The Microbiota of the Human Mammary Ecosystem

Abstract

Human milk contains a dynamic and complex site-specific microbiome, which is not assembled in an aleatory way, formed by organized microbial consortia and networks. Presence of some genera, such as Staphylococcus, Streptococcus, Corynebacterium, Cutibacterium (formerly known as Propionibacterium), Lactobacillus, Lactococcus and Bifidobacterium, has been detected by both culture-dependent and culture-independent approaches. DNA from some gut-associated strict anaerobes has also been repeatedly found and some studies have revealed the presence of cells and/or nucleic acids from viruses, archaea, fungi and protozoa in human milk. Colostrum and milk microbes are transmitted to the infant and, therefore, they are among the first colonizers of the human gut. Still, the significance of human milk microbes in infant gut colonization remains an open question. Clinical studies trying to elucidate the question are confounded by the profound impact of non-microbial human milk components to intestinal microecology. Modifications in the microbiota of human milk may have biological consequences for infant colonization, metabolism, immune and neuroendocrine development, and for mammary health. However, the factors driving differences in the composition of the human milk microbiome remain poorly known. In addition to colostrum and milk, breast tissue in lactating and non-lactating women may also contain a microbiota, with implications in the pathogenesis of breast cancer and in some of the adverse outcomes associated with breast implants. This and other open issues, such as the origin of the human milk microbiome, and the current limitations and future prospects are addressed in this review.

Keywords: biological functions; breast cancer; breast tissue; human milk; mastitis; microbiome; microbiota; vertical transfer.

Figure 1

Historical perspectives of the main milestones in research focused in human milk microbes.

Figure 2

Current and future approaches to study the composition and functions of the human milk microbiota.

Figure 3

Factors affecting the composition of the human milk microbiota. HMO, human milk oligosaccharides; HLA, human leukocyte antigen; BMI, body mass index; IgA, immunoglobulin A.

Figure 4

Potential sources of the microbes present in human milk and interactions with other mother-infant microbiotas. Dashed arrows represent potential translocation through an endogenous pathway.