Table 1.

Key terms for study of the intestinal microbiota

16S rRNA gene	A gene expressed by both Bacteria and Archaea that encodes a subunit of the 30S ribosome. Sequencing of 16S rRNA gene is frequently used in phylogenetic studies of the microbiota. Hypervariable regions of this gene facilitate taxonomic assignment.
Operational taxonomic unit (OTU)	A term used in taxonomic assignments. A group of like microbial sequences. OTUs with ≥97% identity are considered to be of the species.
Microbiome	A specific community of microbes and their environment (eg, a particular body site).
Microbiota	The microbes living within a given environment (eg, the gut).
Metagenome	The total collection of genomes from a community (eg, from microbes from human fecal samples).
Bacteroidetes	One of the two most abundant bacterial phyla in the human intestinal contents
Firmicutes	One of the two most abundant bacterial phyla in the human intestinal contents