I have also seen research stating that there are very toxic quarternary by-products in Grapefruit-Seed-Extract (can't remember which ones - maybe some kind of benzoates also) that are toxic for human consumption and that would also be responsible for the antimicrobial effects. They are normally found in industrial disinfectants etc.
Not inspiring, especially that I had started taking Grapefruit-Seed-Extract again in desparate attempt against cryptosporidium before I go on to chemicals (which are not so effective either from what I read).