[Development, equilibrium and role of microbial flora in the newborn]

Development of the digestive tract intestinal flora is the result of a specific selection process to which the multiple maternal or environmental bacteria that penetrate into the neonatal gut are subjected. In breast-fed infants, Escherichia coli and streptococci are the first bacteria to appear in the gut. They are usually, but not always, followed by a population of Bifidobacterium which quickly becomes predominant. In bottle-fed infants, the intestinal flora is more variable and often includes, in addition to the organisms mentioned above, other enterobacteria and a wider range of obligate anaerobes. Studies of experimental models have shown that the nature of milk fed to the offspring and even the lactating mother's diet have substantial effects on the sequence of development of the neonatal intestinal flora. A large number of factors capable of inhibiting or permitting in vitro growth of various bacterial species have been identified in milk. However, no in vitro activity of these factors added to milk has ever been demonstrated. These factors include "bifidus factors", which promotes the growth of Bifidobacterium, and lactoferrin and immunoglobulins, which prevent colonisation of the gut by pathogenic enterobacteria. Immune factors in milk play a key role in interactions between the microbial flora and gut mucosa. However, they seem to have no effect on the growth of bacterial populations in the gut lumen. A number of pioneer bacteria, which are the first to arrive in the gut, are capable of effectively blocking growth of other bacteria introduced later in the ecosystem. In some instances, these pioneer bacteria also inhibit production of toxins by pathogenic species. Consequently, it is important to adhere to the recommended gradual changes in diet which allow these species to sequentially colonize the gut.