The ‘Appelmans protocol’ is used by Eastern European researchers to generate therapeutic phages with novel lytic host ranges. Phage cocktails are iteratively grown on a suite of mostly refractory bacterial isolates until the evolved cocktail can lyse the phage-resistant strains. To study this process, we developed a modified protocol using a cocktail of three Pseudomonas phages and a suite of eight phage-resistant (including a common laboratory strain) and two phage-sensitive Pseudomona aeruginosa strains. After 30 rounds of selection, phages were isolated from the evolved cocktail with greatly increased host range. Control experiments with individual phages showed little host-range expansion, and genomic analysis of one of the broad-host-range output phages showed its recombinatorial origin, suggesting that the protocol works predominantly via recombination between phages. The Appelmans protocol may be useful for evolving therapeutic phage cocktails as required from well-defined precursor phages.
The use of bacteriophages (phages) to treat bacterial infections, known as phage therapy, has a history substantially longer than that of antibiotics, yet these drugs have been the treatment of choice in the West for over 60 years owing to efficacy, low toxicity and ease of production. Bacteria are becoming increasingly resistant to antibiotics while efforts to discover new agents have drastically reduced. Phages have co-evolved with their hosts over billions of years and have acquired mechanisms to counter bacterial defences such as extracellular biofilm production, which severely reduces the effectiveness of conventional antibiotics. Recent animal and human trials show phages to be safe, well-tolerated agents with a bright future as an alternative to chemical agents.
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