Fungus vs. Hymenoptera (the honey bee edition)

A honeybee colony. Photograph: Haraz N Ghanbari/AP
from  The Guardian

I have already expounded on the awesome power of fungi against mammals (bats), amphibians (frogs), and nematodes (worm), now it is time for the insects to go down...well, maybe. Over the last five or so years, honeybee colonies have been dying off at a dramatic rate, this is referred to as colony collapse disorder (CCD). Basically the bees in a colony go all Roanoke and it is unclear why.

Several studies of CCD have reported viruses associated with bees from infected colonies, suggesting that the CCD is an infectious disease problem. However, a recent study by Bromenshenk and colleagues identified potential microbial agents associated with CCD. Of particular interest in the the approach that the authors took to identify microbial associations.  In short, bees suffering from CCD as well as healthy controls, were gathered and the proteins were isolated and digested using degradative enzymes. Peptide fragments were then identified by mass spectroscopy (MS). Once the peptide fragments are identified, the original protein can be inferred as well as the source of the protein by analyzing available protein databases. Now it amazes me that the MS data was able to identify microbial peptides from the massive amount of bee protein that must have been in the samples. There may have been steps taken to enrich for the microbial sequences, but I doubt it since most microbial species identified were viral, which I am assuming came from infected bee cells and not free virions (although I could be wrong about this).

From these analyses, the authors determined that CCD-affected colonies were associated with co-infection with a virus and a fungus. The less interesting virus appears to be an Iridovirus, but the more interesting fungus is a microsporidian.  The microsporidia used to be classified with the protists, but are clearly fungi. The microsporidians identified included Nosema apsis, which is known to be associated with honey bees, Nosema ceranae and several other Nosema spp. What is interesting is that the authors grouped the Nosema spp. into group 1 and group 2, but group 1, which contains N. apis, was correlated with CCD, whereas group 2, which contains N. ceranae, was not (Table 1, 3, and the discussion). Finally, CCD correlated with co-infection of the Iridovirus and group 1 Nosema, suggesting that one "pathogen" is not sufficient for CCD, but both cause collapse.

Now the authors take their studies further by testing whether co-infection with IIV-6 and N. ceranae promotes CCD-like disease. (The authors are clear that the cannot be sure IIV-6 and  N. ceranae are the actual causative agents since these organisms were implicated by peptide sequences which could easily be found in related but distinct spp.) Low-and-behold they find that co-infection with both spp. is more virulent than infection by either spp. alone or controls (Figure 3). However, looking at the 50% mean survival times, the results appear to be 9.5 days for co-infection, 11 days for either single infection, and ~16 days for uninfected bees (I extrapolated based on the data presented).

I want to point out that this latter experiment is important and is often not done in these kind of studies, so kudos to the authors. However, I am not yet an enthusiastic supporter of the Iridovirus + Nosema promotes CCD hypothesis. First, the data presented, up to Figure 2, is strictly correlative. Maybe honey bees suffering from CCD are more susceptible to viral and fungal infections or even overgrowth. Karposi-sarcoma viral infections in young men was not the cause of, but a symptom of AIDS. Fungi are decomposers, so if the bees are going south the fungi may just have a leg up by being already associated with the healthy honey bees. Second, I find it problematic that the authors used N. ceranae for their infection studies, since N. ceranae was a group 2 member, but group 1 Nosema spp. were more tightly coorelated with CCD. Maybe any number of co-infections would have similar effects. Clearly the control bees go south regardless of infection, so maybe any combination of stresses simply compounds mortality loss.

Regardless, an interesting paper working on a difficult research area (no petri dish cultures for easy manipulations).

Bromenshenk, J., Henderson, C., Wick, C., Stanford, M., Zulich, A., Jabbour, R., Deshpande, S., McCubbin, P., Seccomb, R., Welch, P., Williams, T., Firth, D., Skowronski, E., Lehmann, M., Bilimoria, S., Gress, J., Wanner, K., & Cramer, R. (2010). Iridovirus and Microsporidian Linked to Honey Bee Colony Decline PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0013181