Pollen extracts and constituent sugars increase growth of a trypanosomatid parasite of bumble bees

Phytochemicals produced by plants, including at flowers, function in protection againstplant diseases, and have a long history of use against trypanosomatid infection. Floralnectar and pollen, the sole food sources for many species of insect pollinators, containphytochemicals that have been shown to reduce trypanosomatid infection in bumbleand honey bees when fed as isolated compounds. Nectar and pollen, however, consistof phytochemical mixtures, which can have greater antimicrobial activity than dosingle compounds. This study tested the hypothesis that pollen extracts would inhibitparasite growth. Extracts of six different pollens were tested for direct inhibitory activityagainst cell cultures of the bumble bee trypanosomatid gut parasite Crithidia bombi.Surprisingly, pollen extracts increased parasite growth rather than inhibiting it. Pollenextracts contained high concentrations of sugars, mainly the monosaccharides glucoseand fructose. Experimental manipulations of growth media showed that supplementalmonosaccharides (glucose and fructose) increased maximum cell density, while acommon floral phytochemical (caffeic acid) with inhibitory activity against othertrypanosomatids had only weak inhibitory effects on Crithidia bombi. These resultsindicate that, although pollen is essential for bees and other pollinators, pollen maypromote growth of intestinal parasites that are uninhibited by pollen phytochemicalsand, as a result, can benefit from the nutrients that pollen provides.