Repertoire of human breast and milk microbiota: a systematic review

Breastfeeding is a major determinant of human health. Breast milk is not sterile and ecological large-scale sequencing methods have revealed an unsuspected microbial diversity that plays an important role. However, microbiological analysis at the species level has been neglected while it is a prerequisite before understanding which microbe is associated with symbiosis or dysbiosis, and health or disease. We review the currently known bacterial repertoire from the human breast and milk microbiota using a semiautomated strategy. Total 242 articles from 38 countries, 11,124 women and 15,489 samples were included. Total 820 species were identified mainly composed of Proteobacteria and Firmicutes. We report variations according to the analytical method (culture or molecular method), the anatomical site (breast, colostrum or milk) and the infectious status (healthy control, mastitis, breast abscess, neonatal infection). In addition, we compared it with the other human repertoires. Finally, we discuss its putative origin and role in health and disease.