Assembly of the virome in newborn human infants.

Healthy human infants are typically born without high concentrations of viral particles in their intestines, but after a few weeks of life particle counts typically reach a billion per gram of stool. Where do these vast populations come from? Recent studies support the idea that colonization is stepwise. First pioneer bacteria seed the infant gut. Bacteria commonly harbor prophage sequences integrated in their genomes, which periodically induce to make particles, providing a first wave of viral particles. Later more viruses infecting human cells are detected. Analysis showed that lower accumulation of viruses that grow in human cells is associated with breastfeeding. Thus these studies emphasize the environmental influences on formation of the early life virome, and begin to point the way toward modulating viral colonization to optimize health.