Rage against the mean: a perspective on measuring fitness of individual phage particles

Abstract Traditional bacteriophage methods measure population averages rather than individual particle variation. This perspective advocates for approaches to quantify trait variation in phage particles. Emerging techniques in optical microscopy and flow cytometry can reveal previously-masked phenotypic heterogeneity, offering unprecedented insights across phage infection cycle: binding kinetics, genome entry, replication, coinfection dynamics, and particle stability. This shift from population averages to individual variation represents a critical frontier for phage biology and biotechnology.