EVERYONE HAS a blood group, defined by the characteristic chains of sugar molecules, or antigens, that protrude from their red blood cells like spikes on a hedgehog’s back. Not all these groups are created equal. In the roughly 50% of people with type O, the sugars are harmless, allowing their blood to be transfused into any needy body. In those with A and B blood groups, however, the antigens can trigger harmful immune reactions in someone with a different group. Those with AB blood, who have both types of antigen, can only donate blood to one another.

Supplies of O are therefore in very high demand. If other groups could be transformed to O, the supply of universal donors could be instantly doubled. Now researchers reckon they might have found a way to do it, thanks to a new mix of enzymes found inside gut bacteria.