Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Microbiology and Immunology

Supervisor

Reid, Gregor

Abstract

The use of neonicotinoid pesticides has been implicated in the recent decrease of honey bee (Apis mellifera) populations. In this thesis, a Drosophila melanogaster model was used to characterize immune impairment associated with imidacloprid (neonicotinoid) exposure and test the ability of beneficial bacteria (lactobacilli) to alleviate these harmful effects. The experiments outlined in chapters 2 and 3 demonstrate that imidacloprid alters the gut microbiota by exploiting the cooperation between gut immune pathways. The pesticide reduces expression of Duox via dysregulation of the immune deficiency pathway resulting in decreased hydrogen peroxide production. This contributes to the microbiota changes but also depletes antimicrobial peptide expression through reduced nitric oxide signalling. By supplementing Drosophila with certain strains of lactobacilli, this immune impairment was mitigated. In summary, these studies show how a widely used pesticide contributes to honey bee losses by dysregulating their immune system; however, these effects can be countered by lactobacilli intervention.

Summary for Lay Audience

Honey bees are vital pollinators that help to maintain the global food supply. Despite their benefits to the global community, these insects are experiencing considerable population decline. While numerous causal factors have been identified, pesticides have been recognized for their unintentional toxicity to non-target insects. In particular, neonicotinoid pesticides are widely used despite warnings of collateral damage. The goal of this thesis is to understand the mechanisms whereby low doses of neonicotinoids harm honey bees. As it is difficult to do experiments on bees themselves, Drosophila melanogaster possess similar properties that make it a sufficient model organism. Capitalizing on the genetic tractability of these flies, we showed that neonicotinoids suppress the gut immune system, which then makes the honey bee susceptible to being killed by harmful bacteria. The dual oxidase (Duox) pathway is the first line of defence, which produces hydrogen peroxide to kill invading microorganisms. It was found that a commonly used neonicotinoid—imidacloprid—impaired this pathway by reducing the production of hydrogen peroxide. Imidacloprid induced this by interacting with the immune deficiency pathway, the second line of gut defence. This resulted in insufficient hydrogen peroxide produced to kill harmful bacteria. Additionally, the reduction in hydrogen peroxide causes a decrease in the generation of nitric oxide and subsequent nitric oxide signalling to distal organs, which results in diminished antimicrobial peptide production. It was found that by feeding the flies with specially chosen lactobacilli (beneficial bacteria), the damage caused by the pesticide to the immune system was less severe. This work forms the basis of testing supplementation with beneficial bacteria as a means to reduce the demise of honey bee populations. Development has led to the creation of a BioPatty that contains the lactobacilli plus essential nutrients for the bees. Therefore, by using basic science principles and an appropriate fruit fly model, we can generate a mechanistic rationale to test an intervention in a real-world setting. While cessation of pesticide use should be the ultimate goal, until then, the application of probiotic lactobacilli may contribute to saving the honey bees and our food supply.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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