Multiple genotypic changes in hypersusceptible strains of Pseudomonas aeruginosa isolated from cystic fibrosis patients do not always correlate with the phenotype

Objectives: Although Pseudomonas aeruginosa from cystic fibrosis patients are well known for their antibiotic resistance, isolates that are highly susceptible to multiple drug classes have also been encountered. In this study, hypersusceptible P. aeruginosa isolates were analysed for changes in intrinsic resistance mechanisms to explain the observed phenotype. Methods: P. aeruginosa strains PA30 and PA431 were isolated from the sputa of cystic fibrosis patients and susceptibilities were determined by agar dilution. Isolates were genetically unrelated by PFGE analysis. Expression of efflux pumps, porins, a chromosomal cephalosporinase and a gene, glmS, previously implicated in hypersusceptibility were evaluated by real-time RT-PCR, outer membrane protein analysis and β-lactamase hydrolysis assays. Results: PA30 was hypersusceptible to β-lactams, fluoroquinolones and antimetabolites, with MICs at least 4-fold lower than those for the prototype strain PAO1, while PA431 was hypersusceptible to β-lactams and antimetabolites. Both isolates overproduced the porin OprF but showed down-regulation in the production of the carbapenem channel OprD despite carbapenem hypersusceptibility. PA30 had decreased expression of the mexAB - oprM pump involved with intrinsic antibiotic resistance but overexpressed the mexCD - oprJ and mexEF - oprN efflux systems normally associated with acquired resistance. PA431 showed down-regulation of oprM, the last gene in the mexAB - oprM operon, but overexpressed the mexXY pump. The ampC β-lactamase was weakly inducible in strain PA30, corresponding to cefoxitin hypersusceptibility. Conclusions: The changes in expression of several intrinsic mechanisms in the hypersusceptible strains did not correlate with the observed phenotype. These data highlight the complex interactions of resistance mechanisms in P. aeruginosa and their roles in drug susceptibility.