Tigecycline(TGC)is a recently developed broad-spectrum and highly effective glycylcy-cline antibiotic that overcomes traditional tetracycline resistance mechanisms.It is widely used in the treatment of complex skin and intra-abdominal infections.However,the emergence of TGC re-sistance in recent years poses a significant challenge,and its underlying resistance mechanisms re-main incompletely understood,warranting further investigation.This study induced TGC resistance in Klebsiella aerogenes(ATCC-43864)under in vitro conditions and evaluated phenotypic chan-ges,including growth curves,motility,multidrug susceptibility,and ultrastructural alterations,be-fore and after induction.Whole-genome sequencing(WGS)and transcriptome sequencing(RNA-seq)were employed to analyze resistance-related genetic changes,which were further validated.Ef-flux pump inhibition assays and Red homologous recombination were used to investigate the roles of efflux pumps,lon deletion,and rpsM mutation in TGC resistance.A stable resistant strain,CF-T-32,was obtained,with a 32-fold increase in TGC minimum inhibitory concentration(MIC).CF-T-32 exhibited an enlarged periplasmic space between the cell wall and membrane,reduced growth and motility,and no significant changes in susceptibility to 28 antimicrobial agents,including doxy-cycline,gentamicin,and ciprofloxacin.Genomic analysis revealed no significant differences in ge-nome composition or interference from exogenous plasmids between the parental and induced strains.However,two missense mutations were identified in rpsM and lon.Transcriptomic analysis and qPCR validation showed significant upregulation of efflux pump genes(acrA and acrB)in the AcrAB-TolC system and downregulation of ribosomal protein genes(rpsM,rpsJ,and rpsC).Ef-flux pump inhibition and Red homologous recombination experiments demonstrated that the rpsM C287T missense mutation,leading to a Pro96Leu substitution and reduced expression,is likely the primary factor contributing to TGC resistance in the induced strain.

Tigecycline(TGC)is a recently developed broad-spectrum and highly effective glycylcy-cline antibiotic that overcomes traditional tetracycline resistance mechanisms.It is widely used in the treatment of complex skin and intra-abdominal infections.However,the emergence of TGC re-sistance in recent years poses a significant challenge,and its underlying resistance mechanisms re-main incompletely understood,warranting further investigation.This study induced TGC resistance in Klebsiella aerogenes(ATCC-43864)under in vitro conditions and evaluated phenotypic chan-ges,including growth curves,motility,multidrug susceptibility,and ultrastructural alterations,be-fore and after induction.Whole-genome sequencing(WGS)and transcriptome sequencing(RNA-seq)were employed to analyze resistance-related genetic changes,which were further validated.Ef-flux pump inhibition assays and Red homologous recombination were used to investigate the roles of efflux pumps,lon deletion,and rpsM mutation in TGC resistance.A stable resistant strain,CF-T-32,was obtained,with a 32-fold increase in TGC minimum inhibitory concentration(MIC).CF-T-32 exhibited an enlarged periplasmic space between the cell wall and membrane,reduced growth and motility,and no significant changes in susceptibility to 28 antimicrobial agents,including doxy-cycline,gentamicin,and ciprofloxacin.Genomic analysis revealed no significant differences in ge-nome composition or interference from exogenous plasmids between the parental and induced strains.However,two missense mutations were identified in rpsM and lon.Transcriptomic analysis and qPCR validation showed significant upregulation of efflux pump genes(acrA and acrB)in the AcrAB-TolC system and downregulation of ribosomal protein genes(rpsM,rpsJ,and rpsC).Ef-flux pump inhibition and Red homologous recombination experiments demonstrated that the rpsM C287T missense mutation,leading to a Pro96Leu substitution and reduced expression,is likely the primary factor contributing to TGC resistance in the induced strain.