Eating seaweed can genetically alter the bacteria in your gut as they can acquire genes from marine bacteria for digesting substances unique to seaweeds. It isn’t known if this affects people’s health, but having gut bacteria that can digest carrageenan, a common food additive derived from seaweed, might be harmful.
“Our microbes are naturally genetically engineering themselves,” says Eric Martens at the University of Michigan.
Seaweeds contain unique dietary fibres – large carbohydrate molecules – that neither we nor the bacteria in our guts can normally digest. For instance, the nori wrappers used for sushi are rich in porphyran.
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In 2012, however, a team including Martens found that one gut bacterium had , probably from a bacterium that lives in the sea.
Now a team led by Martens has found several more examples. The researchers started by seeing if any bacteria present in faecal samples from a few hundred volunteers could grow if fed only carbohydrates unique to certain seaweeds. They then sequenced the genomes of these bacteria.
The team found two strains of Bacteroides gut bacteria that can digest carrageenan, which is found in red seaweeds traditionally eaten in parts of Europe and Asia. It is added to many foods to thicken them, and is also found in some sex lubricants.
There has been controversy over the safety of carrageenan because it can break down into a substance called poligeenan, which is toxic. Full-size carrageenan molecules should normally pass through the gut intact, but people with bacteria that can digest carrageenan might produce poligeenan in their guts.
“Whether this happens in enough abundance, we don’t know,” says Martens. Even if poligeenan is produced, it might remain safely inside bacteria. This issue should be investigated further, he says.
A 2018 re-evaluation of carrageenan deemed existing safety data to be inadequate, says a spokesperson for the European Food Safety Authority, and . “Since the process is ongoing, this is all we can say on this matter at this stage.”
Martens also found several more strains of Bacteroides gut bacteria that can digest porphyran. Most had the same set of genes for the enzymes required, showing that it has been transferred among gut bacteria since being acquired from a marine bacterium. However, one species had a different set of genes, showing it acquired its porphyran-digesting genes from another marine bacterium on a separate occasion.
Various metagenomic studies, in which all the DNA in people’s guts is sequenced, show that people living in Japan and China are much more likely to have these porphyran-digesting gut bacteria than people in other regions.
The team also found that strains of a fundamentally different kind of bacterium, Firmicutes, had independently acquired genes for digesting porphyran and a dietary fibre called agarose that also comes from seaweed.
The transfer of genes between bacteria is known to happen as a result of a process called conjugation. “It’s basically bacterial sex,” says Martens.
However, Martens thinks it is very unlikely to happen between gut and marine bacteria. “To me, it would be far out that we would eat live bacteria and that they would conjugate in the digestive tract.” So how these transfers happened remains a mystery.
He suspects there are many more examples of gut bacteria acquiring genes when people eat novel foods such as chia seeds. “There are a lot of regional foods that have these unique polysaccharides,” says Martens. “I am willing to bet we are just scratching the surface.”
These findings could prove useful. So-called probiotic bacteria tend to die off because they can’t compete with established bacteria. Martens’ team found that if mice were fed porphyran after swallowing bacteria with the ability to digest it, these bacteria were much more likely to become established in the gut.
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