Table 1.

Definitions of key terminologies.

Term	Definition
Microbiota	Community of microscopic organisms.
Microbiome	Genes associated with the microbiota.
Amplicon	Product of PCR amplification.
High-throughput Sequencing	DNA sequencing approach that produces large amounts of sequence data rapidly at low cost.
OTU	Operational taxonomic unit: Group of DNA sequences with 97% similarity.
SV	Sequence variant: Individual DNA sequences recovered from a high-throughput marker gene analysis following the removal of spurious sequences generated during PCR amplification and sequencing.
Absolute abundance	Unobservable actual abundance of a taxon in a unit volume of an ecosystem.
Observed abundance	Observed counts of features (OTUs or SVs) in the feature table.
Relative abundance	The fraction of the taxon observed in the feature table relative to the sum of all taxa in the sample. It is between 0 and 1.
Feature Table	A matrix summarizing observed microbial abundances in the sample. Usually, columns represent samples and rows stand for OTUs or SVs.
Library Size	The total number of observed abundances for all taxa in a sample.
Microbial Load	The total number of (unobserved) absolute abundances for all taxa in a unit volume of an ecosystem.