Study On The Tigecycline Resistance Mechanisms And Homology Of Multidrug-resistant Acinetobacter Baumannii

Objective1. To assess the susceptibility of clinical isolates of Acinetobacter baumannii from our hospital to tigecycline and other commonly used antimicrobial agents.2. To assess the possible resistance mechanism giving rise to the reduced susceptibility in tigecycline of Acinetobacter baumannii isolates.3. To assess the molecular epidemiological typing of clinical isolates of Acinetobacter baumannii from our hospital.MethodsFrom January2012to December2012, a total of74non-duplicate A. baumannii isolates including64tigecycline non-susceptible A. baumannii (TNAB) and10tigecycline susceptible A. baumannii (TSAB) isolates were obtained from our hospital.1. MICs of18commonly used antimicrobial agents and tigecycline in74non-duplicate A. baumannii isolates were performed by agar dilution method and broth microdilution method,respectively.2. PCR was used to screen for efflux pump genes, including adeABC, adelJK, adeFGH, abeM, adeDE, adeSR, tetX and tetXl.3. The expression of efflux pump genes including adeB, adeJ, adeG and abeM were analyzed by real-time RT-PCR.4. Efflux pump inhibitors (EPIs) including CCCP, PAβN, NMP, reserpine and verapamil were used to evaluate the efficacy of efflux pumps in reducing tigecycline susceptibility.5. Molecular typing was performed by MLST and PFGE.6. Records from patients with A. baumannii isolated including TNAB and TSAB were retrospectively reviewe.Results1. All clinical isolates of A. baumannii were multidrug resistant, with colistin being the only potentially active agents2. The majority of A. baumannii isolates were detected to be positive for adeABC, adeRS, adelJK and abeM. While the adeE gene was found in one TSAB isolate, which harbored adeB gene as well. Moreover, the tetXl gene was detected in12(18.8%) TNAB isolates.3. Compared with TSAB isolates, the mean expression level of adeB, adeJ, adeG and abeM in TNAB isolates were observed to increase by29-,3-,0.7-and1-fold, respectively.4. PAβN and CCCP could partially reverse the resistance pattern of tigecycline, while NMP, verapamil and reserpine showed no or little efficacy.5. By MLST,10different sequence types (STs) and8novel STs (designated as STnl-STn8) were identified. ST208and ST191which both belonged to clonal complex92were the predominant STs.PFGE analysis showed19different pulsetypes.6. Diverse clinical characteristics were showed in both cohorts, and significantly more carbapenems were used in TNAB patients during the last month before the TNAB isolates were identified.Conclusions1. Since all clinical isolates of A. baumannii from our hospital were multidrug resistant that antimicrobial agents in our hospital should be used in a more reasonable and effective way.2. Given the AdeABC efflux pump seems to play an important role in reduced tigecycline susceptibility, it is necessary to monitor the susceptibility of A. baumannii to tigecycline.3. In our knowledge, this is the first report that tetXl gene was detected in the tigecycline non-susceptibility A. baumannii isolates, and how the tetXl gene contributing to decreased tigecycline susceptibility needs to be a further study.4. The dissemination of the MDRAB isolates in our hospital is mainly attributable to the spread of CC92.