https://phys.org/news/2019-02-unknown-bacteria-human-gut.html

The human gut is home to many species of microbes, collectively referred to as the gut microbiota. Despite extensive studies in the field, researchers are still working on identifying the individual microbial species that live in the gut and understanding what roles they play in human health.

There are many reasons that some microbial species among the gut microbiota have remained unknown for so long, such as a low abundance or an inability to survive outside it. By using computational methods, researchers were able to reconstruct the genomes of these bacteria.