Scientists at the University of Copenhagen have found a way to convert all blood to type O.
There are 2 possible antigens that may appear in blood, A and B. Blood types are thus either A, B, AB, or O, where O has no antigens on it.
Additionally, blood also has a rhesus (Rh) factor associated with it. The presence of Rh creates a +, and the absence creates a -.
All possible combinations of blood types are: A+, B+, AB+, O+, A-, B-, AB-, O-.
For blood transfusions, people cannot accept blood that has antigens that individual does not already have. For instance, a person with O+ blood can only accept transfusions of O+, or O-.
A person with AB- blood can accept: A-, B-, AB-, and O-, but no Rh+ blood. See also wikipedia's blood compatibility table. Blood type O is known as the universal donor, because their blood can be transfused to anyone.
Researchers discovered two bacteria: Elizabethkingia meningosepticum and Bacterioides fragilis, which can be used to strip away the A and B antigens from blood cells. As of yet, they have not found a way to remove the Rh factor yet, so that means that if you are Rh-, you will still need blood from and Rh- donor. The Rh+ can accept blood from both + and -.
If this pans out, it will drastically reduce blood supply problems, and greatly decrease risk of transfusion incompatibilities.