Disruption of arn lipid A aminoarabinosylation confers AmpCmediated cephalosporin resistance in Pseudomonas aeruginosa

Antimicrobial resistance in chronic cystic fibrosis infections caused by Pseudomonas aeruginosa is shaped by pathogen-specific adaptation, yet the mechanisms driving cephalosporin resistance in epidemic clinical lineages remain incompletely defined. Here, we used in vitro experimental evolution to generate ceftazidime-resistant P. aeruginosa LESB58 strains, revealing loss-of-function mutations in the arnBCADTEF-ugd operon, encoding the L-Ara4N lipid A modification system (aminoarabinosylation). Loss of the arn operon in LESB58 conferred high-level resistance to ceftazidime, cefepime, and ceftolozane. Transcriptome sequencing and an ampC-promoter reporter fusion demonstrated strong upregulation of ampC, encoding a β-lactamase, following ceftazidime treatment in the arn deletion strain. Deletion of ampC in the arn mutant restored cephalosporin susceptibility to wild-type levels, confirming AmpC as the primary resistance mechanism. This arn-linked resistance was also observed in LESB65 but absent in laboratory strains PAO1 and PA14. These findings identify a connection between lipid A modification loss and AmpC-driven cephalosporin resistance in clinically relevant P. aeruginosa lineages. How can loss of a lipid A modification pathway increase β-lactam resistance? Hardie Boys, Wardell and @pletzerlab show that disruption of the arn pathway drives AmpC-mediated cephalosporin resistance in clinically relevant Pseudomonas aeruginosa LES backgrounds — but not in PAO1 or PA14. #AMR #Pseudomonas #Microbiology