pH-mediated inhibition of a bumble bee parasite by an intestinal symbiont

Abstract

Non-pathogenic microbes can provide multiple benefits to their hosts, including pathogen inhibition. Gut symbionts can augment resistance to pathogens by stimulating host immune responses, competing for space and nutrients, or producing antimicrobial metabolites. The gut microbiota of social bees, which pollinate many crops and wildflowers, has demonstrated benefits against diverse infections and might help protect against pathogen-related declines. The bumble bee gut microbiota, consisting chiefly of five taxa common to corbiculate bees, has been shown to enhance resistance to the trypanosomatid parasite Crithidia bombi. Specifically, infection intensity was negatively correlated with abundance of Lactobacillus \"Firm-5\" bacteria. However, the mechanism underlying this relationship remains unknown. We tested the hypothesis that the Firm-5 bacterium Lactobacillus bombicola, which produces lactic acid, inhibits C. bombi via a pH-mediated effect.\n\nConsistent with our hypothesis, Lactobacillus bombicola spent medium inhibited C. bombi growth via a reduction in pH that was both necessary and sufficient for inhibition. Inhibition of all parasite strains occurred within the pH range previously observed in honey bee guts, though sensitivity to acidity varied among parasite strains. Spent medium was slightly more potent than HCl, D-, and L-lactic acids for a given pH, suggesting that other metabolites also contribute to inhibitory effects. Our results implicate symbiont-mediated reduction in gut pH as a key determinant of trypanosomatid infection in bees. Future investigation into in vivo effects of gut microbial composition on pH and infection intensity would help determine the relevance of these findings for bees threatened by trypanosomatids.\n\nImportancePollinators such as honey and bumble bees provide services to plants in agricultural and wild ecosystems, but both wild and managed bees are threatened by infection-related declines. The symbiotic gut microbiota of bees provides a naturally occurring defense against infection. For example, the bumble bee microbiota reduces infection with trypanosomatid parasites, but how inhibition occurs remains unknown. We show that the acidic spent medium from a common bumble bee gut symbiont, Lactobacillus bombicola, inhibits in vitro growth of the trypanosomatid gut parasite, Crithidia bombi. The acidity of the spent medium was both necessary and sufficient for parasite inhibition. Inhibitory pH values were within the range documented in honey bee guts, suggesting that pH-mediated parasite inhibition is plausible in live bees. Results suggest that production of acids by sugar-fermenting symbionts confers pH-mediated resistance to infection in bees, whereas depletion of core microbiota could result in low-acid conditions that favor parasite growth.