Expression of Pseudomonas aeruginosa Antibiotic Resistance Genes Varies Greatly during Infections in Cystic Fibrosis Patients

Abstract

The lungs of individuals with cystic fibrosis (CF) become chronically infected with <i>Pseudomonas aeruginosa</i> that is difficult to eradicate by antibiotic treatment. Two key <i>P. aeruginosa</i> antibiotic resistance mechanisms are the AmpC β-lactamase that degrades β-lactam antibiotics and MexXYOprM, a three-protein efflux pump that expels aminoglycoside antibiotics from the bacterial cells. Levels of antibiotic resistance gene expression are likely to be a key factor in antibiotic resistance but have not been determined during infection. The aims of this research were to investigate the expression of the <i>ampC</i> and <i>mexX</i> genes during infection in patients with CF and in bacteria isolated from the same patients and grown under laboratory conditions. <i>P. aeruginosa</i> isolates from 36 CF patients were grown in laboratory culture and gene expression measured by reverse transcription-quantitative PCR (RT-qPCR). The expression of <i>ampC</i> varied over 20,000-fold and that of <i>mexX</i> over 2,000-fold between isolates. The median expression levels of both genes were increased by the presence of subinhibitory concentrations of antibiotics. To measure <i>P. aeruginosa</i> gene expression during infection, we carried out RT-qPCR using RNA extracted from fresh sputum samples obtained from 31 patients. The expression of <i>ampC</i> varied over 4,000-fold, while <i>mexX</i> expression varied over 100-fold, between patients. Despite these wide variations, median levels of expression of <i>ampC</i> in bacteria in sputum were similar to those in laboratory-grown bacteria. The expression of <i>mexX</i> was higher in sputum than in laboratory-grown bacteria. Overall, our data demonstrate that genes that contribute to antibiotic resistance can be highly expressed in patients, but there is extensive isolate-to-isolate and patient-to-patient variation.