Tyramine functions as a toxin in honey bee larvae during Varroa-transmitted infection by Melissococcus pluton

Area of Science:

• Apiculture and Veterinary Entomology
• Microbiology and Biochemistry

Background:

• European foulbrood is a significant disease affecting honey bee larvae.
• The causative agent of European foulbrood has been debated, with Melissococcus pluton identified as a key bacterium.
• Varroa destructor mites are known vectors of honey bee pathogens.

Purpose of the Study:

• To identify the toxic compound produced by Melissococcus pluton isolated from honey bee pupae.
• To determine the toxicity of the identified compound on protozoa and honey bee larvae.
• To elucidate the role of this compound in the pathogenesis of European foulbrood.

Main Methods:

• Isolation and identification of bacteria from infected honey bee pupae.
• Anaerobic culture of bacteria to produce toxic compounds.
• Purification of the toxic agent using XAD columns, gelfiltration, and preparative HPLC.
• Identification of the toxic agent using GC-MS and FTICR-MS.
• Toxicity testing using a protozoan mobility assay and direct feeding to honey bee larvae.
• Comparative analysis of other biogenic amines for toxicity and hemolytic activity.

Main Results:

• Melissococcus pluton was isolated and identified from honey bee pupae wounds.
• The toxic compound produced by M. pluton was identified as tyramine.
• Tyramine exhibited immediate mobility inhibition in Stylonychia lemnae at 0.2 mg/ml.
• The lethal dose of tyramine for 4-5 day old bee larvae was determined to be 0.3 mg/larva.
• Tyramine and histamine showed the highest membranolytic activity against human red blood cells.

Conclusions:

• Tyramine, a characteristic amine produced by Melissococcus pluton, is the causative agent of toxic symptoms in bee larvae.
• The findings suggest that tyramine is responsible for the pathogenesis of European foulbrood.
• Varroa destructor mites may play a role in transmitting the infection that leads to European foulbrood.