**2. Establishment of the first gut microbiota and its modulating factors**

Gut microbiota establishment is determined by several perinatal factors, including gestational pathologies, type of birth, type of feeding, prenatal and perinatal use of antibiotics, complementary feeding, and environmental pollutants [7]. From gestation to the first 2 years of life, these events influence the establishment of the microbiota. Hence, it affects the metabolic and immune response and has a subsequent impact on human health [8].

In the last century, the paradigm dictated that the womb was a sterile environment and that the first microbiota colonized the newborn at the birth time [9]. Even though this is yet a discussion topic, there is evidence pointing toward prenatal exposure to microbes [10]. Despite these, reports of low bacterial abundance and diversity and, in most cases, the lack of culturable bacteria leads to a reasonable doubt about whether it is an established microbiota or only transient exposure to DNA or microbial products that is occurring in the womb [11].

*Could Alterations in the Infant Gut Microbiota Explain the Development of Noncommunicable… DOI: http://dx.doi.org/10.5772/intechopen.105168*

The first major event in microbial colonization for the newborn occurs at birth. Type of birth determines the first gut microbiota composition. Vaginally delivered infant's fecal microbiota is enriched with *Bifidobacterium*, *Bacteroides*, *Clostridium*, and *Lactobacillus* genus. On the other hand, cesarean section is related to a higher abundance of Firmicutes and a lower abundance of Actinobacteria and Bacteroidetes [12]. In the first case, inoculum came mainly from vaginal maternal microbiota, whilst in the second case proceeded from skin and environment, presenting a high abundance of *Staphylococcus*, *Streptococcus*, or *Propionibacteria* [13]. These abundance differences decreased approximately at 6 months of age [12]. Depending on the birth way, different bacterial communities have a competitive advantage, thus first colonizers in infants born by cesarean section delay the establishment of other specific bacterial taxa [14].

First microbiota evolves to adapt to the biochemical environment and in a dependent way on the nutrient availability in the gut [14]. In this sense, whether the infant is breastfed or not, impacts the gut microbiota composition. Ho *et al.* [15], in a meta-analysis study, found higher bacterial diversity and abundance of Bacteroidetes and Firmicutes in non-exclusively breastfed infants compared to those exclusively breastfed at 6 months of age. At the genus level, *Bacteroides*, *Eubacterium*, *Veillonella*, and *Megasphaera* are more abundant in non-exclusively breastfed infants. Bäckhed *et al.* [13] also described differences at genus level between the microbiota of exclusively breastfeed and bottle-feed infants at 4 months of age. The first ones had a microbiota predominated by several species of *Lactobacillus*, whilst the second ones had a high abundance of *Clostridium difficile*, *Granulicatella adiacens*, *Citrobacter* spp., *Enterobacter cloacae*, and *Bilophila wadsworthia*.

Breastfeeding meets all the infant macro and micronutrient requirements during the first 6 months, besides human milk oligosaccharides have a probiotic effect promoting a healthy gut microbiota. Also, human milk provides bioactive compounds that favor immune development, such as immunoglobulins, leukocytes, and antimicrobial peptides. Moreover, human milk harbors its own microbiota, the genera with potential probiotic use as *Lactobacillus, Bifidobacterium*, and *Streptococcus* have been identified as its members [16].

Before 6 months of age, microbial metabolic pathways related to carbohydrate metabolism are higher in non-exclusively breastfed infants [15]. Once other foods aside from breast milk are introduced into the infant diet, functional shifts toward polysaccharides and protein metabolism occur in gut microbiota. However, these changes are not noticeable until breastfeeding cessation. Microbiota composition turns to an adult-like profile with a high abundance of *Bacteroides*, *Bilophila*, *Roseburia*, *Clostridium*, and *Anaerostipes*; but if exclusive breastfeeding continues, *Lactobacillus* and *Bifidobacterium* are higher in gut microbiota at 12 months of age [13].

The evidence on prenatal and perinatal factors influencing the composition of the gut microbiota highlights the importance of microbial colonization as a critical process in early human life. Healthy microbiota is indispensable for immune system shaping and development, and its metabolites promote the integrity of the intestinal mucosa.
