Scientists say breastfeeding mums who use pumps have more potentially disease-causing bacteria in their breast milk, and fewer friendly bugs, than breastfeeding mums who never use pumps. The researchers checked out the bacteria in milk from 393 mums, three to four months after giving birth, and found the milk of mums who pumped contained more Stenotrophomonas and Pseudomonadaceae bacteria – both of which can turn nasty – while the milk of mums who never pumped had a richer diversity of friendly microbes typically found in the mouth.

This may explain why babies fed pumped milk are more likely to develop asthma than those fed directly from the breast, the researchers say. The study, published in Cell Host & Microbe, suggests that these oral bacteria are likely being transferred from the infant’s mouth to the mother’s milk, while the nasties are probably being picked up from the pumps.

“To our knowledge, this is among the largest studies of human milk microbiota performed to date,” says senior study author Meghan Azad, a researcher at Children’s Hospital Research Institute of Manitoba and a Canada Research Chair in Developmental Origins of Chronic Disease at the University of Manitoba. “This study considerably expands our understanding of the human milk microbiota and the factors that might influence it. The results will inspire new research about breastfeeding and human milk, especially related to pumping.”

Although previously considered sterile, breastmilk is now known to contain a low abundance of bacteria. While the complexities of how maternal microbiota influence the infant microbiota are still unknown, this complex community of bacteria in breastmilk may help to establish the infant gut microbiota. Disruptions in this process could alter the infant microbiota, causing predisposition to chronic diseases such as allergies, asthma, and obesity. Although recent studies on human milk microbiota suggest that it might be affected by various factors, these findings have not been reproduced in large-scale studies, and the determinants of milk microbiota are still mostly unknown.

To address this gap in knowledge, Azad and her collaborators carried out bacterial gene sequencing on milk samples from 393 healthy mothers three to four months after giving birth. They used this information to examine how the milk microbiota composition is affected by maternal factors, early life events, breastfeeding practices, and other milk components. The researchers found a high degree of variability in the milk microbiota across mothers. Among the many factors analyzed, the mode of breastfeeding – whether mothers provided milk with or without a pump — was the only consistent factor directly associated with the milk microbiota composition.

In future studies, the researchers will further explore the composition and function of the milk microbiota. In addition to bacteria, they will profile fungi in the milk samples. They also plan to investigate how the milk microbiota influences both the gut microbiota of infants and infant development and health. Specifically, their projects will examine the association of milk microbiota with infant growth, asthma, and allergies.

“This work could have important implications for microbiota-based strategies for early-life prevention of chronic conditions,” Azad says.

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