And Andrew, to answer your question, that would be affirmative. I have heard of both of those compounds, and if you go back to post #25 in this discussion you will find a study that I linked to that involved metronidazole & ajoene.
http://www.ncbi.nlm.nih.gov/pubmed/22677132
Here's another study from a few years ago that did not include metronidazole.
http://www.ncbi.nlm.nih.gov/pubmed/21056027
These chemically synthesized derivatives of garlic (allicin and ajoene) only proved to inhibit growth of Spironucleus vortens, and only at very high concentrations far beyond what one will find on the market in garlic oil, fresh garlic, dry garlic, or garlic products sold in the aquatic industry.
In the latter study the researchers concluded:
All these concentrations are much higher than those reported to be required for the inhibition of most bacteria, protozoa and fungi previously investigated, indicating an unusual level of tolerance for allium-derived products in S. vortens. However, chemically synthesized derivatives of garlic constituents might prove a useful avenue for future research.