The guts of breast-fed babies often contain bacteria of the subspecies Bifidobacterium longum infantis, which keep at bay harmful microorganisms that might otherwise take up residence. The genome of this subspecies has now been published.
Of its 2,423 recognized protein-coding genes, 702 are not found in related bacterial taxa sequenced so far, write David Mills of the University of California, Davis, and his colleagues. The bacterium feeds on specific sugars in human milk that infants do not use, and Mills and his team have identified several genes that make this possible. Another gene cluster makes enzymes that allow the microbes to consume milk-borne urea in the nitrogen-poor environment of the infant bowel.