Got bacteria? The astounding, yet not-so-surprising, microbiome of human milk

• The milk microbiome is generally dominated by Staphylococcus, Streptococcus, and Pseudomonas.
• Milk microbiome varies across studies, but methodologic differences may contribute to this.
• The origin of milk microbes probably includes transfer from the maternal gastrointestinal tract.
• How variation in the milk microbiome influences acute and chronic infant health is not understood.

Contrary to long-held dogma, human milk is not sterile. Instead, it provides infants a rich source of diverse bacteria, particularly microbes belonging to the Staphylococcus, Streptococcus, and Pseudomonas genera. Very little is known about factors that influence variation in the milk microbiome among women and populations, although time postpartum, delivery mode, and maternal factors such as diet and antibiotic use might be important. The origins of the bacteria in milk are thought to include the maternal gastrointestinal tract (via an entero-mammary pathway) and through bacterial exposure of the breast during nursing. Currently, almost nothing is known about whether variation in microbe consumption by the infant via human milk and that of the mammary gland, itself, impacts short-term and/or long-term infant and maternal health although several studies suggest this is likely. We urge the clinical and public health communities to be patient, however, in order to allow human milk and lactation researchers to first understand what constitutes ‘normal’ in terms of the milk microbiome (as well as factors that impact microbial community structure) prior to jumping the gun to investigate if and how this important source of microbes impacts maternal and infant health.

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