In Vivo and In Vitro Evaluation of Probiotic and Postbiotic Strategies Against Foulbrood in Honeybees

American and European foulbroods, which are among the most severe honeybee diseases, are caused by Paenibacillus larvae and Melissococcus plutonius, respectively. Both bacteria pose a significant threat to larval health and colony survival. Due to the lack of safe and effective treatments, there is growing interest in using beneficial bacterial supplements as a promising alternative to antibiotics. This study evaluated both the probiotic and postbiotic potential of selected bacterial strains against foulbrood pathogens with the aim of gaining a better understanding of their modes of action. An initial screening of 25 strains for anti-foulbrood activity using agar diffusion techniques was conducted to select the most active candidates for further investigation. We investigated the role of their cell-free supernatants (CFS) and initiated the characterisation of their activity and mechanisms of action. The minimal inhibitory dose of the CFS, the nature of the active compounds, and their antibacterial effect were determined. Finally, the probiotic and postbiotic properties were assessed using P. larvae-infected larvae reared under laboratory conditions. Five lactic acid bacteria exhibited antagonistic activity against one or both pathogens, with two of them showing particularly strong inhibitory effects through their CFS. The CFS of Lactobacillus crispatus and Lactiplantibacillus plantarum achieved complete inhibition of P. larvae at a concentration of 12.5% (v/v). Subsequent characterisation suggested that their activity was primarily bacteriostatic, likely mediated by lactic acid. In vivo assays demonstrated a significant 42.5% increase in larval survival when supplemented with live L. plantarum, whereas CFS treatments failed to rescue infected larvae. These results emphasise the potential of probiotic- and postbiotic-based strategies as a sustainable approach to managing foulbrood in beekeeping.