Bacteriophage-based therapy in cystic fibrosis-associated Pseudomonas aeruginosa infections: rationale and current status

Abstract

Pulmonary infections involving Pseudomonas aeruginosa are among the leading causes of the deterioration of the respiratory status of cystic fibrosis (CF) patients. The emergence of multidrug-resistant strains in such populations, favored by iterative antibiotic cures, has led to the urgent need for new therapies. Among them, bacteriophage-based therapies deserve a focus. One century of empiric use in the ex-USSR countries suggests that bacteriophages may have beneficial effects against a large range of bacterial infections. Interest in bacteriophages has recently renewed in Western countries, and the in vitro data available suggest that bacteriophage-based therapy may be of significant interest for the treatment of pulmonary infections in CF patients. Although the clinical data concerning this specific population are relatively scarce, the beginning of the first large randomized study evaluating bacteriophage-based therapy in burn infections suggests that the time has come to assess the effectiveness of this new therapy in CF P. aeruginosa pneumonia. Consequently, the aim of this review is, after a brief history, to summarize the evidence concerning bacteriophage efficacy against P. aeruginosa and, more specifically, the in vitro studies, animal models, and clinical trials targeting CF.

Keywords: bacterial infection; multidrug resistance; pneumonia; pulmonary infection.

Figure 1

In vitro steps before phage use for in vitro or in vivo models. Notes: (A) Specific bacterial activity: the Pseudomonas aeruginosa strains of interest (eg, Strain 1 and Strain 2) are co-cultured on agar plates with a phage suspension to verify the lytic activity of the phages against known bacterial strains. (B) Phage suspension is titrated with the double agar overlay plaque assay according to Gratia’s method. Briefly, serial dilutions of a phage suspension are added to bacteria broth in dilute agar or agarose matrix (“top agar” or “overlay”). After uniform mix, solidification is obtained on standard agar plates (“bottom agar” or “underlay”). After incubation, lysis is visualized as clear areas called plaques and corresponding to decreased density of bacterial lawn due to the lytic activity of the phages. Extrapolation of the initial phage suspension contents is obtained by extrapolation from the last dilution giving a plaque. Phage quantity is therefore expressed as plaque-forming units. Abbreviations: LB, Luria-Bertani; CFU, colony-forming units.

Figure 2

Example of Pseudomonas aeruginosa-specific high-titer phage-production protocol. Target strains of P. aeruginosa are incubated separately for 24 hours in the early exponential phase (OD 600 nm =0.1) with a known phage cocktail at a 0.035 multiplicity of infection (= volume of phage suspension/volume of bacterial suspension). After 24 hours, the obtained lysates are centrifuged at 5,000× g for 30 minutes at 4°C. Supernatant is filtered through a 0.2 μm membrane and stored at 4°C. Abbreviations: CFU, colony-forming units; OD, optical density.