9. Exposure bioassays using in vitro rearing of larvae

Bioassays can be used to determine the biological activity of a substance by its effects on a test organism. Differences in virulence of a pathogen are best analysed in exposure bioassays, and such methods involving in vitro rearing of honey bee larvae (see the in vitro rearing paper of the BEEBOOK (Crailsheim et al., 2013) have been used for both Paenibacillus larvae and M. plutonius (McKee et al., 2004; Genersch et al., 2005; Giersch et al., 2009). Virulence tests using this technique show that M. plutonius strains collected in different geographic places in Europe present important variations in the mortality rate and how fast the larvae die (Charrière et al., 2011). Three common measurement results can be obtained from exposure bioassays: the dose (LD50) or concentration (LC50) of the pathogen it takes to kill 50 % of the hosts tested, and the time (LT50) required for killing 50 % of infected individuals. For the purpose of determining the LD50 or LC50, a reliable estimation of the concentration of bacterial cells used in the exposure bioassay is crucial.