Table 1.

Overview of physiological and pathological factors influencing the composition of the intestinal microbiota in adult subjects

Involved factors	Comment
Physiological factors
Dietary habits	Influence on the enterotype Influence on microbial diversity Influence on the relative abundance of some taxa by particular metabolic substrates (eg waxes, fibers) or sensitive to different concentrations of bile acids
Geographic origin	Influence mediated by dietary habits, methods of food storage, exposure to animals, domestic hygiene
Physical activity	Increase in microbial diversity and in the concentration of health-promoting bacteria
Type of childbirth, breastfeeding/lactation, age of weaning	They can influence the overall composition of the microbiota in childhood, leaving a fingerprint even in adulthood
Presence of cohabitants and pets	Over the time the microbiota of people and pets that live in close contact tends to resemble each other in the global composition
Genetic factors	The presence of some taxa depends on the types of receptors expressed by epithelial cells of the mucosa
Living environment (home vs. institution)	Reduction of microbial complexity with high inter-individual variability in institutionalized subjects
Age	The microbiota is stable in adulthood up to 65-70 years Then there is an increase in inter-individual variability with a reduced number of species and a tendency to dysbiosis
Pathological factors
Direct exposure (therapy) or indirect (environmental contamination) to antibiotics	It causes dysbiosis with profound changes in the composition of the microbiota that are not necessarily associated with a decrease in the number of bacteria Dysbiosis depends on the type of antibiotic taken, the dose and duration of therapy
Chronic pharmacological therapies	The main evidence is for antiblastic chemotherapy. On overall, polypharmacy is related to dysbiosis
Immunological alterations	Immunosuppression promotes the growth of pathogenic strains