**2. Gut microbiota**

Our body hosts a vast, diverse community of stable and varying microorganisms that are referred to as microbiota. The gut is the niche with the highest number and diversity of micro-organisms, containing over 1014 microbial cells, 10 times the amount of somatic and germinal cells in our body. The microbes that inhabit our gut are known as gut microbiota [4].

Gut microbiota is an open ecosystem that contains a broad diversity of metabolically active microbes that coexist in space and time and play a relevant role in the health of their host. The gut microbiota is considered a metabolic organ that is adaptable and rapidly renewable. There is a mutually beneficial interplay between the host and gut microbiota [4, 5].

#### **2.1 Gut microbiota and immunity**

The relationship between the lymphatic system and gut microbiota in early stages of life is crucial to the appropriate development of interactions between mucosal cell communities and systemic immunomodulation [5]. Animals with a sterile gut have been proven to be highly vulnerable to infections, which demonstrate the important role that gut microbiota plays in the immune system [4].

Bacterial colonization of the newborn's gut is influenced by a variety of factors such as gestation and delivery and breast-feeding mode [6]. HBM is an excellent continuous source of commensal bacteria for the infant gut. Evidence has been provided of a vertical transfer of bacteria from mother to child via breast milk [7, 8]. The fact that facultative anaerobic bacteria in newborn's gut are the predominant bacterial community in HBM microbiota is not a chance. These bacteria play a key role in the prevention of infections in the newborn [9]. Gut microbiota disorders (dysbiosis) in the first stages of life reportedly precede the development of atopy [10].

During the first week of life, the total bacterial count and, more specifically, anaerobic bacteria count progressively increase. The feeding mode of the newborn has a decisive impact on bacterial gut colonization. Bifidobacteria, lactobacilli, and Gram-positive cocci predominate in the feces of breastfed infants, whereas the bifidobacteria count is lower in the feces of formula-fed infants, with the predominance of bacteroides, clostridia, and coliforms [11–13]. Differences in the composition of newborn's gut microbiota based on the type of feeding could be the clue to identifying the bacteria that exert protective effects to breastfed infants [4].
