively, while colistin/meropenem combination showed synergistic and additive effect in 63.6% and 36.4% of P. aeruginosa, respectively. Conclusion: Carbapenemase genes carriage accounts for high level carbapenem resistance in our isolates. Colistin/tigecycline and colistin/meropenem combinations can be considered for treatment of severe infections by CR A. baumannii and P. aeruginosa, respectively. Keywords: meropenem, multidrug resistant, synergy, fractional inhibitory concentration, intensive care unit