**3.1 Factors affecting gut microbial composition**

Several factors can influence microbiota composition. These factors include the mode of infant delivery and breastfeeding, diet, intake of antibiotics and other drugs, stress, disease, smoking, drinking, aging, and race, among others [18]. The main influencing factors are described below:

a.**Mode of infant delivery and breastfeeding.** The mode of delivery significantly affects gut colonization in newborns. Passage through the birth canal affords the neonate a microbiota like that of the mother's vagina, whereas for infants born via Cesarean-section, the microbiota resembles the mother's skin and environmental microorganisms [19, 20]. Breastfeeding also provides beneficial genera, such as *Bifidobacterium* and *Lactobacillus*, with a lower colonization rate by *Escherichia coli*, *Clostridium*, and *Bacteroides* [21, 22].

