Mechanisms of tigecycline resistance in carbapenems-resistant Acinetobacter baumannii strains

Abstract

Objective To investigate the mechanisms of tigecycline nonsusceptibility in carbapenems-resistant Acinetobacter baumannii (CRAB) strains in order to provide a theoretical basis for a reasonable use of antibiotics and the control of nosocomial infection. Methods Susceptibility testing of 120 non-duplicate CRAB strains to tigecycline was performed by using the broth microdilution method. Minimal inhibitory concentrations (MIC) of tigecycline against the A. baumannii strains were determined by using the broth microdilution method before and after exposing the strains to Carbonylcyanide-m-chlorophenylhydrazone (CCCP), which was the efflux pump inhibitor. Polymerase chain reaction (PCR) was used to amply the efflux pumps genes including adeB, adeJ, adeG, abeM, adeE, adeRS, tetX and tetX1. The real-time PCR was performed to measure the expression of efflux pumps genes including adeB, adeJ, adeG, abeM and adeE. Results A total of 120 CRAB strains were collected including 13 (10.8%) tigecycline non-susceptible A. baumannii (TNAB) strains and 107 (89.2%) tigecycline susceptible A. baumannii (TSAB) strains. The MIC values of tigecline to the 120 CRAB strains were in a range of 0.25 μg/ml to 8 μg/ml. The adeR and adeJ genes were detected in 90.0% and 92.5% of the 120 CRAB strains, respectively. The positive rates of adeB, adeS, adeG and abeM genes among the 120 CRAB strains were all 94.2%. None of the three genes including adeE, tetX and tetX1 were detected. The mean expression levels of adeB and adeJ in TNAB strains were respectively increased by 18.69 folds and 5.46 folds as compared with those in sensitive strains. No significant increase in the expression of adeG and abeM genes was observed in TNAB strains. A 4-fold decrease in the MIC was observed in 8 out of 13 TNAB isolates treated with 10 μg/ml of CCCP. The CCCP could partially reverse the resistance pattern of tigecycline. Conclusion The efflux pump systems of adeABC and adeIJK rather than the abeFGH and abeM systems might play an important role in reducing the tigecycline susceptibility in carbapenems-resistant A. baumannii strains. Key words: Acinetobacter baumannii; Tigecycline; Efflux pumps; Resistance mechanisms