In each kingdom, there are subcategories: phylum, class, order, family, genus and species. With each subcategory, the criteria for grouping organisms become progressively more specific and ...

Officially, a "species" is the bottom of the taxonomic ranking system that starts with domain and works its way down through kingdom, phylum, class, order, family and, finally, genus and species.

Researchers can now detect and identify many of the different species of bacteria in your gut — not just phylum, class, order, family or genus. But, that's created a new problem, Pollard says.