That's hotly debated. Even the study you cite cannot rule out skin contamination.
But unequivocally showing the existence of entero-mammary pathway (translocation of maternal gut bacteria from the gut to the mammary glands) would be very exciting news indeed.
Yes, because if you are buying breast milk from a woman who used a breast pump, then the skin contamination is very very little compared to actual breast suckling.
But unequivocally showing the existence of entero-mammary pathway (translocation of maternal gut bacteria from the gut to the mammary glands) would be very exciting news indeed.