The largest number of bacteria and -species exist in the human adult gut, compared with any other area of the body. The infant gut is oppositely regarded as sterile during fetal life. Bacterial colonization of the gut begins at the time of birth and continues during the next 18 months. Bacterial load and composition mature toward that of the normal adult gut after 18 months of age.
Colonization pattern during the first one and a half year of life is important for maturation of gut functions and immune development. Modulation of the microbiota is influenced by various factors such as gestational age at birth of the infant, delivery method, use of antibiotics, how the infant is fed and exposure to microbes in the surrounding environment.
Some bacteria are fermented in the colon to form organic acids. This is important for normal gut cell function and cell regulation in the gut, whereas accumulating levels of organic acids may be unusable and can cause damage, for instance, as diarrhea or more persistent disease. Bacterial phyla such as Firmicutes and Bacteroidetes and Proteobacteria are dominating in the human gut and are regarded as commensal or good bacteria. Bacteria such as Candida, Saccharomyces and Escherichia Coli have been related to illness and, therefore, may be detrimental if present in the gut in high quantities.
Changes in microbiota can be caused by changes in diet, which is well known today. Ingestion of specific bacteria can help modulate or keep a healthy microbiota, at least while given. Examples of dietary changes include the first enteral feeding after birth as well as during weaning, when supplements and more solid food ingredients are introduced to the infant’s diet.