The Neonatal Intestinal Microflora and the Immune System

Directly after birth, bacteria commence to colonise the skin and the mucous membranes of the respiratory, urogenital and intestinal tract. With time, more or less complex bacterial ecosystems are established at various body surfaces, of which the normal intestinal microflora is the largest and most diverse. Thus, an adult individual harbours more than 400 bacterial species at this site. Most of these bacteria are obligatory anaerobic. The establishment of the intestinal microflora after birth precedes in a sequential manner, and a complete microflora is not obtained until after several years of age. A number of factors may influence the establishment of the microflora, including delivery and feeding mode, social contacts and the degree of environmental hygiene. Many bacteria that inhabit the intestine, especially facultatively anaerobic bacteria, are potential pathogens, since they may spread to extra-intestinal sites. High population levels of these bacteria in the intestine in early life make the new-born infant vulnerable to infections, for instance septicaemia. On the other hand, these bacteria may be of great importance for the maturation of the developing immune system. Bacteria colonising the intestine activate both mucosal and systemic immunity, and may contribute to development of oral tolerance towards harmless antigens. This paper describes the establishment of the intestinal microflora in early life, factors affecting the colonisation process, and influence of the intestinal microflora on the infant's immune system.