Human milk represents a source of bacteria for the initial establishment of the oral (and gut) microbiomes in the breastfed infant, however, the origin of bacteria in human milk remains largely unknown. While some evidence points towards a possible endogenous enteromammary route, other authors have suggested that bacteria in human milk are contaminants from the skin or the breastfed infant mouth. In this work 16S rRNA sequencing and bacterial culturing and isolation was performed to analyze the microbiota on maternal precolostrum samples, collected from pregnant women before delivery, and on oral samples collected from the corresponding infants. The structure of both ecosystems demonstrated a high proportion of taxa consistently shared among ecosystems, Streptococcus spp. and Staphylococcus spp. being the most abundant. Whole genome sequencing on those isolates that, belonging to the same species, were isolated from both the maternal and infant samples in the same mother-infant pair, evidenced that in 8 out of 10 pairs both isolates were >99.9% identical at nucleotide level. The presence of typical oral bacteria in precolostrum before contact with the newborn indicates that they are not a contamination from the infant, and suggests that at least some oral bacteria reach the infant’s mouth through breastfeeding.

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