Mechanisms of resistance to carbapenems in meropenem- resistant Acinetobacter isolates from clinical samples

Purpose: To analyze the resistance mechanisms in Acinetobacter species by phenotypic methods. Methods: Antibiotic susceptibility profile for 150 clinical isolates of Acinetobacter was determined by the standard disk diffusion method. Isolates detected to be meropenem resistant were tested further by broth microdilution minimum inhibitory concentration (MIC) for meropenem. The resistant isolates were also tested for metallo β-lactamase (MBL) production by the double-disk approximation test, for AmpC beta-lactamase production and efflux pump detection by agar microdilution MIC with and without reserpine. Results: Twenty-one isolates were found resistant to meropenem by the standard disk diffusion method. Nine samples were from patients admitted in intensive care units (ICUs). Broth microdilution MICs of the isolates revealed low-level resistance to meropenem. MBL was not produced by any of these isolates. AmpC β-lactamases were produced by nine (43%) isolates. ‘Efflux pump’-mediated resistance to meropenem was detected in two out of nine random isolates tested for the same. Conclusions: Carbapenem resistance is not uncommon in Acinetobacter isolates. AmpC production may cause carbapenem resistance. MBL and efflux pump may not be important causes of carbapenem resistance.