Check this article out: Grapefruit Seed Extract or Citricidal has no antimicrobial properties
It is the chemical used to make grapefruit seed extract or citricidal that has the anti-biotic, anti-viral and anti-fungal effect.
The chemical is very toxic to animals and humans although the manufacturers of grapefruit seed extract suggest that there is little chemical left after the process.
However I asked a friend who works in a Laboratory to test organic cold-pressed grapefruit seed extract for its claim that it was anti-biotic.
The test showed no anti-biotic effect but worse it grew bacteria.
The article in the link above shows emails from manufacturers who eventually admit themselves that cold-pressed grapefruit seed extract (in other words the seeds have been mechanically pressed for their extract and no chemical has been used) has NO antibiotic effect.
Which proves to me that grapefruit seeds on their own do not contain antimicrobial properties and that it is the chemical used to make Citricidal or Grapefruit Seed Extract that has the antimicrobial properties especially as it is a chemical that is known and used because of its antimicrobial properties.