Re-evaluating what makes a phage unsuitable for therapy

One of the biggest challenges in phage therapy is selecting the ‘right’ phage. What constitutes a ‘good’ phage is unclear, but regulators are converging on exclusion criteria for ‘bad’ phages. Here, we examine three commonly applied criteria: the lack of virulence/antibiotic resistance, the inability to transduce, and being strictly lytic (virulent). We assess the risk posed, the tools for determining the criteria, and the potential impact of the criteria.