The Bacterial Ecosystem of Mother's Milk and Infant's Mouth and Gut

Abstract

The progressive building of the infants' gut microbiota is pivotal for educating their immune system. Human breast milk is among the first sources of microbes for the assembly of the infant's microbiota, but research struggles to give a demonstration for the origin of bacteria in milk. Aiming at contributing to the knowledge on assembly of the mother's milk and infant's microbiome, here we characterized the oral, gut and milk ecosystems in a homogeneous cohort of 36 healthy mother-infants pairs, by 16S rRNA next-generation sequencing. A limited number of operational taxonomic units (OTUs) was shared among the three ecosystems, including not only OTUs assigned to the well-known immune-modulating Bifidobacterium genus, but also specific Streptococcus and Staphylococcus OTUs, which were dominant in the infant's mouth ecosystem. The high conservation of these OTUs among the three ecosystems seems to call for a worth exploring ecological role through targeted and/or culture-dependent techniques. Notwithstanding the limitations of a 16S rRNA gene-based molecular characterization, we might hypothesize that the baby's mouth, being the transition point for the milk to reach the intestine, could play a role in both the gut microbiota assembly, via deglutition, and mother's milk duct colonization, during suction.

Keywords: breastfeeding; infant gut microbiota; infant oral microbiota; microbiota assembly; milk microbiota; term infants.

FIGURE 1

Diversity in the bacterial ecosystem of the mother’s milk, and infant’s feces and mouth. PCoA based on unweighted (A) and weighted (B) UniFrac distances of the microbiota of mother’s milk (light blue), infant feces (yellow), and infants mouth (pink). Samples are identified by filled circles. In both PCoA first and second principal components (PCo1 and PCo2) are plotted. The percentage of variance in the dataset explained by each axis is reported. Box and whiskers distribution of the Shannon α-diversity index (C), intra-group unweighted UniFrac distances (D), and intra-group weighted UniFrac distances (E), calculated for milk (light blue), fecal (yellow) and oral (pink) samples. Significant differences between datasets are indicated, as calculated using Tukey post hoc test after Kruskal–Wallis test for multiple comparisons (^∗P ≤ 0.001, ^∗∗P ≤ 0.0001).

FIGURE 2

Average composition of the bacterial community in mother’s milk and infant’s mouth and feces. For each group of samples, a pie chart based on the average relative abundance (%) at family level is plotted. Bacterial families with relative abundance ≥0.2% in at least 10% of the samples are depicted. Colors for each family are reported in the legend.

FIGURE 3

Relationship between pre- and post-breastfeeding infant oral microbiota. PCoA based on unweighted UniFrac distances of the microbiota of the infant’s mouth sampled before (empty circles) and after (filled circles) breastfeeding. Samples from the same subject are connected by a black line. The first and second principal components (PCo1 and PCo2) are plotted. The percentage of variance in the dataset explained by each axis is reported.

FIGURE 4

Operational taxonomic unit (OTU) sharing between mother’s milk, and infant’s fecal and oral bacterial ecosystems. Venn diagrams showing the average number of OTUs shared between the bacterial communities of the mother’s milk (light blue), the infant’s feces (yellow) and the infant’s mouth (pink), the latter sampled before (A) and after (B) breastfeeding. The total number of OTUs for each ecosystem is reported in the boxes outside the circles, expressed as mean and range (in brackets).