Probiotic and postbiotic strategies against foulbrood in honeybees: in vitro and in vivo insights

Among the most severe honeybee diseases, American and European foulbroods caused by Paenibacillus larvae and Melissococcus plutonius , respectively pose a significant threat to larval health and colony survival. Due to the lack of safe and effective treatments, interest is growing in the use of beneficial bacterial supplements as a promising alternative to antibiotics. This study evaluated the probiotic and postbiotic potential of selected bacterial strains against foulbrood pathogens. An initial screening of 25 strains for anti-foulbrood activity led to the selection of the most active candidates for further investigation. The inhibitory effect of their cell-free supernatants (CFS) was assessed and their mode of action was investigated. The probiotic and postbiotic properties were further evaluated using P. larvae -infected larvae reared under laboratory conditions. Five lactic acid bacteria exhibited strong antagonistic activity against one or both pathogens, as their CFS displayed inhibitory effects. Notably, the CFS of Lactobacillus crispatus and Lactiplantibacillus plantarum completely inhibited P. larvae at a dose of 12.5% (v/v). Further characterisation of these CFS, suggested a bacteriostatic effect, mainly attributed to organic acids. In vivo assays demonstrated a significant increase in larval survival when supplemented with live L. plantarum , whereas CFS treatments failed to rescue infected larvae. These findings highlight the potential of probiotic and postbiotic-based strategies as sustainable alternatives for managing foulbrood in beekeeping.