Glucosinolate Bioactivation by <i>Apis mellifera</i> Workers and Its Impact on <i>Nosema ceranae</i> Infection at the Colony Level

The microsporidian fungus <i>Nosema ceranae</i> represents one of the primary bee infection threats worldwide and the antibiotic fumagillin is the only registered product for nosemosis disease control, while few alternatives are, at present, available. Natural bioactive compounds deriving from the glucosinolate–myrosinase system (GSL–MYR) in Brassicaceae plants, mainly isothiocyanates (ITCs), are known for their antimicrobial activity against numerous pathogens and for their health-protective effects in humans. This work explored the use of <i>Brassica nigra</i> and <i>Eruca sativa</i> defatted seed meal (DSM) GSL-containing diets against natural <i>Nosema</i> infection in <i>Apis mellifera</i> colonies. DSM patties from each plant species were obtained by adding DSMs to sugar candy at the concentration of 4% (<i>w</i>/<i>w</i>). The feeding was administered in May to mildly <i>N. ceranae</i>-infected honey bee colonies for four weeks at the dose of 250 g/week. In the treated groups, no significant effects on colony development and bee mortality were observed compared to the negative controls. The <i>N. ceranae</i> abundance showed a slight but significant decrease. Furthermore, the GSL metabolism in bees was investigated, and MYR hydrolytic activity was qualitatively searched in isolated bee midgut and hindgut. Interestingly, MYR activity was detected both in the bees fed DSMs and in the control group where the bees did not receive DSMs. In parallel, ITCs were found in gut tissues from the bees treated with DSMs, corroborating the presence of a MYR-like enzyme capable of hydrolyzing ingested GSLs. On the other hand, GSLs and other GSL hydrolysis products other than ITCs, such as nitriles, were found in honey produced by the treated bees, potentially increasing the health value of the final product for human consumption. The results are indicative of a specific effect on the <i>N. ceranae</i> infection in managed honey bee colonies depending on the GSL activation within the target organ.