Table 2.

Factors affecting colonization of gut microbiota in neonates and infants or children, specific microbial effect, and the resultant health conditions.

Factor	Observed effect on microbiota	Specific health condition/disorder/disease	Reference
Intrauterine environment	Presence of bacteria in the uterus	Remote history of antenatal infections such as urinary tract infection during the first trimester Preterm birth	(18–25)
	Presence of bacteria in the amniotic fluid
	Presence of bacteria in the meconium
Stress during pregnancy	Low counts of beneficial bacteria (e.g., Bifidobacteria, Lactobacillus)	Allergic reactions	(26)
Probiotic use during pregnancy	Increased colonization by beneficial bacteria	Reduced incidence of allergic reactions	(27)
	Increased bacterial diversity
Antibiotic use during pregnancy	Delayed colonization or reduced abundance of beneficial bacteria	Increased allergic reactions (asthma, allergic sensitization, allergic rhinitis)	(7, 28–36, 47, 80–83)
		Irritable bowel syndrome (IBS)
		Inflammatory bowel disease (IBD)
Smoking during pregnancy	Microbial dysbiosis (decrease in Firmicutes and Actinobacteria and an increase in Bacteroidetes and Proteobacteria	Increased risk of IBD	(37, 38)
Length of gestation period – preterm	Slow rate of bacterial colonization	Necrotic enterocolitis (NEC)	(6, 29, 39, 40, 86, 87)
	Reduced bacterial diversity
	High interindividual differences in colonization
	Increased level of potential pathogenic bacteria
Length of gestation period – term	Increased abundance of beneficial bacteria	Lower incidence of NEC	(6, 29, 39–41, 86, 87)
	High bacterial diversity
Cesarean delivery	Reduced bacterial richness and diversity	Increased risk of asthma, allergic reactions, Type 1 diabetes, atopic eczema, obesity and NEC	(6, 7, 17, 42, 46, 47, 51, 77, 78, 80, 88)
	Reduced colonization by beneficial bacteria
	Increased colonization by potential pathogens
		Low levels of Thl responses
Vaginal delivery	Increased microbial diversity	Decreased risk of asthma, allergic	(48–50, 81, 82)