What does it take to satisfy Koch's postulates two centuries later? Microbial genomics and Propionibacteria acnes.

For two centuries, Koch's postulates have set the gold standard for establishing the microbiological etiology of infection and disease. Genomic sequencing now brings finer resolution to both bacterial strain variation and the host genetic state that may predispose to disease. In this issue of the JID, Fitz-Gibbons and colleagues present strain-based resolution of Propionibacterium acnes and its association with the common teenage malady acne vulgaris. Here I examine how Koch's postulates were envisioned and incorporate this finer resolution of both host and microbial states.