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Friday, 31 March 2017

In the guts of bees

By Nicola Temple

We hear a great deal about the beneficial bacteria that live in our digestive system and commonly referred to as the microbiome, which help us turn indigestible materials into nutrients that we can absorb. There are countless probiotic products on the market that are meant to introduce more of these beneficial bacteria into our system, enriching our microbiome. However, humans and indeed mammals are not alone in having helpful microflora in the gut.

The microbes that inhabit the guts of social bees has been of particular interest recently. These microbial communities have been studied for their role in bee health, but also as a model organism to help understand the relationship between hosts and their gut microbes, potentially providing insight into our own system.

The specialised cast of microbes

The microbiome of bees is relatively simple, but very specialised. There are about eight to ten bacterial species, but different species of bee will carry different strains of these bacterial species. The bacteria are so specialised that a strain from one bee genus isn’t able to colonise the gut of a bee from a different genus. This suggests that these bacterial strains have been evolving with their hosts over a very long period of time.

Nest entrance of the stingless bee, Geniotrigona thoracica, is
from Malaysia. Photo credit: Eunice Soh.
Like us, these bacteria help the bees break down complex molecules through fermentation in order to make the nutrients available to the hosts. There’s also evidence that they might help to neutralise toxins in the gut. These friendly microbes also outcompete nastier pathogenic species that can make the host ill. For example, the gut microbes in bumblebees have been linked to lower levels of the parasite Crithidia bombi.

The gut microbes of non-social insects, including solitary bees, aren’t as specialised because they acquire them from their environment rather than from other members of their species. Among social bees, it is behaviours such as passing food between individuals and feeding larvae, that allow an exchange of microbes. However, these exchanges pass along the bad microbes as well as the good.  Beekeepers are painfully aware that pathogens can pass through a colony like wildfire. Social insects therefore need a very responsive system that helps keep these pathogens in check. And the key to this might be a very ancient relationship between the good microbes and the host bees themselves, which allows the bee’s immune system to quickly identify the less desirable critters.

A long-term relationship

Research published this week in the journal Science Advances suggests that five of the species of gut bacteria found in modern social bees have been evolving along with their hosts for about 80 million years. It was around this time that the first solitary bees began socialising with other bees - sharing nests and food resources and making concerted defence efforts. The descendants of these first social bees are the hundreds of species of honey bees, bumblebees and stingless bees that are alive today.
This finding not only shows that social creatures, such as bees and humans, transfer bacteria among each other during the same lifetime, they pass them along generations, enabling the microbiome and host to evolve together.

"The fact that these bacteria have been with the bees for so long says that they are a key part of the biology of social bees," says Nancy Moran, a professor of integrative biology at the University of Texas who co-led the research with postdoctoral researcher Waldan Kwong. "And it suggests that disrupting the microbiome, through antibiotics or other kinds of stress, could cause health problems."
The co-evolution of the gut bacteria and the bees is so closely linked, in fact, that the researchers found that when a new species of bee branches off in the evolutionary tree, a new strain of bacteria branches off with it. The result being that each of the hundreds of species of social bees alive today has its own specialised strains of gut microbes.

Human influence on this long-term relationship

It’s currently unknown how toxins introduced by humans, including pesticides, might affect the bee microbiome. There is recent evidence, however, that the prophylactic use of antibiotics by bee keepers in the US has resulted in some gut bacteria in honeybees developing antibiotic resistance.


References

Engel, P. et al. 2016. The bee microbiome: impact on bee health and model for evolution and ecology of host-microbe interactions. mBio 7 (2): e02164-15.

Kwong, W.K., Medina, L.A., Koch, H., Sing, K-W., Soh, E.J.Y., Ascher, J.S., Jaffe, R. & Moran, N.A. 2017. Dynamic microbiome evolution in social bees. Science Advances 3: e1600513.

Kwong, W.K., Engel, P., Koch, H. & Moran, N.A. 2014. Genomics and host specialization of honey bee and bumble bee gut symbionts. PNAS 111 (31): 11509-14.


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