Abstract
The development of all honey bee castes proceeds through three different life stages all of which encounter microbial infections to a various extent. We have examined the immune strength of honey bees across all developmental stages with emphasis on the temporal expression of cellular and humoral immune responses upon artificial challenge with viable Escherichia coli bacteria. We employed a broad array of methods to investigate defence strategies of infected individuals: (a) fate of bacteria in the haemocoel; (b) nodule formation and (c) induction of antimicrobial peptides (AMPs). Newly emerged adult worker bees and drones were able to activate efficiently all examined immune reactions. The number of viable bacteria circulating in the haemocoel of infected bees declined rapidly by more than two orders of magnitude within the first 4–6 h post-injection (p.i.), coinciding with the occurrence of melanised nodules. Antimicrobial activity, on the other hand, became detectable only after the initial bacterial clearance. These two temporal patterns of defence reactions very likely represent the constitutive cellular and the induced humoral immune response. A unique feature of honey bees is that a fraction of worker bees survives the winter season in a cluster mostly engaged in thermoregulation. We show here that the overall immune strength of winter bees matches that of young summer bees although nodulation reactions are not initiated at all. As expected, high doses of injected viable E.coli bacteria caused no mortality in larvae or adults of each age. However, drone and worker pupae succumbed to challenge with E.coli even at low doses, accompanied by a premature darkening of the pupal body. In contrast to larvae and adults, we observed no fast clearance of viable bacteria and no induction of AMPs but a rapid proliferation of E.coli bacteria in the haemocoel of bee pupae ultimately leading to their death.
Highlights
IntroductionHoney bees (Apis mellifera) maintain three castes within a colony
As eusocial insects, honey bees (Apis mellifera) maintain three castes within a colony
In Europe, the problems started with the introduction of the ectoparasitic mite Varroa destructor leading to a newly acquired virulence of several bee viruses [41].In the USA, the colony collapse disorder (CCD), a phenomenon characterized by a yet unexplained hive abandonment of the total worker honey bee population has resulted in a loss of 50 to 90% of colonies [42,43]
Summary
Honey bees (Apis mellifera) maintain three castes within a colony. A single queen (fertile female), up to 50,000 workers (sterile females) and about 2,000 drones (males) are found in hives during the early summer season [1]. Queens and drones differ markedly in morphological, physiological and behavioural features. A characteristic property of honey bees is their specific way to survive the winter season as compared to other eusocial insects. The bulk of workers does not die in the autumn (like bumble bees) or hibernate in caves below the frost line (like ants and termites), but about 1/5 to 1/10 of the members of a bee colony cluster together when the temperatures drop. Summer and winter bees differ in many ways, markedly in longevity. Summer bees live 20 to 40 days, whereas winter bees remain alive up to 230 days [3]
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