The Study Of Tigecycline Resistant Mechanisms In Acinetobacter Baumannii

Multidrug-resistant Acinetobacter baumannii has been a serious problem in clinical anti-infections, and tigecycline has effective antimicrobial activity to it. Tigecycline has broad antimicrobial spectrum, strong antimicrobial activity, less adverse reaction, and application not limited by the patient’s age, gender and disease. So that, in the treatment of multidrug resistant A. baumannii, tigecycline is especially impressed. However, the using of antibiotics always goes with the emergence of bacterial resistance. Since US FDA approved tigecycline in clinical application in2005, tigecycline-resistant A. baumannii was separated in2006. Now more and more strains of tigecycline-nonsusceptible and tigecycline-resistant pathogenic bacteria have been isolated in clinic, and A. baumannii is the most common bacteria among them. The present research is mainly divided into three parts which are gradually in-depth study of tigecycline-resistance mechanism in A. baumannii.In part I, PCR, Real-time PCR, efflux pumps inhibitor phenotype analysis, mutation analysis methods were used to study the influence of RND efflux pump to tigecycline resistance in multidrug resistant A. baumannii. The results revealed that the presence of efflux pump genes of adeB, adeG and adeJ were widely existed. The expression level of efflux pumps was significantly higher in tigecycline-resistant strains than that of in tigecycline-susceptible strains, mainly for the adeB expression level increased significantly, which was due to the insertion of ISAba-1in the two component regulation system adeS. The presence of efflux pump inhibitor phenotype was65.6%in tigecycline-nonsusceptible strains （MIC>4mg/L）. In part Ⅱ, the gene A1S₃356which is a homologous gene of tetX was cloned from A. baumannii standard strain ATCC17978, and its role in tetracyclines and tigecycline resistance was studied. The results showed that the tetracyclines and tigecycline sensitivity had no change in the strains with over-expression of A1S₃356, which meant A1S₃356didn’t mediate the resistance of A. baumannii to tetracyclines and tigecycline resistance.In part Ⅲ, the A. baumannii standard strain ATCC19606was induced by a serial passage experiment using tigecycline in vitro, and the tigecycline-resistant strain19606-T8was isolated. Then the whole genome was sequenced by high-throughput sequencing technologies. The comparative genomics, comparative transcriptome, gene complementation experiment and functional verification test were performed to explore some unknown tigecycline-resistant mechanisms in A. baumannii and the influence of tigecycline to A. baumannii. The result revealed that the deletion mutation of trm gene was responsible for tigecycline nonsusceptible, and the wild-type trm could restore the susceptibility of tigecycline. The trm gene as a novel mechanism plays an important role in the process of tigecycline resistance, except RND efflux pumps. Meanwhile there were some changes of transcriptome in A. baumannii under the action of tigecycline, such as high expression of some drug resistance genes, efflux pump associated genes, porins and transcription factors, and the expression of metabolic related genes were also changed.