Breast milk contains bacterial populations thought to influence microbiota composition in the infant gut. However, the origin of bacteria in milk and the process of milk microbe-mediated seeding of the infant gut remains relatively unknown. In this study we hypothesized that breast milk and infant stool microbiota are influenced by sites of mom-infant interaction, especially by the breastfeeding process. To test out our hypothesis we used 16S rRNA and shotgun metagenomic sequencing to identify core breast milk microbes of mom-infant pairs (breastfed and never-latched) and their potential sources. Our results show that microbial communities from various body sites had distinct microbial communities. Breast milk samples have the lowest overall biomass and feces the largest. Overall, detection of fungal and viral sequences was limited, with dominance of bacterial populations on all sampled sites. In the breastfed group, 27 operational taxonomic units (OTUs) were found in 85% of all breast milk samples, and were defined as ‘core’ breast milk bacteria. These included Bifidobacteria, Enterobacteria, Corynebacterium, Lactobacillus, Neisseria, Haemophilus, Staphylococcus, and Streptococcus. Our sourcetracker analysis of latched breast milk correlated bacterial communities with those primarily derived from areolar skin and infant oral sites. To assess retrograde microbial transfer from the infant’s mouth to breast milk, 16S rRNA sequencing was performed on never-latched mom-infant pairs. Breast milk, maternal areolar skin, and infant stool communities were significantly less diverse in never-latched individuals compared to latched pairs. There was remarkable dominance of Staphylococcus in never-latched milk. To clarify the abundance of Staph in these milk samples, we will quantitatively measure the amount of Staphylococcus by utilizing genus specific qPCR measured relative to total bacterial DNA. The microbiota of infant stool was sequenced and analyzed with SourceTracker in an attempt to identify the contributions of breast milk microbiota. During the first month of life, 26±10% of gut microbes are thought to be attributed from breast milk in breastfeeding infants. All genera present in the core breast milk microbiome were found in infant stool. Mode of delivery explained 5% of the bacterial variance in milk and infant stool (p=0.023). In one mom-infant pair, a distinct strain of Bifidobacteria breve was identified that is identical in maternal breast milk, rectum, and infant stool suggesting direct transmission. Interestingly, our data indicate a connection between Bifidobacteria breve in maternal gut and breast milk consistent with previous murine studies suggesting that intestinally-derived bacteria translocate to the mammary gland. Our study showed that although breast milk had a lower overall biomass relative to other body sites, it appears to play an important role in seeding the infant. Additionally, our data suggest that breastfeeding is a potentially important mechanism in the propagation of breast milk microbes through retrograde flux via infant oral and areolar skin contact.