**2.2 Stability of HIM**

Bacterial colonization of the human gut likely occurs at the time of birth, when infants born via vaginal delivery are inoculated with a complex mixture of maternal vaginal microorganisms. According to Dominguez-Bello et al. [65], those infants had colonizing *Lactobacillus*, *Prevotella*, or *Sneathia* species in their skin and mucosae, which resembled their own mother's vaginal microbiota. In contrast, infants delivered by cesarean section had predominantly *Staphylococcus*, *Corynebacterium*, and *Propionibacterium* species, akin to their mothers' skin microbiota. Thus, there is concern that babies delivered via cesarean section may receive an insufficient maternal bacterial load [17].

After birth, breastfeeding is the main factor defining the composition of newborn's GI microbiota, since breast milk provides a variety of specific antibodies and immediate immunity molecules that neutralize pathogenic bacteria. Breast milk also contains more than 200 oligosaccharides (prebiotics) that favor the growth of bifidobacteria [66, 67], which have been reported to prevent gastrointestinal infections by competitive exclusion of pathogens based on common binding sites on epithelial cells [67]. Therefore, in breastfed children, bifidobacteria reaches up to 90% of GI microbiota, followed by lactobacilli, *Bacteroides*, coliforms, and clostridia. In contrast, infants fed with infant formula have predominance of *Bacteroides*, enterococci, coliforms, and clostridia, with much lesser bifidobacteria, resembling the more diverse GI microbiota of adults [66, 67].

The initial breastfeeding-driven colonization is essential for induction of adaptive immunity and for early metabolic programming. After the introduction of complementary feeding, the microbiota differences between breastfed children and those fed with formula tend to disappear. It is assumed that the predominant bacteria in the intestinal microbiome of 3-year-olds are similar to those of adults and remain relatively stable lifelong [66, 67].

Daily variability of the HIM composition has been assessed in controlled feeding studies, specifically short-term administration of extreme amount of fat and fiber intake, which revealed disturbance of the intestinal microbiome, but this effect was of low-scale and transient that not changed the individual's enterotype designation [66, 67].
