In this week's PNAS Early Edition, investigators describe the intestinal microbiome of a premature infant, in which they characterized "strain-level genomic divergence during the first three weeks of life." During colonoziation, the team identified three compositional phases, and in addition, genotypic variation that could be "responsible for divergent strain ecologies, including hotspots of sequence variation in regulatory genes and intergenic regions, and in genes involved in trans