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Gut bacteria seem to vary according to the blood groups of their hosts, but the reason for this is not yet clear.

Your ABO blood type is determined by a type of sugar on the surface of your red blood cells. Type A individuals have a different sugar from type B individuals, while AB people have both sugars. O people 鈥� who are known as universal donors 鈥� have neither.

These sugars are called antigens and help tell your immune system that your blood cells belong to you and shouldn鈥檛 be attacked. If an A person were to accidentally receive a transfusion of B blood, antibodies made by their immune system would react with the B sugar and flag these cells for destruction.

Other parts of the body 鈥� including the intestines 鈥� carry these antigens too, prompting researchers to wonder if the bacteria that live in our body might as well.

To see if our gut bacteria match our blood type, Zhinan Yin at Jinan University in Guangzhou, China, and his colleagues took gut bacteria samples from 149 volunteers from across the four blood groups.

The team found that blood type wasn鈥檛 linked to any differences in the kinds of bacteria a person had. However, they noticed that bacteria seemed to be recognised by antibodies from different blood types, in a similar way to when antibodies detect incompatible blood cells.

This suggests that gut bacteria make sugars that match their host鈥檚 blood type. 鈥淲e were very surprised to see this,鈥� says Yin.

While some bacteria are already known to carry molecules that are similar to B antigens, this is the first indirect evidence that bacteria can have sugars that behave like A antigens too.

We don鈥檛 yet know how gut bacteria match their antigens to their hosts, or why. 鈥淚t鈥檚 possible that our gut bacteria may get protection by masking themselves to look more like their host,鈥� says Laura Cooling, at the University of Michigan. 鈥淲e are just at the beginning of understanding this symbiotic relationship,鈥� she says.