Does Bacillus larvae produce an antibacterial substance in infected honey bee larvae?

Abstract - An effort was made to answer the question of whether production of antibacterial substance by Bacillus larvae, the cause of American foulbrood (AFB) disease, is the reason why bee larval cadavers (scales) contain a pure culture of that bacterium. Laboratory bioassays of antibacterial activity indicated the presence of an antibacterial agent in spent Bailey's culture medium used to grow B larvae, whereas antibiotic activity was not detected in larval scales naturally infected with B larvae. The antibacterial activity appears at the onset of B larvae sporulation in artificial medium and its potency rises during sporulation of the pathogen. The agent has a broad spectrum of antibacterial activity against Gram-positive bacilli, micrococci and Gram-negative bacteria including Pseudomonas aeruginosa. The activity of the antibacterial substance was unaffected by proteolytic enzyme digestion and by pretreatment with proteolytically active, aqueous scale filtrates. Since the antibacterial compound is not liberated during infection, the presence of pure cultures of B larvae found in bee cadavers cannot be explained simply by antibiosis. Because Apis mellifera larvae contain little or no indigenous microflora, pure cultures of B larvae in cadavers from AFB infected brood could result from little or no competition for the infecting bacillus.