Breast milk may contain more than 700 species of bacteria, according to a Spanish study.
And slimmer mothers pass on more bacteria, according to the analysis at the Spanish Institute of Agrochemistry and Food Technology.
Researchers tested the colostrum – the first milk expressed by mothers – and milk delivered after six months of breast-feeding.
As well as milk-based bacteria, such as lactococcus, the researchers found common organisms such as Staphylococcus and Streptococcus in the colostrum.
The study also showed that women who are able to undergo natural birth the milk is "rich" in micro-organisms – and this also applies to unplanned pregnancies.
The researchers say they need to establish what role the bacteria perform.
They may be helping the baby’s digestive system or they may mainly be "tuning" its immune system.
The findings have been reported in the American Journal of Clinical Nutrition.
Researcher María Carmen Collado said: "This is one of the first studies to document such diversity using the pyrosequencing technique (a large scale DNA sequencing determination technique) on colostrum samples on the one hand, and breast milk on the other, the latter being collected after one and six months of breastfeeding."
The human milk microbiome changes over lactation and is shaped by maternal weight and mode of delivery. American Journal of Clinical Nutrition 96(3):544-551, 2012.
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