OTU's are used to categorize bacteria based on sequence similarity. An operational taxonomic unit (OTU) is an operational definition used to classify groups of closely related individuals. OTU number, the Shannon index, and the observed phylum number are shown for five gut microbiomes from orangutans (domestic zoos), humans (Japanese. From each OTU, a single sequence is selected as a representative. The operational taxonomic unit (OTU) is widely used in microbial ecology. High-throughput bacterial 16S rRNA gene sequencing followed by clustering of short sequences into operational taxonomic units (OTUs) is.