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.