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

Summary

Human milk is an important source of microbes that colonize the infant gut in early life contributing to immune system maturation and protection against pathogen invasion [1, 2]. Emerging evidence shows that human milk viruses are also transmitted from mother to infant via breastfeeding [3]. These viruses include eukaryotic viruses, bacteria-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 patho- genic challenges and protection. The early infant virome is dominated by bacteriophages that likely contribute to a highly dynamic microbiome in early life. These bacteriophages have the ability to kill bacteria or supply them with potentially beneficial gene functions, thereby shaping the microbiome. Thus, the eukaryotic virus, bacteriophages, and bacteria coexist in the infant gut in an interdependent and dynamic relationship [4, 5]. Figure 1 shows the hypothesized trajectory of the microbial diver- sity during the first 2–3 years of life based on the few existing studies. 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 define the human milk virome and its functional impact on the development of the growing infant better.

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