Milk Microbiome and Neonatal Colonization: Overview

Summary

Breastfeeding confers the infant long-term health benefits by reducing the risk of obesity and noncommunicable diseases in later life. These effects may at least partially be mediated by the considerable impact breastfeeding has on the developing infant gut microbiome. Human milk contains molecules such as human milk oligosaccharides, one of the main functions of which is to selectively promote the growth of specific bacteria in the infant gut. In addition, human milk has relatively recently been discovered to harbor a distinct microbiome.

The origin, function, and significance of the bacteria in human milk
remain poorly understood. The milk microbiome includes obligate anaerobic bacteria, such as bifidobacteria, often considered to be characteristic of the intestinal microbiome (Table 1) [1]. Data from experimental and clinical studies suggest that the bacteria in milk may indeed originate in the maternal gut, and the existence of a specific enteromammary pathway has been suggested based on data from experimental and clinical studies [1, 2]. Increased bacterial translocation from the intestinal lumen to mesenteric lymph nodes has been observed during the perinatal period in mice [2]. In line with this observation, the human milk microbiota in mothers who have delivered by elective cesarean section is reportedly significantly different from that of mothers who have undergone labor (Table 2) [3]. In addition, the milk microbiome is affected by maternal health, metabolic state, and antibiotic use.

The functions of the milk microbiome and particularly its potential contribution to infant gut colonization are an area of active scientific research. The overall composition of the milk microbiome is clearly distinct from that of the infant gut microbiome. Results from clinical studies indicate, however, that specific bacterial taxa are detectable both in
human milk and infant feces [1]. The infant gut microbiome resembles more closely the microbiome in the milk of the infant’s own mother than that in the milk of an unrelated women [4]. Finally, based on source tracking analyses of samples from 107 mother-infant pairs, approximately 15%

of the fecal microbiome in predominantly breastfed infants originate from the bacteria in milk during the first 30 days of life [4]. Taken together, these data strongly suggest that the bacteria in human milk are the source of bacteria colonizing the neonatal gut.
The biological or clinical significance of the human milk microbiome and its role in infant gut colonization remain open questions. If the human milk microbiome is shown to contribute to the beneficial effects of breastfeeding, interesting new therapeutic avenues may be discovered.

Modulating or mimicking the milk microbiome may provide a novel means to affect early gut colonization and reduce the risk of noncommunicable diseases associated with aberrant gut colonization or suboptimal breastfeeding.

References
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