Glyphosate perturbs the gut microbiota of honey bees

Glyphosate, the most heavily used herbicide in the world, targets a specific enzyme found in plants and in some microorganisms - including some beneficial bacteria found in the gut of honey bees. The researchers set out to assess possible impacts glyphosate exposure might have on these bacteria, which contribute to food processing, immune system regulation, and defense against pathogens. They found that glyphosate exposure reduced the protective effect of gut microbiota against opportunistic pathogens.

The first experiment looked at changes to the composition of bee microbiomes following glyphosate exposure. Adult worker bees were exposed to 5 mg/L or 10 mg/L of glyphosate for five days, then returned to the hive. These glyphosate concentrations were chosen to mimic environmental levels, which have been detected between 1.4 and 7.6 mg/L in the field. A subsample of bees from each treatment group were captured and tested for changes in gut microbiota on the day of reintroduction and 3 days after reintroduction to the hive.

Gut bacteria numbers decreased for both exposure groups but was more pronounced in the group exposed to the lower dose of 5 mg/L. The bees exposed to the lower dose also experienced more severe compositional shifts in their gut microbiome. The relative lack of effect at the higher exposure is unexplained although the researchers hypothesized that it could be due to the recapture methods, which recovered fewer than 20% of the bees as it failed to sample bees that died or abandoned the hive. The eight dominant bacteria in honey bee guts differed in sensitivity to glyphosate, with some species declining significantly in relative abundance following glyphosate exposure.

The second experiment looked at the interaction of glyphosate exposure and pathogen infection. Newly emerged workers, which acquire their gut microbial community in the first few days after emergence, were exposed to 5 mg/L or 10 mg/L of glyphosate for five days, after which point the bees were exposed to the opportunistic pathogen Serratia marcescens. Exposure to glyphosate and then the pathogen led to increased mortality relative to honey bees only exposed to the pathogen or only to glyphosate. Mortality effects from glyphosate exposure were thus not direct effects of glyphosate on bees, but due to the increased vulnerability to pathogen infection. The authors suggest that glyphosate may interfere with the ability of newly emerged workers to assemble a normal gut microbial community, making them more susceptible to harmful pathogens.

Authors: 
Motta, E.V.S., K. Raymann, and N.A. Moran
Journal: 
PNAS
Year published: 
2018
DOI: 
https://doi.org/10.1073/pnas.1803880115