It may not feel like it, but your insides are a biodiversity hotspot, brimming with hundreds of different living species you notice more when they're not there. We've known for a long time that the human gut hosts many species of bacteria, but a step towards a comprehensive catalog has shown just how much is going on in there.

Dr Samuel Forster of Australia's Hudson Institute is first author of a paper in Nature Biotechnology reporting the majority of species present in fecal samples from 20 UK and Canadian residents. This can only give only a small indication of the diversity in the human population. Nevertheless, 273 species of bacteria were identified, of which 105 were completely unknown previously, and another 68 had not been sequenced before. Many species come in diverse strains, with 737 identified in this study.

Forster told IFLScience estimates for the number of bacterial species living in humans around the world range as high as 5,000, but each of us probably only hosts around 200. Some are shared across most of humanity, others specific to small localities or unusual diets. Even with the relatively homogenous background of the participants, Forster said the number of species expanded dramatically when the study moved from six to 20 individuals.

The ecosystem in our intestines is so little understood because it is so hard to culture. Forster explained growing a single pathogen that has made us sick by temporarily dominating is usually easy. Persuading the diversity of microbes flourishing inside someone to grow on the outside is like replicating the diversity of a rainforest from scratch. Different gut bacteria feed on different nutrients, often not on the raw ingredients we put in our mouths, but the waste products of other microbes.

While he may have only identified a small proportion of the single-celled organisms that make their home in the human gut, Forster said the work is a major advance. "This study has led to the creation of the largest and most comprehensive public database of human health-associated intestinal bacteria,” he said in a statement. 

Examining the DNA from bacteria in fecal samples is like “finding a collection of pieces from multiple jigsaw puzzles.” Forster said to IFLScience. The database, he added, will be like the pictures on the puzzles' boxes, allowing scientists to work out which puzzle a piece belongs to.

In recent years, the bacteria in our gut, once largely ignored except during serious infections, have come to be increasingly recognized as playing an important part in our general health. From their control over our diet to their influence on mental health and possibly even neurodegenerative diseases, these cells matter, so it's time we started to understand what's there.