Ecology of Pollen Storage in Honey Bees: Sugar Tolerant Yeast and the Aerobic Social Microbiota

Historically, the storage of collected pollen by honey bees was thought to rely on microbes to enhance pollen nutrition. However, this hypothesis has found little empirical support. More recent experiments that quantified pollen storage time, microbial load relative to pollen mass, and variation in the microbiota clearly indicate that honey bees do not rely on microbial enzymes to alter the nutritional quality of collected pollen. Here, we quantified abiotic factors that suppress microbial growth in stored pollen and determined microbial abundance relative to pollen mass using both culturing and molecular assays. We found that microbial growth is quickly suppressed by added honey- and host-supplied enzymes, but that sugar tolerant yeasts subsist longer than bacteria in stored pollen. This work contributes to our understanding of host-microbial interactions in the honey bee and highlights the aerobic social microbiota, a symbiotic and omnipresent collection of native bacteria and yeasts that dominate the social resource space of the honey bee colony and hive. Honey bee colonies are resource rich and densely populated, generating a constant battle to control microbial growth. Honey is relatively sterile in comparison with beebread: a food storage medium comprising pollen mixed with honey and worker head-gland secretions. Within colonies, the microbes that dominate aerobic niches are abundant throughout social resource space including stored pollen, honey, royal jelly, and the anterior gut segments and mouthparts of both queens and workers. Here, we identify and discuss the microbial load in stored pollen associated with non-Nosema fungi (primarily yeast) and bacteria. We also measured abiotic changes associated with pollen storage and used culturing and qPCR of both fungi and bacteria to investigate changes in stored pollen microbiology by both storage time and season. Over the first week of pollen storage, pH and water availability decreased significantly. Following an initial drop in microbial abundance at day one, both yeasts and bacteria multiply rapidly during day two. Both types of microbes then decline at 3-7 days, but the highly osmotolerant yeasts persist longer than the bacteria. Based on measures of absolute abundance, bacteria and yeast are controlled by similar factors during pollen storage. This work contributes to our understanding of host-microbial interactions in the honey bee gut and colony and the effect of pollen storage on microbial growth, nutrition, and bee health.