It's in the Honey! - Bee Diet and Defense Against Pathogens

Intro: The state of the honey bee

Honey bees have been in the news a lot lately, and unfortunately most of it hasn't been good. Colony collapse disorder, or CCD, is a phenomenon where beehives are mysteriously found empty. This has made understanding the cause of CCD challenging, on top of the growing consensus that CCD probably arises from a multitude of risk factors¹.

As both a beekeeper and biology student, I care a lot about CCD. I keep bees as a hobby with a group of students at my college, and in the three years I have been beekeeping two of our hives have fallen prey to CCD. (We don't have a very big operation to begin with, ranging from one to four hives at a time, and currently three).

A popular model for CCD is that many stressors, while individually insufficient to cause colonies to collapse, in combination are too much for the bees to handle. The list of potential culprits includes microbes, pesticides, parasites, and beekeeping practices like transporting bees over large distances.

There is a lot of focus on the varroa mite, appropriately named Varroa destructor, which latches onto honey bee adults, pupae, and larvae, feeding off their hemolymph and in the process acting as a vector for disease. The mite has invaded Europe, the Americas, and Asia, but not Africa or Australia. In the southern United States, where I keep bees, varroa mites are in every beehive.

The notable absence of CCD in Australia, which has the same species of honey bee as in Europe and the Americas (no coincidence, since the Europeans colonizing the two continents brought bees with them) suggests that varroa mites might have an important role in CCD. It also raises doubts about the pesticide-dominant model that has been posited by some³.

Understanding how honey bees respond to pathogen and pesticide exposures could point to breeding or therapeutic strategies to combat CCD.

Part 1: It's in the honey!

When beekeepers finish a honey extraction, the honey centrifuge, buckets, and various tools are all coated with a layer of honey. As you can imagine, cleaning honey from the nooks and crannies of oddly shaped machinery is a nightmare. But lucky for us, bees love honey. They love it so much that if you leave the equipment outside for several hours, bees in the surrounding area will do the cleaning for you.

Honey is so integral to the life of a honey bee, yet we don't understand very much about how eating honey affects bees. A recent study by a group at the University of Illinois makes a stab at understanding how components of honey can directly affect gene expression of worker bees, in particular genes that aid in defense against pathogens and pesticides².

Oddly enough, bees appear to start out handicapped in the fight against pathogens, as far as insects go. While most insects have around 80 cytochrome P450 genes, encoding a family of proteins responsible for neutralizing foreign substances (these are why drugs wear off!), honey bees have only a modest 46.

P450s are also responsible for breaking down phytochemicals (plant-made chemicals), and not surprisingly honey is full of them. What's more, we already know that honey induces the expression of two P450 gene families: CYP6AS and CYP9Q.

This latest study goes a step further by identifying the compound responsible. They separated components of honey extracts using chromatography (HPLC for the chemistry savvy) and seeded sugar candies with the purified compounds. Then they looked to see if expression of P450s and other genes was altered in workers fed the candy.

Ultimately the study identified the phytochemical coumaric acid as a strong inducer of CYP9Q3 and four members of the CYP6AS family of P450s. It also induced higher levels of expression of a number of other honey bee genes with antimicrobial or drug-neutralizing functions.

Conventional wisdom is that honey is better for bee health than feeding with sucrose or high fructose corn syrup, which are currently widespread practices (including by me). This feeding is necessary to support young or weak hives. However, some beekeepers feed with honey from stronger hives, rather than using substitutes. If those hives are less prone to collapse, it would support a model of induction of honey bee defense by honey phytochemicals.

Since replacing sugar syrup feed stocks with pure honey is probably not feasible for the large-scale beekeepers who make up the bulk of the industry, coumaric acid supplements might capture some of the benefits of honey while remaining practical. It remains to be investigated whether the gene expression changes induced by coumaric acid are actually consequential for bee defense against CCD-relevant assaults. If so, a simple nutritional supplement could be a powerful tool in our fight against CCD.

Stay tuned for part 2, where I look at a bee strain that fights varroa mites head-on.

Image Credit: JJ Harrison (via Wikimedia)

References:

1. Broder, J. M. "Study finds no single cause of honeybee deaths." The New York Times. May 2, 2013.

2. Mao, W., Schuler, M. A., & Berenbaum, M. R. Honey constituents up-regulate detoxification and immunity genes in the western honey bee Apis mellifera. PNAS. Published online April 29, 2013.

3. Kolbert, E. "Silent hives." The New Yorker. April 20, 2012.

4. Australian Museum. Animal species: honey bee.

5. Shapiro, L., editor. Varroa honey bee mite. Encyclopedia of Life.

6. Goodsell, S. Cytochrome p450. Structural View of Biology, PDB.