In their first month of life, breastfed babies get around a third of their gut bacteria from breastmilk, according to US research.

A mother's breast milk contains good bacteria that is planted in the gut of breastfed babies, a study has found.

US researchers say the finding highlights the importance of breastfeeding in establishing the life-long digestive health of infants.

The microbiota, a community of trillions of good and bad bacteria living in the gut, has been linked to numerous health outcomes, including asthma, autism and cancer.

But little is known about the transfer of breast milk microbes from mother to infant.

Scientists at the University of California conducted a study of 107 healthy mother-infant pairs where bacterial composition was identified through the sequencing of the 165 ribosomal RNA gene in breast milk, areolar skin and infant stool samples.

They found the breastfed babies got around a third of their gut bacteria from breastmilk in the first month of life.

Another 10 per cent comes from the skin around the nipple, called the areola.

The more a baby was breastfed, the more bacteria they had from breastmilk in their guts.

"Our study confirms a bacterial community in breast milk and tracks that community from mothers into the infant gut. Breast milk bacteria influence the establishment and development of the infant microbiome with continued impact after solid food introduction.

"Our results emphasise the importance of breastfeeding in the assembly of the infant gut microbiome," the authors wrote.

The study was published in medical journal JAMA Pediatrics.