London - Babies fed milk expressed with a breast pump may be at a higher risk of asthma, research suggests.
This is because potentially harmful bacteria found on the surface of pumps could end up in the mother's milk.
A study found those who exclusively breastfeed are more likely to produce milk containing "good" bacteria.
But those who express milk have higher levels of two potentially harmful bacteria, according to researchers from the University of Manitoba in Canada. These may cause changes in a baby's gut bacteria linked to asthma.
Previous research has shown that 12.5 percent of babies fed using a breast pump develop asthma by their third birthday. This compares with 8.8 percent of babies exclusively breastfed.
Part of the reason could be the bacteria Stenotrophomonas and Pseudomonadaceae, which might increase the risk of asthma. If breast pumps are not properly sterilised, they may carry these bugs.
The Canadian researchers conducted genetic analysis on the milk of 393 women with babies aged three and four months. The milk of those who exclusively breastfed their babies was compared with that of those who had pumped at least once in the previous fortnight.
Almost half of exclusively breastfeeding mothers produced milk containing "good" Bifidobacterium thought to balance the gut compared with fewer than a third of mothers who used breast pumps.
This bacteria is vital for growing babies and is thought to get into their guts through creamy breast milk that protects it from being destroyed by stomach acid.
It could prevent asthma, which is caused by an overreaction of the immune system, by "training" the baby's gut to react normally to bacteria. Disruption of gut bacteria is also linked to allergies and obesity in children.
The findings, published in the journal Cell Host and Microbe, suggest it is both lower levels of good bacteria and higher levels of the potentially harmful ones in breast pump milk that could put babies at greater risk.
The good bacteria are thought by many experts to come from the mouths of the babies themselves, getting into the breast via their saliva. If a baby drinks pumped milk from a bottle, instead of suckling at the breast, this cannot happen.