What is already known on this topic
The human gut microbiota breaks down polysaccharides found in fruits, vegetables and grains. But the polysaccharides found in edible seaweed, which many populations now consume in sushi and as a food additive, have different chemical structures. How gut bacteria have adapted to digest these nutrients remains unclear.
What this research adds
By analyzing stool samples from dozens of people, researchers have found that human gut bacteria are able to degrade several seaweed-derived polysaccharides. Genes for seaweed degradation, which were originally discovered in Bacteroidetes, have entered the human gut microbiota through a process known as lateral gene-transfer. The researchers also found other marine bacteria-derived genes that are involved in the digestion of seaweed polysaccharides. The gut bacteria Firmicutes appear to have picked up the ability to break down seaweed polysaccharides.
Conclusions
The study offers insights into how the microbiota processes seaweed and it highlights the interplay between diet and the adaptation of human gut microbes.
About ten years ago, researchers found the genes that allow ocean bacteria to degrade seaweed in a microbial sample from the gut of a Japanese adult. New research reveals that genes of oceanic origin are more common in the human microbiota than previously recognized.
The work, published in Cell Host & Microbe, highlights the interplay between diet and the adaptation of human gut microbes.
Scientists have known that the human gut microbiota breaks down polysaccharides found in fruits, vegetables and grains. But the polysaccharides found in edible seaweed, which many populations now consume in sushi and as a food additive, have different chemical structures. How gut bacteria have adapted to digest these nutrients remains unclear.
Previous studies have shown that gut bacteria belonging to the genus Bacteroidetes harbor genes for degrading seaweed-derived polysaccharides. Closely related genes have been found in ocean-dwelling Bacteroidetes.
To investigate how common seaweed genes are in gut bacteria, researchers led by Jan Hendrik Hehemann at the Max Planck Institute for Marine Biology and Eric Martens at the University of Michigan analyzed stool samples from dozens of people.
Seaweed digestion
The team used culture-based approaches to characterize several dozens of human gut bacteria that are able to digest various seaweed-derived polysaccharides, including laminarin, alginate and porphyran.
Genes for processing laminarin were common in the stool samples and were present in members of 22 different species, the researchers found. This is likely due to the bacteria’s ability to process beta-glucans, a type of sugars found in oats and whole grains.
However, other seaweed polysaccharides were used by only a few bacterial species. Genes for processing alginate were the second most prevalent in the stool samples, and only one isolate of Bacteroides plebeius — the bacterium that had been originally found in a Japanese adult — was able to grow on porphyran. “The genes to process agarose and porphyran, two of the more commonly consumed seaweeds in Southeast Asia, tended to be enriched in the people living there,” Martens says.
Gene transfer
In their analyses, the researchers found other marine bacteria-derived genes that are involved in the digestion of seaweed polysaccharides in humans. The genes entered the human gut microbiota through lateral gene-transfer — a process through which genetic material is transferred between organisms.
The team also found that Firmicutes, which are among the most abundant bacteria in the human gut, appear to have picked up the ability to break down seaweed polysaccharides. “Firmicutes are known to live in fish intestines and the closest ancestors of the genes that appear to have jumped into human gut Firmicutes were ones found in fish,” Martens says.
The findings offer insights into how the microbiota processes seaweed, but many questions remain open. “Whether [the genes] came directly from an oceanic bacterium someone just happened to consume or through a more complex path into the human gut is still a mystery,” Martens says.
