Milk Microbiota: What Are We Exactly Talking About?

Abstract

The development of powerful sequencing techniques has allowed, albeit with some biases, the identification and inventory of complex microbial communities that inhabit different body sites or body fluids, some of which were previously considered sterile. Notably, milk is now considered to host a complex microbial community with great diversity. Milk microbiota is now well documented in various hosts. Based on the growing literature on this microbial community, we address here the question of what milk microbiota is. We summarize and compare the microbial composition of milk in humans and in ruminants and address the existence of a putative core milk microbiota. We discuss the factors that contribute to shape the milk microbiota or affect its composition, including host and environmental factors as well as methodological factors, such as the sampling and sequencing techniques, which likely introduce distortion in milk microbiota analysis. The roles that milk microbiota are likely to play in the mother and offspring physiology and health are presented together with recent data on the hypothesis of an enteromammary pathway. At last, this fascinating field raises a series of questions, which are listed and commented here and which open new research avenues.

Keywords: enteromammary pathway; mammary gland; metagenomics; microbial community; milk microbiota; offspring gastrointestinal microbiota.

FIGURE 1

Factors influencing milk and milk-associated microbiota and technical biases.

FIGURE 2

Milk and milk-associated microbiota in humans and animals: sampling sites and major taxa. Figure based on Supplementary Tables S1–S4. Red and orange taxa are shared between all human and animal species or present in three species out of five, respectively. For humans and bovines, taxa size reveals citation frequency.