Probiotic potential and inhibitory properties of bacteria and yeasts derived from water kefir against the honey bee pathogen Paenibacillus larvae

In this study, bacterial and yeast strains from water kefir were evaluated for their inhibitory activity toward Paenibacillus larvae, the causative agent of American foulbrood. Using agar well diffusion, 16 bacteria and 25 yeasts consistently inhibited growth of at least one of eight strains tested. Bacterial inhibition of the pathogen was associated with lactic acid-producing genera (Lacticaseibacillus, Liquorilactobacillus, Lentilactobacillus), while yeast inhibition—the first such effect identified from in vitro assays—was associated with Saccharomyces cerevisiae and Pichia kluyveri. Although the mechanisms remain unclear, metabolomics screening implicated acetate, lactate, ethanol, and other metabolites as potential contributors to growth suppression. Our findings highlight the promise of developing both bacterial and yeast-based probiotics as novel tools to control American Foulbrood in managed honey bee colonies.