Investigation Of Polymyxin B Resistance Mechanism In Carbapenem-resistant Acinetobacter Baumanii

Objective:To detect the commonβ-lactamse genes in Acinetobacter baumannii and investigate the resistance mechanism among multiple-drug resistant A. baumannii strains for polymyxin B.Method:Ninety-eight clinical isolates of carbapenems-resistant A. baumannii were collected from eight hospitals in Zhejiang Province and one hospital in Henan Province. Agar dilution and E-tset were used to determine the MIC of 14 antibiotics. The homology of those isolates was analyzed by pulse-field gel electrophoresis(PFGE). Genes of carbapenemase were detected by specific PCR. Derivative clones resistance against polymyxin B were obtained by using in vitro induced experiment, Cyanide chlorophenyl hydrazone (CCCP) inhibition experiments were conducted to determine whether there is efflux pump. PCR, DNA sequencing and real-time quantitative PCR were used to analyze polymyxin B resistance-associated gene mutations and expression differences. Results:Among 14 antibiotics, all of the 98 strains were susceptible to polymyxin B, some antibiotics including tigecycline, amikacin, ciprofloxacin, resistant rates were 89%, 94.4%, and 97.7%, respectively, the others were 100%; The 48 strains selected by resistance phenotype were classified as a single clone according to PFGE profiles.Ninety-four of 98 strains were positive for OXA-23 gene,41 of 98 strains for VIM-2,1 for IMP-2, none for IMP-1 and VIM-1. The CCCP inhibition test confirmed the activation of broad spectrum efflux pump. The derivative strains were not found gene mutations, which were compared with their mother strains in PmrA-PmrB gene sequence.Conclutions:Carbapenem-resistant A. baumannii most produced OXA-23 carbapenemase, half produced VIM-2 type metal-β-lactamase, seldom produced IMP-1.The resistant staints against polymyxin B can be induced in vitro, whose resistance mechanism is maybe associated with the activation of efflux pump.