A new study reveals that babies in Western countries like the UK, US and Sweden rarely have the Bifidobacterium longum infantis microbe in their guts by two months old, unlike infants in South Asia and sub-Saharan Africa. This difference may stem from variations in mothers' diets affecting breast milk composition. The findings could help tailor probiotics for premature babies based on region.
Researchers at the Wellcome Sanger Institute in the UK sequenced over 1,900 genomes of Bifidobacterium longum from stool samples collected in the UK, Sweden, the US, and seven countries in South Asia and sub-Saharan Africa: Bangladesh, Pakistan, Kenya, Malawi, Burkina Faso, Uganda and Zimbabwe.
The study focused on two subspecies: B. longum longum and B. longum infantis, both of which support immune system development and help prevent gut and blood infections. By two months old, about 70 percent of infants from the African and South Asian countries had B. longum infantis in their guts, compared to fewer than 2 percent of babies in the Western countries. "Infantis is basically missing in Western settings," said Yan Shao, the lead researcher.
In contrast, B. longum longum was present in roughly one-third of Western infants by the same age, but in fewer than 10 percent of those from Africa and South Asia. Shao suggested this regional divide likely arises because the subspecies thrive on different diets. "Bifidobacteria help to digest nutrients from breast milk, and the composition of this varies with the mother’s diet, so it could be that longum [longum] is better adapted to a Western diet while infantis thrives on diets in other regions," he explained. Infants lacking both subspecies likely have other similar bacteria, such as Bifidobacterium breve.
The research expanded the analyzed B. longum genomes from South Asia by about 17 times and from Africa by around 11 times. Lindsay Hall at the University of Birmingham called it "a huge step forward for underrepresented populations."
Probiotics, sometimes given to premature infants, could be customized by region based on these insights. For example, B. longum infantis might benefit babies in Africa and Asia but may not persist well in Western infants. The study appears in Cell (DOI: 10.1016/j.cell.2026.01.007).
