Could vaccinating baby bees reverse the global decline in bee numbers? Dalial Freitak at the University of Helsinki and her colleagues think so. They found a common yolk protein that nourishes the developing bee larvae, also kickstarts their immune system. Their discovery was published in PLOS Pathogens in July.
“Finding a link between this molecule and what’s happening in the field – that is extremely exciting,” says Boris Baer, director of the University of Western Australia’s Centre for Integrative Bee Research.
Reports of rapid declines in bee populations date back as far as the Roman Empire. But this time it’s different, says Baer. “We don’t see bee populations recovering”. In the US, for instance, the number of managed honey bee colonies have declined from six million in 1947 to 2.5 million today. The global phenomenon dubbed “colony collapse disorder” threatens our food supply. Bees are our main crop pollinators – if they go, so might we.
The general view is that colony collapse disorder is a perfect storm of pesticides, pathogens and fewer wildflowers. The European Union restricted neonicotinoid pesticides in 2013 and varroa mites, which suck bees dry of haemolymph – their version of blood – can be controlled with chemicals if caught early. Freitak and her colleagues took a different tack and explored the natural immunity of bees.
Mammals acquire immunity by developing antibodies. Bees can’t make antibodies like we do, but they do share one feature with mammals. Mammalian mothers transfer antibodies to their offspring via their placenta and milk. It turns out bee mothers must also pass on some type of immunity to the egg. Scientists have known since the mid-2000s that baby bees can resist bacteria such as E. coli and fungi such as yeast the moment they hatch.
But they had no idea how the larvae received this immunity boost. Freitak and her team set out to investigate. Bee eggs contain large amounts of the protein vitellogenin. (So do the eggs of fish and birds where it provides much of the yolk’s nutrient content.) Freitak’s team had a hunch this protein might also play a role in bee immunity.
To test that idea in the lab, they removed six queen bee ovaries and laid them in a dish containing bacterial particles tagged with fluorescent molecules. The particles stayed put outside the ovaries. But when vitellogenin was added to the mix, those particles were shuttled into the ovaries. Exactly how, the researchers don’t know. But no vitellogenin; no particle transfer.
Freitak thinks the bee’s natural immunisation cycle starts with forager bees, which pick up pathogens as they collect pollen and nectar for the queen’s royal jelly. Her gut enzymes break down the pathogens and the fragments move into energy stores known as “fat bodies” in her abdomen and head. This is also where vitellogenin is made. As the vitellogenin moves from the fat bodies to the ovaries, they pick up and shuttle along pathogen fragments too. The whole mix is deposited into unlaid eggs.
After the queen lays her eggs, the developing larvae feast on the mix. The fragments trigger their immune system to make antimicrobial peptides that target those particular pathogens, so that when they emerge from the egg, they’re already armed.
Could Freitak’s discovery offer a way for us to help bees?
Her team has already filed a patent, and begun field trials, for a bee vaccine against American foulbrood, the most destructive bacterial infection of honeybee larva; it turns them into an infectious goo. When other bees clean up, they spread the parasite to the entire hive.
The infection can take out a queen and her hive before she can digest the bacterium and immunise the next generation. The bacteria also produce tough spores so the only solution for beekeepers is to burn the hive before the disease spreads.
Freitak’s vaccine contains pre-digested fragments of the bacterium that commercial beekeepers can add to bee food. The bacterium will end up in the queen’s dinner, and then, thanks to vitellogenin, inside all her eggs. Freitak hopes the vaccine could be given as a treatment on demand. “It would work like a yearly flu shot,” she says.