Rapid and quantitative measurement of bacteriophage infectivity via fully automated droplet digital PCR.

The clinical translation of phage therapy for multidrug-resistant infections is constrained by the lack of rapid, standardized therapeutic phage selection. Here, we introduce digital phage susceptibility testing (dPhaST), an automated droplet digital PCR workflow that quantifies phage-induced DNA release as a molecular signature of lysis. By targeting conserved 16S rRNA regions, dPhaST measures lytic activity across diverse bacterial pathogens within 3 h. Across 122 phage-host combinations involving 19 bacterial strains from six species, dPhaST shows 95.9% concordance with spot tests while resolving weak and heterogeneous lytic activities that are not readily distinguished phenotypically. It remains robust during the early infection window despite phage-encoded nuclease activity and tolerates phage cross-contamination better than spot tests. The method captures defense-mediated interactions involving CRISPR-Cas and Sir2-HerA systems. In this work, we show that automated digital quantification enables rapid and mechanistically informative profiling of early phage lytic efficacy across Gram-positive and Gram-negative pathogens.