Longer lifespan and improved quality of life for patients with cystic fibrosis (CF) have been
attributed to, at least in part, advances in treating pulmonary infection in this disease. The
acquisition of ubiquitous environmental organisms and the development of inflammation and airway scarring as a result is a major hurdle to improving the lives of patients with CF. This is
especially true for Pseudomonas aeruginosa (Pa), the dominant respiratory pathogen in CF
patients, due to the development of antibiotic resistance as well as the development of a
biofilm mode of growth. The problem posed by this organism has necessitated exploration of
novel therapies, including bacteriophage.
As phage therapy in the context of CF is unlikely to be used in isolation, the first part of this
thesis explores the efficacy of a cocktail of 4 lytic bacteriophages when used as adjunctive
therapy to the commonly used antibiotics ceftazidime and tobramycin in high-throughput
models of Pa infection. Reduction in both biofilm biomass and cell viability was demonstrated
when combining phage with the two antibiotics in a biofilm model of infection across 16
different clinical Pa strains. To interrogate the effect of combination therapy more
thoroughly, checkerboard experiments combining phage and ceftazidime or tobramycin
individually in both planktonic and biofilm Pa culture were performed. Although there were
individual examples of synergy, there were also examples of antagonism. Efficacy of the
phage cocktail varied based on Pa strain, type of culture (biofilm vs. planktonic), as well as the
antibiotic used. For 3 of the 16 clinical strains used in these experiments, there was an
observed conversion to mucoid phenotype on Congo Red agar. As a small pilot study,
transcriptomic analysis on 1 of these strains was undertaken and demonstrated upregulation
of genes involved in alginate production with phage in both planktonic and biofilm culture,
mirroring the phenotypic findings seen.
Lastly, bacteriophage sensitivity was tracked over time with the objective of answering
questions pertaining to the recruitment of participants to prospective clinical trials involving phage. With the objective of knowing whether recruitment of patients could be based on historical sensitivity to phage, I selected 24 patients with chronic Pa infection and looked at
serial isolates over the course of 12 months, performing antibiotic sensitivity testing, plaque
assays as well as variable number tandem repeat (VNTR) typing of each isolate. Results
showed that even when representing a single Pa strain, there was a high degree of variability
in sensitivity to both bacteriophage and antibiotics. There were high rates of sensitivity to
bacteriophage – even in multi-drug resistant (MDR) strains -which supports the development
of this therapy. However, the observed variability implies that selection of patients in future
trials should be on the basis of current, not past, isolate testing.
Broadly speaking, my results with relation to the development of bacteriophage therapy in
CF lung disease are promising, showing broad efficacy in a large number of clinical Pa strains
in both biofilm and planktonic culture, both as adjunctive therapy with use of commonly used
anti-Pa antibiotics, and in MDR strains. The variability of results, both across strains, but also
in individual strains at different time points, should be taken into consideration as trials with
phage are developed.