Breast milk contains more than 700 species of bacteria, which is more than previously believed, according to a new study.

Breast milk from overweight mothers or those who put on more weight than recommended during pregnancy, however, had fewer species of bacteria, according to the Spanish researchers.

They also found that mothers who had a planned cesarean-section delivery had fewer species of bacteria in their breast milk than those who had a vaginal birth, while mothers who had an unplanned C-section had about the same number of species of those who had a vaginal birth.

This suggests that a mother's hormonal state at the time of birth may affect the diversity of bacteria species in her breast milk, according to the study by Raul Cabrera-Rubio and colleagues, which was published in the American Journal of Clinical Nutrition.

Breast milk plays an important role in determining the composition of the bacterial community in a child's digestive system, noted a news release from the Spanish Foundation of Science and Technology.

The study authors are now trying to determine if bacteria in breast milk help babies digest the milk or if they play a role in immune system development. The investigators said their research could lead to improved child nutrition.

"If the breast milk bacteria discovered in this study were important for the development of the immune system, its addition to infant formula could decrease the risk of allergies, asthma and autoimmune diseases," they wrote.