Establishment of a Normal Intestinal Microflora in the Newborn Infant

The intestinal microflora of an adult person is a complex ecosystem, estimated to harbor about 400 different bacterial species. Anaerobic bacteria outnumber aerobic and facultative bacteria by a factor of 100 to 1000. The population level of each species is thought to be strictly regulated by the competition for nutrients and space. Thus, potential pathogens such as Escherichia coli and other enterobacteria are kept at moderate numbers. Since the normal microflora makes it difficult for newly arrived bacteria to become established, it also provides a first line of defense against pathogenic bacteria, a function described as colonization resistance (1). Although colonization of the skin and mucosal surfaces of the infant begins directly after birth, the establishment of the microflora ecosystem is a slow and gradual process that takes several years. The development of the ecosystem can be followed by assessing biochemical reactions performed by certain species of indigenous intestinal bacteria, for example, the production of short-chain fatty acids, the conversion of cholesterol to coprostanol, and the degradation of mucin. Yet at 2 years of age, some of these functions are not fully established in all children, which shows that several bacterial species do not populate the gut until later (2). The intestinal microflora in early life has certain features that may make the infant vulnerable to infections. At the same time, the early intestinal microflora might be of importance for the maturation of the gut immune system, including the development of oral tolerance to harmless antigens such as food proteins. In this chapter, I review neonatal intestinal colonization and its relation to infant morbidity.