Scientists say breastfeeding mums who use breast pumps have more potentially disease-causing bacteria in their breast milk, and fewer friendly bugs, than breastfeeding mums who never use pumps. However, the researchers behind the study say that it doesn’t follow that expressed milk is bad for infants.

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, the researchers, led by Meghan Azad, a researcher at Children’s Hospital Research Institute of Manitoba, 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.

The study, published in Cell Host & Microbe, found the milk of mums who pumped contained more Stenotrophomonas and Pseudomonadaceae bacteria while the milk of mums who never pumped had a richer diversity of friendly microbes typically found in the mouth.

However, Azad emphasised that the study doesn’t show that pumped milk is bad.

“Breast milk is beneficial for many reasons, and for some moms, pumping might be the only way they can provide breast milk to their babies for a variety of reasons,” she told INSIDER. “So we certainly don’t want to discourage pumping, but rather raise the question of what does this mean and what further research needs to be done.”

One line of future research is likely to be investigating whether babies who are fed pumped milk are more likely to develop asthma than those fed directly from the breast.

The researchers intend to further explore the composition and function of the milk microbiota and 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.

Editor’s note: This article has been updated. The original version incorrectly conveyed a negative slant towards expressed breast milk.

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