Journal of Gastrointestinal Disorders and Liver Function

Composition of the intestinal microbiota The number of bacteria that make up the microbiota is greater than the number of cells in a human being. In its composition four families of bacteria stand out: Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. Up to several years ago, it was estimated that a newborn had a sterile digestive system. However, the presence of microorganisms has been demonstrated in the meconium, the placenta, and the umbilical cord, which suggests the intrauterine beginning of the colonization[1]. Nevertheless, this colonization happens mainly at the postnatal stage and completes its maturation between 2 and 4 years of age. After birth, there exists a predominance of aerobic or facultative anaerobic microorganisms such as Staphylococcus, Enterococcus, and Lactobacillus, among others, which consume the intraluminal oxygen. This favors the habitat for strictly anaerobic bacteria such as Bifidobacterium, Bacteroides, and Eubacterium[1-3]. The bacterial density varies according to the locality and exponentially increases as the digestive tube advances[1,4]. In the duodenum and in the proximal jejune, gram-positive aerobics or facultative anaerobics (Enterococcus and Lactobacillus) predominate. The distal ileum is a transition zone where aerobic bacteria, typical of the small intestine, and anaerobic ones, typical of the colon, are found[1,4]. The composition and the profile of the microbiota are influenced by different factors, such as the manner of birth, maternal breastfeeding and its duration, the use of infantile formu-