A cystic fibrosis epidemic strain of Pseudomonas aeruginosa displays enhanced virulence and antimicrobial resistance

Abstract

The Liverpool epidemic strain (LES) of Pseudomonas aeruginosa is a transmissible aggressive pathogen of cystic fibrosis (CF) patients. We compared transcriptome profiles of two LES isolates with each other and with a laboratory and genetic reference strain (PAO1) after growth to late exponential phase and following exposure to oxidative stress. Both LES isolates exhibited enhanced antimicrobial resistances linked to specific mutations in efflux pump genes. Although transcription of AmpC beta-lactamase was up-regulated in both, one LES isolate contained a specific mutation rendering the ampC gene untranslatable. The virulence-related quorum-sensing (QS) regulon of LES431, an isolate that caused pneumonia in the non-CF parent of a CF patient, was considerably up-regulated in comparison to either isolate LES400, associated with a chronic CF infection, or strain PAO1. Premature activation of QS genes was detected in isolates from both non-CF parents and the CF patient in a previously reported infection episode. LES isolates lacking the up-regulated QS phenotype contained different frameshift mutations in lasR. When fed to Drosophila melanogaster, isolate LES431 killed the fruit flies more readily than either isolate LES400 or strain PAO1, indicating that virulence varies intraclonally. The LES may represent a clone with enhanced virulence and antimicrobial resistance characteristics that can vary or are lost due to mutations during long-term colonization but have contributed to the successful spread of the lineage throughout the CF population of the United Kingdom.

FIG. 1.

PFGE comparisons of LES isolates and strain PAO1. M, pulse marker, 50 to 1,000 kb (Sigma-Aldrich); lane 1, PAO1; lane 2, LES400; lane 3, LES431.

FIG. 2.

Expression of QS-regulated phenotypes. Production of elastase (a), LasA (b), and pyocyanin (c) in P. aeruginosa PAO1 (▴), LES400 (○), LES431 (⧫), LES416 (▪), LES417 (x), and LES430 (□). ▵, LB control.

FIG. 3.

Virulence of P. aeruginosa in fruit flies. Deaths of flies over time when fed P. aeruginosa PAO1 (•), LES400 (▾), LES431 (▪), or buffer (⧫) are shown.