Phage susceptibility testing methods or 'phagograms': where do we stand and where should we go?

Phage therapy is a highly promising approach to address the challenge that is presented by the global burden of antimicrobial resistance. Given the natural specificity of phages, phage susceptibility testing (PST) is a prerequisite for successful personalized therapy, allowing the selection of active phages from large and diverse collections. However, the issue of an easy-to-use and standardized technique remains. In this review, we describe the principles, advantages and drawbacks of two routinely used PST techniques: plaque and growth kinetic assays. These are labour-intensive and time-consuming methods that require automation of one or more steps, including preparation of test panels, incubation, reading and analysis of results. In addition to automation, there is an urgent need to establish a reference method to enable efficient of PST techniques selection of therapeutic phages. We discuss knowledge gaps and parameters that need to be investigated to work towards this goal.