New bacteria found in human gut

Scientists have identified a new bacteria in the human gut which is more abundant in people with plant-rich diets and may play a role in digesting fibre.

Researchers from Cornell University and University of California, Berkeley, have sequenced the bacteria's genome and describe the microbe's appearance and function based on genetic clues.

The newly discovered Melainabacteria, found in groundwater and in mammalian guts, is more common in herbivorous mammals and appears to add value to diets by synthesising vitamins B and K for their hosts.

The human immune system recognises specific sequences in the flagella of commensal and symbiotic bacteria, including Melainabacteria, offering evidence that the bugs are common gut residents.

Melainabacteria are close relatives of billion-year-old Cyanobacteria - often called blue-green algae - ancient photosynthesising microbes that helped raise atmospheric oxygen in prehistoric eras, and likely led to early plant cells.

Only about a quarter of all bacteria can be cultured in the lab, so researchers look for a signature gene sequence – called 16S rRNA - to identify new types of bacteria.
The human gut contains between 10 trillion and 100 trillion bacterial cells, and most of those fall into five different phyla, or lineages.

"We started picking up sequences [for Melainabacteria], but when we put them in context with other bacteria, they were on the branch of Cyanobacteria," said Ruth Ley, assistant professor of microbiology and a senior author of the study.

Melainabacteria belong on a common lineage with Cyanobacteria, but they diverged and do not photosynthesise, Ley said.

When scientists used computer programmes to analyse genetic sequences of gut samples, the computers classified Melainabacteria as Cyanobacteria. But then researchers began asking, why are they in the gut?

They used a new technique for stitching bacterial genomes together from aquifer groundwater and human stool samples.

They picked four samples, three human stools and one from an aquifer, where the new bacteria accounted for up to 4 percent of the community of microbes.

Using the new technique, they pieced together eight genomes, one of them from the aquifer sample. The genomes also confirm that the gut and environmental Melainabacteria belong in separate subphyla.

The researchers also believe that Melainabacteria facilitate fermentation in the gut, probably to break down plant fibres, which produces hydrogen gas as a byproduct. But when hydrogen accumulates, it stops the fermentation process.

The new bug is likely to rely on a partner microbe that processes hydrogen, Ley said.
The research was published in the journal eLife.