UC San Diego

The European honey bee, Apis mellifera, plays an important role in agriculture worldwide, but managed colonies have faced considerable losses in recent years, including a 41% winter loss of U.S. managed colonies in 2019. A widespread contributing factor is a microsporidian pathogen, Nosema ceranae, which has spread to colonies worldwide, developed a resistance to antibiotic treatment, and can alter the host’s immune response and nutritional uptake. These obligate gut pathogens share their environment with a natural honey bee microbiome whose composition can affect pathogen resistance. We tested the effect of N. ceranae infection on this microbiome by feeding 5 d old adult bees (with developed natural microbiomes) with live N. ceranae spores (40,000 per bee) or a sterile 2.0 M sucrose solution. We caged and reared these bees in a controlled lab environment and tracked their mortality over 12 d, after which we dissected them, measured their infection levels (spore counts), and prepared samples for future microbiome analyses. Bees fed live spores had two-fold higher mortality by 12 d, as well as 36.5-fold more spores per bee as compared to controls. In addition, we found strong colony effects on infection, confirming that susceptibility to N. ceranae varies widely between colonies. By analyzing the microbiome of the most resistant bees, future studies can be performed to determine how these microbes affect resistance of honey bees to microsporidian infections.