The Beekeeper’s Companion Since 1861
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Notes from the Lab

Prospecting for plant toxins that will keep bees healthy

- June 1, 2022 - Scott McArt - (excerpt)

COVID has made me hypervigilant about the slightest tickle in my throat. But even in pre-COVID times, I and pretty much everyone I know had something they would take when they felt that tickle. Green tea, echinacea, garlic, cloves, perhaps a tincture of propolis?

Each of these herbal remedies has something in common: They contain phytochemicals (i.e., plant toxins) with known antimicrobial capabilities. Let’s take cloves, for example. That strong “clove taste” comes from eugenol, an aromatic oil that can kill bad microbes, including Leishmania, a parasite that’s transmitted by sand flies and causes Leishmaniasis in humans. If you didn’t know, Leishmaniasis is a major threat to human health in the tropics, subtropics, and southern Europe.

As with anything in toxicology, the dose makes the response. If people consume too much eugenol, it will cause severe liver damage. But a small amount can be good, especially if you’re trying to stay healthy by controlling a microbial infection.

What does this have to do with bees? Well, of course bees also have to ward off harmful microbes, including viral, bacterial, fungal, and protozoan parasites. Since bees are constantly consuming phytochemicals in pollen and nectar, is there any evidence some of these toxins are antimicrobial and therefore potentially ward off disease? If so, are there phytochemicals that are particularly promising in terms of their therapeutic potential for in-hive treatments? These are the topics for the fifty-fourth Notes from the Lab, where I summarize “Punch in the gut: parasite tolerance of phytochemicals reflects host diet,” written by Evan Palmer-Young and USDA Bee Research Lab colleagues and published in the journal Environmental Microbiology [2022].

For their study, Palmer-Young and colleagues focused on three trypanosomatid parasites: Crithidia mellificae, Lotmaria passim, and Crithidia fasciculata. Crithidia mellificae and L. passim are protozoan gut parasites of honey bees (see Photo 1) that are transmitted in feces. Infection is associated with reduced nutrient absorption and increased mortality in honey bee workers. Crithidia fasciculata is a parasite of mosquitoes and is similar to the bee-associated trypanosomatids in morphology and fecal-oral transmission. Many mosquitoes consume nectar as adults, potentially exposing them to phytochemicals. However, C. fasciculata also infects mosquito larvae and as a result, the parasite is likely exposed to a variety of phytochemicals in woody debris-rich aquatic breeding habitats.

The authors’ goal was to test how a wide variety of plant phytochemicals did or didn’t inhibit growth of each parasite. To do this, they cultured each parasite in the lab and performed inhibition assays using 25 different phytochemicals (see Photo 2, Table 1). Each phytochemical was chosen based on previously demonstrated inhibitory activity against Leishmania, which is the most well-studied trypanosome parasite due to its serious risk to human health.

To assess inhibitory activity, the authors measured parasite growth rates when each parasite was exposed to eight different concentrations of each of the 25 phytochemicals. Using these data, they were able to calculate IC50 values for each phytochemical-parasite combination. IC50 is the concentration of a chemical that inhibits parasite growth by 50% compared to controls.

So, what did they find? Did any of the phytochemicals kill parasites? Yes. As seen in Table 1, 12 of the 25 phytochemicals (48%) inhibited growth of at least one of the three parasites. All chemicals that inhibited a parasite are colored yellow through orange, corresponding to increased inhibitory activity. Low numbers mean a small amount of the chemical was required to reach the IC50, high numbers mean a large amount of the chemical was required, and unshaded cells with a ‘>’ sign followed by a number indicate there was no inhibition at the highest tested concentration.

The three chemicals with the strongest inhibitory effects (IC50 < 100 µg/ml for all species and strains) were the antileishmanial Streptomyces metabolite amphotericin B (IC50 range from 0.59 for L. passim to 2.32 for C. mellificae) and the terpenoids thymol (from 28.3 for L. passim to 54.1 for C. mellificae) and its isomer carvacrol (from 41.1 for C. fasciculata CFC1 to 65.7 for C. mellificae).

Were there differences between the honey bee vs. mosquito parasites in susceptibility to phytochemicals? Yes. Differences were found for the flavonoid chrysin (IC50 > 50 µg/ml for both C. mellificae and L. passim vs. 6.65 µg/ml for C. fasciculata CFC1 and 11.7 µg/ml for C. fasciculata Wallace), meaning there was more than a four-fold higher tolerance among the bee parasites. Interestingly, chrysin is a flavonoid found in nectar, pollen, and plant resin-derived propolis. Perhaps the bee parasites have evolved tolerance to ….