Listen to your gut — bumblebee edition

I like bees, and for a brief and mostly happy period was the host of the Stern/Rudner Swarm — a pair of hives maintained by Bodo Stern and David Rudner that resided in my garden.   My job was to grow the flowers and take a share of the resulting honey.  Sadly they failed to make it through last winter.  I suspect that the problem for these particular hives was the lack of a convenient water source rather than one of the many diseases that seem to be decimating bees worldwide, but it’s no secret that bees are in trouble— and therefore, perhaps, so are we.  Bees are not only responsible for making honey; they also

My worries are different from Hannelore's. Hannelore is from the cartoon Questionable Content: http://questionablecontent.net/view.php?comic=1855.

pollinate agricultural crops, most types of fruit, and many wild plants.  In my list of apocalyptic worries, the collapse of the bee population is not at the top — climate change and the risk of sterilizing the oceans are both up there — but it’s not at the bottom either.  And so I was interested to see a recent paper (Koch and Schmid-Hempel, 2011: Socially transmitted gut microbiota protect bumble bees against an intestinal parasite.  PNAS doi/10.1073/pnas.1110474108) that suggests that the gut microbiota of social bees may give them some protection against certain parasites.

The parasite studied in this paper is a nasty little item called Crithidia bombi, which — if it infects a young queen bee — leads to a ~50% loss in the queen’s ability to found her own colony.  In previous work, the authors found that treating bees with antibiotics led to a very high C. bombi infection rate; but these experiments were hard to interpret since bees apparently really don’t like antibiotics.  About half of the bees in the antibiotic-treated group died just from the antibiotic treatment, before they ever saw the parasite.  So, was the higher infection rate simply caused by the fact that the bees were not very healthy?  Or was there some specific interaction between the microbiota associated with the bees and the parasite?

In this paper, Koch and Schmid-Hempel avoid antibiotics entirely, instead raising young bumblebees in semi-sterile conditions and introducing bacteria to them (or not) by feeding.  Two things about bees are interesting in the context of this experiment.  The first is that during metamorphosis the entire larval gut is

Honey from the late, lamented Stern/Rudner hives

shed and replaced, so the new gut is sterile.  The second is that bees have a rather simple gut microbiome, dominated by just two bacterial taxa: gammaproteobacteria and betaproteobacteria. The authors made an attempt to culture the bacteria from bee guts and were somewhat successful with the gammaproteobacteria (though not with the betaproteobacteria); so they fed some bees with the cultured gammaproteobacteria and compared the results with those for bees fed with complete bee microbiota (i.e. bee feces), or fed with sterile sugar water.   Bees raised in this way, whether sterile or fed with bacteria/feces are perfectly healthy, with mortality levels identical to normal bees.

What the authors found was that although feeding with feces led to the establishment of microbiota that are indistinguishable from those of wild bees, feeding with cultured gammaproteobacteria didn’t appear to do anything — it seems that these bacteria can’t establish themselves in the bee gut without the other members of the community.  Bee gut microbiota are simple relative to other microbial communities, it seems, but still complex enough to be tricky to manipulate and analyze.  And yes, the presence of bacteria matters: when Koch and Schmid-Hempel infected sterile bees, feces-fed bees and gammaproteobacteria-fed bees with C. bombi, the feces-fed bees, which had established gut microbial communities in the correct gamma:beta proportion, ended up with a 10-fold lower parasite load than the other groups. Consistent with this, the authors also found a negative correlation in wild-caught bumblebees between the proportion of betaproteobacteria in an individual’s gut and the level of parasite infection.

In social bees, these beneficial microbial communities are passed from generation to generation in the normal business of the hive.  In solitary bumblebee species, however, the set of bacteria you acquire as you leave your pupal state behind you is more of a matter of chance, leaving the bee potentially exposed to parasite infection. Because of the relative simplicity of their gut microbiota, bees may turn out to be an interesting model system for studying how microbial symbionts help protect their hosts against pathogens.

Koch H, & Schmid-Hempel P (2011). Socially transmitted gut microbiota protect bumble bees against an intestinal parasite. Proceedings of the National Academy of Sciences of the United States of America PMID: 22084077