Bacterial Agents for Biocontrol of American Foulbrood (AFB) of Larvae Honey Bee

Bee colonies are constantly threatened by a bacterial larvae disease called American foulbrood, caused by the Gram-positive bacterium <i>Paenibacillus larvae</i>. This is a highly specialized pathogen with only one established host, the honey bee larvae. The current state of legislation throughout the European Union prevents the inclusion of pharmacotherapy treatment, and the only control is based on the physical elimination of infected colonies upon veterinary orders. The purpose of this study was to investigate the antimicrobial activity of selected bacteria with proven probiotic potential with typical characteristics meeting the definition of a probiotic that could reduce the American foulbrood pressure by promoting the development of the larvae microbiome that competes with and blocks the excessive proliferation and production of <i>P. larvae</i> endospores. The antimicrobial activity of inoculants of the following selected strains was studied: <i>Bacillus pumilus</i>, <i>Bacillus licheniformis</i>, <i>Streptomyces narbonensis</i>, <i>Lysinibacillus fusiformis</i>, <i>Levilactobacillus brevis</i>, <i>Bacillus megaterium</i> against <i>Paenibacillus larvae</i> ATCC 9545 (formerly <i>Paenibacillus larvae </i>sub sp. <i>larvae)</i>, and <i>Paenibacillus larvae</i> CCUG 48973. Analyses were performed by the well diffusion method according to EUCAST standards (clinical breakpoints and dosing of antibiotics) with modifications due to the specificity of the bacteria used. The largest zone of growth inhibition of <i>P. larvae</i> was confirmed with <i>S. narbonensis</i>, <i>B. licheniformis</i>, and <i>B. megaterium</i>, and medium activity was observed with <i>L. brevis</i> and <i>B. pumilus</i>. Negligible activity was shown by <i>L. fusiformis</i>. Differences were noticed in the resistance of indicator strains of <i>P. larvae</i> and between the media and carriers used.