Characterization of intrinsic resistance of Apis mellifera colonies against the European Foulbrood in Switzerland. Application for breeding protocols.

European foulbrood (EFB) is caused by Melissococcus plutonius, a bacterium that infests the intestinal tract of honey bee larvae and leads, in combination with secondary microbial invaders, to their death. In severe cases, EFB can lead to the collapse of the whole colony, due to massive brood loss. The number of EFB cases in Switzerland has increased considerably during the last decade. Local and environmental factors as colony and apiary density, different virulence levels of M. plutonius strains, colony constitution, beekeeping and agricultural practices have been

hypothesized to play a major role in disease susceptibility and outbreak. Because of their high economic

relevance, honey bees in general and honey bee diseases in particular are well studied, but an understanding of pathogenetical processes and defense mechanism involved in an EFB infection is still lacking. Therefore, more knowledge on the epidemiology of EFB is needed to find new and more effective strategies to control this disease. I will study the problem EFB with three different approaches: First, by investigating the environment of M. plutonius, my goal is to conduct a metagenome analysis of the intestinal tract of honey bee

larvae and perform tests to study the inhibitory potential of beneficial gut bacteria (e.g. lactic acid bacteria) on the multiplication of M. plutonius. Second, to further characterize the epidemiology of M. plutonius I will a) focus on the virulence of different M. plutonius strains using molecular tools, with the aim of determining and mapping virulence genes; b) monitor the infection process of M. plutonius in the larval gut, using genetic fluorescence labeling tools for visualization and quantification. Third, by investigating the defense

mechanisms of the honey bee at the individual and colony level, I am going to a) determine which immune

genes are expressed by individual honey bees infected with M. plutonius; b) search for active compounds in the larval food, which might repress bacterial multiplication; c) verify if a more effective hygienic behavior can

prevent EFB; d) study if colony and individual resistance to EFB is linked to the genotype of honey bees. For

this purpose, I will verify whether there are initial patrilineal differences in EFB resistance within colonies and establish two bee populations presenting very distinct tolerance levels to compare them for immune responses, composition of larval food, beneficial gut bacteria and hygienic behavior. The goal of my work is to consolidate our knowledge on the factors of EFB defense mechanisms and to promote EFB resistance by

honey bees across Switzerland by providing selection criteria that could be used in breeding protocols for beekeepers to obtain EFB resistant honey bees.