Humoral immune response of Galleria mellonella larvae after infection by Beauveria bassiana under optimal and heat-shock conditions

Natural infection of Galleria mellonella larvae with the entomopathogenic fungus Beauveria bassiana led to antifungal, but not antibacterial host response. This was manifested by induction of gallerimycin and galiomicin gene expression and, consequently, the appearance of antifungal activity in the hemolymph of the infected larvae. The activity of lysozyme increased at the beginning of infection and dropped while infection progressed. Exposure of the naturally infected animals to 43 °C for 15 min extended their life time.Galleria mellonella larvae were injected with 104, 105 and 106 fungal blastospores, resulting in the appearance of strong antifungal activity and a significant increase in lysozyme activity in larval hemolymph after 24 h. Antibacterial activity was detectable only when 105 and increased when 106 blastospores were injected. The number of the injected B. bassiana blastospores also determined the survival rate of animals. We found that exposure of the larvae to 38 °C for 30 min before infection extended their life time when 103 and 104 spores were injected. The increase in the survival rate of the pre-heat-shocked animals may be explained by higher expression of antimicrobial peptides and higher antifungal and lysozyme activities in their hemolymph in comparison to non-heat-shocked animals.