Fatty acids and their derivatives from <i>Chlorella vulgaris</i> extracts exhibit <i>in vitro</i> antimicrobial activity against the honey bee pathogen <i>Paenibacillus larvae</i>

The green microalga <i>Chlorella vulgaris</i> Beijerinck (Chlorellaceae) is widely used as a food supplement for humans and animals. In beekeeping practice, <i>Chlorella vulgaris</i> has potential as a pollen supplement. We studied whether <i>Chlorella</i> extracts display antimicrobial properties against <i>Paenibacillus larvae</i>, the causative agent of the honey bee bacterial disease American foulbrood. We identified components responsible for antimicrobial activity and evaluated the added values of <i>Chlorella</i> as a food supplement for honey bees. Different extracts (water, acetone, methanol) were prepared from <i>Chlorella</i> biomass (phototrophically and heterotrophically cultivated) and screened for antimicrobial activity against ERIC I and ERIC II genotypes of <i>P. larvae</i>. Active acetone extracts of phototrophically cultivated <i>Chlorella vulgaris</i> biomass were fractioned via preparative reverse-phase chromatography. Antimicrobial activity was detected for 9 of the resulting 33 fractions. Further analysis revealed the chemical composition of the active fractions. <i>C. vulgari</i>s extracts showed a significant antimicrobial effect against vegetative cells and spores of <i>P. larvae</i> strains of ERIC I and ERIC II genotypes. The lowest MIC of the most active acetone extract was 6.3 µg/mL for both tested genotypes. In the majority of the active fractions, monolinolenin, fatty acid linoleic acid, and methyl esters of linoleic and/or palmitic acid were identified via high-performance liquid chromatography coupled with high-resolution mass spectrometry analysis. Based on our results, we concluded that algal <i>C. vulgaris</i> food supplements not only contain nutritional but also potential prophylactic properties for honey bee health.