The Mechanisms Of Tigecycline Resistance Mediated By PlsC And AbrP In Acinetobacter Baumannii

Multidrug-resistant Acinetobacter baumannii has become a severe challenge in clinical anti-infections treatment. Tigecycline has attracted attention in the treatment of infection because it has shown an effective antimicrobial activity to MDRA. baumannii. However, with increasing usage of tigecycline, resistance to this antibiotic has inevitably developed in clinical A. baumannii strains. Currently, the existing resistant mechanisms can not fully explain the cause of clinical strains resistant to tigecycline.This research is mainly divided into three parts to gradually explore tigecycline resistant mechanism in A. baumannii.In part I, a tigecycline-resistant strain,19606-M24, was obtained by increasing the concentration of the antimicrobial in liquid culture to explore the mechanism of tigecycline resistance in A. baumannii. Three mutations were identified by the whole genome sequencing and the whole genome comparison, including one synonymous substitution in a hypothetical protein and a frameshift mutation in plsC and omp38. The plsC genewas confirmed to cause decreased susceptibility to tigecycline by a complementation experiment and cellular membrane change was detected by flow cytometry. In addition, the fitness cost of plsC was estimated to be approximately 8% by measuring the relative growth rate. In conclusion, plsC was found to play an important role in tigecycline resistance in A. baumannii. The minor fitness cost of plsC indicates a high risk for the emergence and development of tigecycline resistance in A. baumannii.In part Ⅱ, we characterized the biological functions of a novel gene (abrP) associated with decreased susceptibility of tigecycline and fitness in A. baumannii. Inactivation of abrP resulted in decreased susceptibility to antibiotics such as tetracycline, doxycycline, minocycline, chloramphenicol and fosfomycin. Increased cell membrane permeability in the strain of inactive abrP was determined using flow cytometry and transmission electron microscropy. The mutant strain also displayed slower cell growth and reduced cell fitnessin an in vitro competition and a non-competition experiment. Results from the present study show that abrP played an important role in intrinsic resistance to different classes of antibiotics and cell fitness in A. baumannii. The change of antibiotic sensitivities may result from modifications to the cell membrane permeability of A. baumannii.In part Ⅲ, we determined the transcriptional profile of A. baumannii in the presence or absence of tigecycline. The results showed that MFS family efflux pump and multiple transcriptional regulators possibly involved in the stress response and drug-resistance were upregulated in response to tigecycline. Strong suppresses in aerobic phenylacetate catabolic pathway and a number of ABC transporters were also observed after exposed to tigecycline. Furthermore, A.baumannii showed lower MIC of Ceftazidime in the presence of tigecycline due to the down-regulated OXA-23 and AmpC. These finding suggested that the response of A. baumannii to tigecycline was multifactorial and included MFS family efflux pump transcriptional regulators and metabolism pathway, like phenylacetate catabolic pathway.