There have been a number of reports recently that mother's breast milk may be a source of beneficial bacteria that colonise her infant's gastrointestinal tract.1,–,16 The human gut is the home of a large community of bacteria which plays a part in a range of activities which contribute to our health. Occupying the colon, where their collective number of cells exceeds that of their host by a factor of 10, the ‘gut microbiota’ enjoy an intimate and mutually beneficial relationship with the multicellular organism they inhabit. With metabolic activities as diverse and complex as those of the liver, they can be considered an organ in their own right.17 These ‘friendly’ bacteria are crucial for the maintenance of health at all stages of life, contributing to immune function and defence against infections, protecting against some cancers and digestive diseases, synthesising micronutrients and concluding the digestion of food components which escape assimilation in the small intestine.18

The microbiota of the gastrointestinal tract are acquired at birth and their rate, type and pattern of colonisation may have immediate and long-term positive effects on health. While it has been assumed that they are acquired chiefly during vaginal delivery from mother's lower bowel, new reports suggest that they may also have their origin inside her breast. Mother's milk supplies the newborn with a large number of non-nutritional bioactive substances which assist in adaptation of the fetus to extrauterine life,19 and account for the superiority of human milk over artificially synthesised alternatives. While it is plausible that certain ‘probiotic’ bacteria (defined as endogeneous bacteria that confer health benefits to their host20) are among them, certain criteria must be fulfilled before they can be added to the long list of maternally derived, milkborne substances that benefit the newborn. …