Species Colonization in Infancy: A Global Cross-Sectional Comparison by Population History of Breastfeeding.

species are beneficial and dominant members of the breastfed infant gut microbiome; however, their health benefits are partially species-dependent. Here, we characterize the species and subspecies of in breastfed infants around the world to consider the potential impact of a historic dietary shift on the disappearance of subsp. in some populations. Across populations, three distinct patterns of colonization emerged: (1) The dominance of subspecies , (2) prevalent of multiple species, and (3) the frequent absence of any These patterns appear related to a country's history of breastfeeding, with infants in countries with historically high rates of long-duration breastfeeding more likely to be colonized by subspecies compared with infants in countries with histories of shorter-duration breastfeeding. In addition, the timing of infant colonization with subsp. is consistent with horizontal transmission of this subspecies, rather than the vertical transmission previously reported for other species. These findings highlight the need to consider historical and cultural influences on the prevalence of gut commensals and the need to understand epidemiological transmission patterns of and other major commensals.