Immunogenic profiling of structurally distinct bacteriophages and their interaction with human cells

The increase of multi-resistant bacteria highlights that the golden era of antibiotics is ending and that alternative treatmentsare urgently needed. Phages have been historically used to treat bacterial infections prior to the discovery of antibiotics and have gained renewed interest in the past decade. Despite the advantages of phage therapy over traditional antibiotic usage, a number of concerns persist over their clinical application centring on their efficacy and safety. This thesis presents four papers that focus on the isolation and characterization of phages that target reference strains and drug-resistant strains of E. coli as well as their infection dynamics and kinetics. In Paper I, six of thirty isolated phages were selected to be characterized for their growth parameters and host range using two commonly used methods. The study showed that the host range (an important selection criteria for phages) of the phages can change based on the assessment method and that the lysis efficiency of phages is host-dependent. The study suggests that standardised methods to assess the host range and lytic activity of phages are required to reduce result variability between research groups. Paper II investigated a rare phage with C3 morphotype from the Podoviridae family and characterised it via genomic, proteomic, morphologic and phylogenetic analysis. The study revealed previously unseen aspects including the formation of a honeycomb structure comprised of phage head during DNA packaging, the possible contractile nature of the tail and the 280 million year co-evolution between the major head protein and the scaffolding protein. Paper III highlights the need to take the immune system into consideration when designing phage therapeutics. In the study, four purified structurally distinct phages (selected from the three main phage families) were exposed to human cells (HT-29 and Caco-2 immortalised intestinal epithelial cell lines and donor-derived peripheral blood mononuclear cells) and the immunogenicity of the phages determined. Phage immunogenicity was shown to vary in a concentration and phage dependent manner with SU63 (a Myoviridae) being the most immunogenic phage and SU32 (a Siphoviridae) the least immunogenic. In the presence of human cells and a suitable host, phages were shown to maintain their killing efficacy as well as the ability to proliferate. Paper IV studies the infection dynamics of an experimental two-phage cocktail against a single bacterial host in vitro and in silico. However, in silico analysis and in vitro analysis produced conflicting results, in which mathematical modelling predicted the complete clearance of bacteria for all treatment scenarios whereas experimental results showed a 1-3log10 reduction in bacterial content. Practical experiments also showed increased anti-bacterial activity when the time between the additions of each phage was varied. This discrepancy suggests that the current mathematical model is unsuitable due to the inability to account for discrete variables such as interference.