digestive tract 860 host-associated 860 animal host 860 mammalian host 847 intestine 831 human host 353 industrialized 245 adult 136 rural 86 non-industrialized 82 child 81 rumen 33 elderly 30 adolescent 17 urban 17 hunter-gatherer 11 infant 11 traditional agriculturalist 2 full term birth 1 ancient microbiome 1