A worker of the bumble bee species (Bombus terrestris) used in the study.
Credit: Paul Schmid-Hempel
CAMBRIDGE: A certain socially transmitted gut bacteria that plays an essential role in bee health and resistance to parasites has been identified, according to a new study.
The paper emphasises sociality as the key element in establishing these bacterial communities (or 'micriobiota'). The findings have implications for our general understanding of bee health and disease, as populations are declining worldwide.
"To understand what keeps bees healthy, we should start to look beyond the bee host immune system and consider the microbiota as an integral part of the defense of bees against their parasite," said Hauke Koch, co-author of the study published in Proceedings of the National Academy of Science today, and biologist at the Swiss Federal Institute of Technology Zurich in Switzerland.
Protecting pollination
Bees and their pollinating 'services' are vital for the existence of agriculture and wild plants. Yet bee populations - particularly the bumble bee and honey bee - are declining at rapidly across the globe.
Although the increasing rarity of some bumble bee species is now widely documented, due to the devastation caused by the deadly, parasitic varroa mite, there is still limited information on the underlying processes or severity of these declines. So the importance of understanding bee immune systems is to better protect them against the pathogens that are wiping them out.
Recently, there has been renewed interest in the idea that a transfer of microorganisms - such as bacteria - might play a role in insect defense systems. Scientists have suspected that, in addition to the bee's internal system, there might be an equally essential 'external immune system', facilitated by social or environmental factors. However, there has been little evidence so far to suggest that any specific 'bacterial transfers' are, for instance, protective against harmful parasites.
Parasites and pathogens
Koch and colleague Paul Schmid-Hempel, also from the Swiss Federal Institute of Technology Zurich, have now analysed gut bacteria from bumble bees as a potential protecting agent against a familiar gut parasite. The parasite (Crithidia bombi) is both ubiquitous and detrimental: it impedes bee queens from achieving a maximally healthy colony, which decreases their chances of survival and the greater sustainability of the species.
To test the bacteria's effects, the scientists separated and raised worker bee pupae in three groups. The first was fed their regular diet of faeces from their nest mates, the second was fed the same type of faeces - but artificially cultivated - and the third was fed sugar water. Separating those fed the gut bacteria of their nest mates from those fed the same gut bacteria from external sources allowed the researchers to also test the influence of sociality in ensuring protection against parasites.
Gut instincts
Koch and Schmid-Hempel found that the first group that were fed faeces from nest mates were significantly less affected by the Crithidia bombi parasite, with an average infection rate almost an order of magnitude less than the other two groups. They appeared to have established a 'normal' or ideal cocktail of microorganisms in their gut.
This led to both a low level of infection and, relatedly, better protection against even the most diverse and intense strains of the parasite. Moreover, the sugar water group had a virtually undetectable amount of the protective gut bacteria, indicating that the acquisition of this essential 'cocktail' comes solely through social interaction. Combined with previous studies, which show the absence of these microbes in solitary bee species, the role of sociality in conferring parasitic defense mechanisms is solidified. According to the researchers, the social environment of bumble bees is essential for establishing a healthy external immune system.
