**1. Introduction**

The human gastrointestinal tract is inhabited by hundreds of thousands of microorganisms that represent highly diverse microbiota. Gut microbiota comprises a metabolically active and complex ecosystem (bacteria, archaea, viruses, and unicellular eukaryotes) that colonizes the digestive tract soon after birth [1]. It has a crucial role in establishing a dynamic association with the human organism, having crucial roles in several physiological and pathological processes [2, 3]. It protects the host from the colonization of pathogens and is linked to nutrient digestion and absorption as well as to immunological, metabolic, and motor functions [4].

In the adult gut microbiota, four major microbial phyla are known to represent over 90% of the bacterial components: *Firmicutes*, *Bacteroides*, *Proteobacteria*, and *Actinobacteria* [5]. *Firmicutes* phyla mainly include *Ruminococcus*, *Clostridium*,

*Lactobacillus*, *Eubacterium*, *Faecalibacterium*, and *Roseburia*, while *Bacteroides* include *Prevotella* and *Xylanibacter*. Fewer representatives are from the phyla *Actinobacteria* and *Proteobacteria* [6].

The development and maturation of gut microbiota is a dynamic process that starts in early life. Its composition may be affected by several intrinsic and extrinsic factors, like mode of delivery, mother's age, diet and metabolic status, type of feeding, family genetics, lifestyle, exercise, immunological factors, drugs like antibiotics, and availability of nutrients [7–13]. Colonization of the infant's gut was thought to begin at birth, but scientific evidence has provided indications of bacterial presence in the placenta, umbilical cord, and amniotic fluid in healthy full-term pregnancies [14–16]. These findings suggest that microbial exposure may start before delivery, allowing colonization of the fetus with maternal microbiota. Another driver affecting the microbial colonization of the infant's intestine represents the delivery mode [17]. Vagina-associated microbes such as *Lactobacillus* and *Prevotella* colonize the neonatal gut of vaginally delivered infants [18, 19], while infants which are not directly exposed to maternal microbes during C-section become colonized by environmental microorganisms from maternal skin, the hospital staff, and the hospital environment [7, 11, 19–21]. The introduction of solid food to an infant's diet changes the microbiota, and by the age of three, it resembles a relatively stable adult-like profile with a dense microbial population [12]. The composition of the gut microbiota in the adult population is relatively stable and is only transiently altered by different external factors. It is now evidenced that dietary factors, particularly the amount, type, and balance of the main dietary macronutrients (carbohydrates, proteins, and fats), and different types and intensities of exercise play an important role in shaping the gut microbiota composition [22, 23]. The preservation of healthy gut microbiota has an important role in maintaining good health, with crucial effects on mucosal barrier fortification, motility of the gut, conversion of food into required nutrients, immune system homeostasis, and protection against pathogenic microorganisms [24].
