What honey bees eat, and the bacteria in their gut

Fuel and building material

A honey bee colony runs on two foods that do different jobs. Nectar, stored as honey, is carbohydrate: the fuel for flight, heat and wax-making. Pollen is everything else — protein, lipids, sterols, vitamins and minerals. Adult workers can live on sugar for a while, but brood cannot be reared without protein, and the protein comes from pollen eaten by young nurse bees and turned into the glandular food they feed to larvae.

Quantity is not the only question. In a caged-bee experiment, different single-plant pollens changed nurse bees' physiology — the development of the glands that make brood food and the level of vitellogenin, a storage protein central to bee health — and changed how well bees tolerated the gut parasite Nosema ceranae. A mixed pollen diet made no difference to healthy bees, but parasitised bees fed mixed pollen outlived those fed any single pollen except the richest in protein.

Bee bread: stored, and perhaps not fermented

Pollen that is not eaten at once is packed into cells with nectar, honey and bee secretions and sealed — bee bread. Most accounts, including earlier versions of this site, say lactic acid bacteria then ferment it over a week or so into a more digestible food. A 2014 study from the University of Arizona and a USDA bee research centre tested that and did not find support. Bees preferred to eat pollen stored for less than three days; freshly collected pollen carried few bacteria, and their numbers fell after four days of storage; stored and fresh pollen carried the same bacterial community; and there was roughly one microbe to a million units of pollen-grain surface.

Its authors concluded that the cell is a preservative environment, kept so by the added honey, nectar and secretions and by pollen itself, rather than a place where microbes transform the food. HoneyHQ treats the fermentation account as contested rather than wrong, because it rests on older culture-based work the 2014 study did not overturn on every point — but it no longer states it as fact.

The gut community

Compared with a mammal's, the honey bee's gut microbiome is strikingly simple. It is dominated by only about nine bacterial species clusters that are found in bees and nowhere else, with a great deal of variation between strains within them. Among the best studied are Snodgrassella alvi, Gilliamella apicola and Frischella perrara.

1. 1Born without itA newly emerged worker has no gut bacteria at all.
2. 2Caught from nursesWithin four to six days in the hive, characteristic communities form in the ileum and rectum. In experiments, typical communities formed only when nurse bees or their faeces were present; food sharing alone, or contact with comb, honey or bee bread, produced atypical ones.
3. 3Put to workBees with their normal gut bacteria gained more weight than bees without. The bacteria produce short-chain fatty acids, lower the gut's acidity and oxygen, and appear to break down plant material in pollen into compounds that reach the bee's blood.

What disrupts it

• Antibiotics. Beekeepers in some countries use them against bacterial brood disease. In one study, antibiotic treatment changed the size and make-up of the gut community for a long time afterwards and lowered survival, in the hive and in the laboratory when bees were then exposed to opportunistic bacterial pathogens.
• Glyphosate. The herbicide works by blocking an enzyme, EPSPS, that plants and some bacteria depend on. Most bee gut bacteria carry a version of it, and Snodgrassella alvi carries the sensitive kind. Bees exposed to glyphosate at concentrations documented in the environment lost dominant gut species, and young workers so exposed died more often after later exposure to the pathogen Serratia marcescens.