Chapter 17 - Human Milk Microbes – Summary and Research Gaps

Abstract

Previous chapters in this book have detailed the history, presence, origin, and variation in the complex ecosystem of bacteria found in human milk. Based on the variety of studies presented, it is clear that human milk is not sterile and instead is a potential bacterial inoculum to the infant. However, current methodologies for determining the presence, identification, and amount of bacterial taxa in milk may provide bias in what we know; this area remains a fertile realm of important investigation. Furthermore, variation of bacteria found in human milk has been related to a number of factors, including maternal genetics, body composition, nutrition, delivery mode, antibiotic use, and concentration and profile of human milk oligosaccharides. However, very little is known about whether these relationships are causal in nature. Additionally, there are many potential sources of the bacteria in milk including specific trafficking of bacteria from the maternal gastrointestinal tract, reverse flow from the infant's oral cavity, and migration from the skin of the breast. It is also likely that infants fed expressed human milk potentially receive other bacteria in their feedings from milk collection, storage, and suckling devices—all of which are exposed to environmental bacteria. It is noteworthy that, to date, much of our knowledge regarding bacteria in human milk reflects the pathogenic properties of bacteria during mastitis, a state of mammary inflammation with significant impact on lactation success. The characteristics of the organisms causing mastitis and their relationship to other bacterial community members is an important field of study in our understanding of the human milk microbiome in healthy and ill women. Treatment and prevention of mastitis through probiotics may support lactation better than antibiotic therapy. In summary, the emerging field of the human milk microbiome represents a paradigm shift in terms of both maternal and infant health. As with the focus that has been placed on understanding the microbial communities in other niches of the human body, we recommend substantial resources be directed to understanding the human milk microbiome, its origins and variability, and how it is related to maternal and infant health around the globe.