Allicin, a.k.a. Diallyl thiosulfinate (or Diallyl disulphide-oxide), has been identified as the chief active pharmaceutical ingredient in garlic (Allium sativum). Allicin is a broad-spectrum agent against both Gram+ and Gram- bacteria (Cavallito, 1944; Adetumbi & Lau, 1983; Ankri & Mirelman, 1999) with a scope of applicability about as wide as penicillin's, though less potent.
Allicin also works against at least some viruses (Weber et.al., 1992), and some parasitic protozoans (Lun et.al., 1994; Ankri et.al., 1997) Lastly, some of its rapid-breakdown products, ajoene in particular, possess considerable anti-fungal properties (Yoshida et.al.,1987). Of course, garlic also yields other active compounds, and they too have some pharmaceutical value, but are said to be decidedly less potent than the chief active ingredient.
Allicin is naturally produced when garlic is damaged or crushed, allowing the release and inter-reaction of two substances, the non-protein amino acid alliin and the enzyme alliinase (Krest & Keusgen, 1999). In that sense, allicin is an all-around defense of the garlic plant, manufactured fresh, right where trauma to the plant occurs and where the threat of invasion or infection is imminent. Not surprisingly, it reportedly has insect repellent properties as well.
Allicin further has a remarkable ability to permeate living tissue (Miron et.al., 2000). That is why it is hard to get garlic's smell off your skin after contact with the raw material --it really digs in, penetrating tissue with comparative ease, and this has implications on its potency.
Hobbyist attention presently seems to center on garlic’s value as therapy versus infestations of C. irritans, and so I shall get right to such a scenario, focusing on allicin as the active ingerdient of garlic.
III. Allicin Wonderland
In a parasitic protozoan infestation similar to “Marine Ich”,there are usually two major insults to the fish's physiology:
1) the gross tissue damage committed by the protozoans themselves;
2) and the secondary infections that crash the party.
The applicability of such a broad-spectrum antiseptic as contained in garlic towards fending off secondary infections should be obvious, so I will set aside such secondary infections at this point.
Allicin has been observed to suppress the efficacy of cysteine proteinase and alcohol dehydrogenase, two tissue-demolition agents produced by another protozoan parasite, Entamoeba histolytica, (Ankri et.al., 1997), and one can easily extrapolate how garlic medication might limit the invasive and predatory damage caused directly by C. irritans.
Yet what generates as much (if not more) speculation is not the ability of garlic to restrict damage, but rather garlic's apparent ability to deliver damage to the parasites themselves -with numerous claims of outright detachment of C .irritans trophonts (the burrowing protozoans) and tomonts (the "egg cysts") as a result.
The aforementioned ability of allicin to permeate tissue and mucus enables it to invest an afflicted area thoroughly with its partially sulfurous chemical signature. The potential is there to mask the chemical cues that enable a parasite's recognition of the host, potentially confusing the invader and further suppressing the havoc it wreaks. This means garlic therapy can benefit even uninfested fish, allowing them to escape detection by host-hunting C. irritans tomites
Definitely, allicin brings outright chemical assault to the parasite. In one test, allicin’s cytotoxicity fell heavily against the parasitic protozoans Trypanosoma spp. and Giardia lamblia in concentrations that were well within the tolerance of sample-host fibroblasts (Lun et.al., 1994). The same penetrating power that ensures thorough investiture of the contested tissue, with protective and camouflaging agents, can also ensure thorough delivery of allicin's antagonism to --indeed, into-- invading parasites.
Lastly (though certainly, other properties may yet be discovered), allicin is said to reinforce the cues for cellular apoptosis, the mechanism of programmed cell-death (Thatte et.al., 2000). An innate mechanism such as apoptosis, guided with care, has the potential to severely limit the spread of say, cancer, by motivating cancerous cells to quickly self-terminate before they can multiply. If such a 'scorched earth' defense can be triggered by an infestation episode and be guided/reinforced by allicin, then yet another way may be revealed how garlic hinders both the ability of parasites to feed on host tissue and any opportunity for secondary infections to spread.
