Contrary to traditional belief, breast milk is not a sterile fluid, even in healthy women. Breast milk may contain pathogenic bacteria that could cause serious infections especially in premature infants. There is no recommendation to evaluate breast milk routinely for pathogenic bacteria. However discontinuation of breastfeeding is not recommended even the mother had mastitis, because it is believed this will not pose a risk to infant. This is the report of a premature infant born at the 32nd gestational week who was readmitted to neonatal intensive care unit with late-onset Staphylococcus aureus sepsis. While searching for the transmission route of the infection, the breast milk was assessed. Although the mother did not have any symptoms of breast inflammation, S. aureus isolates, genetically 99% similar to those from the infant blood samples, were documented in the breast milk.
Conclusion: Breast milk may contain pathogenic bacteria, even when expressed, stored and transported properly. When evaluating the source of a S. aureus infection in preterm infants, breast milk might be the source of the infection, even if the mother has no sign of mastitis.