A substance in honey bee larvae inhibits the growth of Paenibacillus larvae larvae

Paenibacillus larvae larvae, a Gram-positive and spore-forming bacterium, is the cause of American foulbrood. We investigated the resistance of larvae of different ages from different colonies against P. larvae larvae. We prepared ethanol-water-extracts from two, three, four and five day old larvae and also larvae in two capped stages to test the ability of the homogenates to inhibit the growth of P. larvae larvae in vitro. There were age and colony dependent differences in the inhibiting potentials of larvae. The results suggest that the concentration of the inhibiting substance(s) in the extract might be responsible for the known different degree of resistance against P. larvae larvae in vivo. Our results further show that the extracts of the two-, three- and four-day-old larvae have the ability to inhibit the growth of P. larvae larvae very well, while starting around day five the inhibiting effect is gradually reduced