OBJECTIVE To observe the in vitro induced drug resistance mechanisms of polymyxin B-resistant Acin-etobacter baumannii and explore the in vitro antimicrobial activity of polymyxin B combined with melatonin.METHODS Totally 10 strains of polymyxin B-sensitive A.baumannii were clinically isolated from the Affiliated Li Huili Hospital of Ningbo university between Jan.2021 to Dec.2021 and were induced to the polymyxin B-resistant strains by means of in vitro induction method.The drug resistance mechanisms of the polymyxin B-resistant A.baumannii strains were explored by polymerase chain reaction(PCR)combined with sequencing analysis.The minimum inhibitory concentrations(MICs)of the single and joint use of polymyxin B and melatonin were detected by using broth microdilution checkerboard.The antibacterial effect of polymyxin combined with melatonin was e-valuated by calculating fractional inhibitory concentration index(FICI).The strains that showed the remarkable synergistic effect for the antimicrobial activity were chosen to determine the time-kill curves.RESULTS Among the polymyxin B-induced drug-resistant A.baumannii strains,there were gene mutations at various loci of polymyxin resistance genes pmrB,lpxA and lpxD.The MIC of polymyxin B against A.baumannii strains declined after the combined use of polymyxin B and melatonin.The combined use of the two drugs that showed synergistic effect for antimicrobial activity accounted for 80.00％(8/10),the additive effect 20.00％(2/10).The time-kill curve showed that the combined use of polymyxin B and melatonin showed remarkable antibacterial effect at the 24th hour.CONCLUSIONS The drug resistance gene mutations that are mediated by chromosomes are the major drug resist-ance mechanisms of the A.baumannii strains to polymyxin B.The combined use of polymyxin B and melatonin may reverse the drug resistance of the A.baumannii to polymyxin B.

OBJECTIVE To observe the in vitro induced drug resistance mechanisms of polymyxin B-resistant Acin-etobacter baumannii and explore the in vitro antimicrobial activity of polymyxin B combined with melatonin.METHODS Totally 10 strains of polymyxin B-sensitive A.baumannii were clinically isolated from the Affiliated Li Huili Hospital of Ningbo university between Jan.2021 to Dec.2021 and were induced to the polymyxin B-resistant strains by means of in vitro induction method.The drug resistance mechanisms of the polymyxin B-resistant A.baumannii strains were explored by polymerase chain reaction(PCR)combined with sequencing analysis.The minimum inhibitory concentrations(MICs)of the single and joint use of polymyxin B and melatonin were detected by using broth microdilution checkerboard.The antibacterial effect of polymyxin combined with melatonin was e-valuated by calculating fractional inhibitory concentration index(FICI).The strains that showed the remarkable synergistic effect for the antimicrobial activity were chosen to determine the time-kill curves.RESULTS Among the polymyxin B-induced drug-resistant A.baumannii strains,there were gene mutations at various loci of polymyxin resistance genes pmrB,lpxA and lpxD.The MIC of polymyxin B against A.baumannii strains declined after the combined use of polymyxin B and melatonin.The combined use of the two drugs that showed synergistic effect for antimicrobial activity accounted for 80.00％(8/10),the additive effect 20.00％(2/10).The time-kill curve showed that the combined use of polymyxin B and melatonin showed remarkable antibacterial effect at the 24th hour.CONCLUSIONS The drug resistance gene mutations that are mediated by chromosomes are the major drug resist-ance mechanisms of the A.baumannii strains to polymyxin B.The combined use of polymyxin B and melatonin may reverse the drug resistance of the A.baumannii to polymyxin B.