Beyond the Bacterial Microbiome: Virome of Human Milk and Effects on the Developing Infant

Human milk microbes play an important role in infant health and disease. Emerging evidence shows that human milk viruses are also transmitted from the mother to the infant via breastfeeding. These viruses include eukaryotic viruses, bacterium-infecting viruses called bacteriophages, and other viral particles. Human milk viruses are instrumental in shaping the infant gut virome and microbiome. Eukaryotic DNA and RNA viruses contribute to pathogenic challenges and protection. Bacteriophages have the ability to kill bacteria or supply them with potentially beneficial gene functions, thereby shaping the microbiome. The early infant virome is dominated by bacteriophages that likely contribute to a highly dynamic microbiome in the early life. There is a critical window of early childhood growth with rapid maturation of metabolic, endocrine, neural, and immune pathways. The colonization of microbes in the infant body during this time plays an important role in the establishment and maturation of these pathways. The virome transmitted via breastfeeding may also be particularly important at these critical time points of immune development. More longitudinal studies of mother-infant pairs will help to better define the human milk virome and their functional impact on the development of the growing infant.