Bacteria Increases Pollen Protein Digestibility

  • Honey bees must have protein in their diet

  • Pollen is a source of protein

  • Proteins are broken down into amino acids

  • Bacteria assists in preserving pollen, extracting and breaking down protein from pollen

  • A healthy gut contains hundreds of non-core bacterial species


Honey bees foraging for pollen store pollen on a specialized area on their hind legs.


Honey Bees Need Protein in their Diet

While we usually think of honey bees collecting nectar, an average-size colony may bring in 100 pounds of pollen in a season. Pollen is an essential part of the honey bee diet, providing a wide range of nutrients: protein, carbohydrates, lipids, vitamins, and minerals.

Honeybee food is digested in the ventriculus and ileum. Honey bees need to break down dietary proteins into smaller units, amino acids, with the help of other specialized proteins, called proteolytic enzymes. Honey bees secrete proteolytic enzymes such as trypsin, but gut microbes are a great source of proteolytic enzymes that support digestion.


What is Protein?

Proteins are large biological molecules that perform many different functions in living cells. Proteins are building materials in body tissues, muscles, and glands. Proteins are made up of smaller units called amino acids. There are twenty different types of amino acids. If a correct amino acid can’t be found, whole proteins can’t be synthesized. The most abundant protein in honey bees is vitellogenin, a protein that influences stress resilience and is important to honey bee social organization. Honey bees can’t make some amino acids, and these amino acids, called essential amino acids, must come from food (pollen and bee bread). The essential amino acids for honey bees are arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (De Groot, 1953). The nutritional composition of the honey bee diet affects survival.


Role of Bateria in the Digestive Process in Honey Bee Gut

Honey bee gut bacteria contribute to digestion of pollen and other honeybee food within the ileum. Bacteria ferment pollen and nectar to produce nutritional molecules called Short-Chain Fatty Acids (SCFAs). SCFAs are small fuel molecules that can be metabolized (absorbed into hemolymph and used for energy) by honey bees directly.

In 2017 study1, Zheng and colleagues treated honeybees with an antibiotic to remove bacteria (GF) and compared these honey bees to conventional (CV) bees with normal microbiomes. They showed that microbes increase the production of SCFAs (acetate, propionate, lactate, succinate, and butyrate) in the ileum, rectum, and for their absorption into the hemolymph. These SCFAs are decreased when bacteria are removed from honey bees (GF). In conventional (CV) honey bees, SCFAs levels are higher in the hemolymph, where these fuel molecules support honey bee development, hormonal signaling, and weight gain.


Levels of SCFAs in the gut and hemolymph of GF and CV honey bees © Zheng et al4


Role of Bacteria in Stored Pollen/Bee Bread

Through the addition of nectar and glandular secretions and certain bacteria, honeybees store pollen as beebread. This processing changes how pollen looks and increases digestibility and nutritional value of stored pollen (Herbert and Shimanuki, 1978; Human and Nicolson 2006). Figure from COLOSS BEEBOOK2 (below) shows the appearance of pollen grains before (A) and after (B) addition of nectar and glandular secretions.


In 2015, Anderson et al3 reported that stored pollen does not undergo long-term nutrient conversion and those microbes do not predigest the pollen prior to consumption by bees. In their study, researchers analyzed fresh pollen and bee bread, and found pollen grain shells intact and containing very few bacteria in bee bread. In contrast, pollen found in honey bee hindgut had many more bacteria attached. These observations were interpreted to mean that the bacteria on pollen are associated with preservation, not digestion, of nutrients. In 2021, another study4 revealed a previously unseen effect of bacteria on pollen. Christiansen et al inoculated fresh pollen with Nectar-dwelling Acinetobacter bacteria and waited 45 minutes to see that the bacteria “stimulated protein release by inducing pollen to germinate and burst.” A precise measurement of available protein indicated that bacteria doubled available protein from the pollen grains in the study. Therefore, previously unnoticed, bacteria likely play an important role in increasing available protein from pollen, both before and after pollen grains are consumed by honey bees.


Photograph of pollen grain that germinated and burst © Christiansen et al3


Which Microbes?

Our understanding of microbes in the honey bee gut is growing. The microbes best characterized within the adult honey bee gut, also called core gut microbiome, are the five most consistent, ubiquitous, and abundant bacterial taxa: Snodgrassella, Gilliamella, Lactobacillus Firm-4, Lactobacillus Firm-5, and Bifidobacterium. These core bacterial types coexist alongside a high number of less-frequent bacteria acquired from the environment, such as Erwinia, Serratia, Citrobacter, Acinetobacter, Lactobacillus not belonging to Firm-4 or Firm-5, Streptococcus, and Bacillus. A recent study5 showed that the core bacterial community is made up of 32 species of bacteria, while 164 species of non-core bacteria are found in the same honey bee samples. Non-core bacteria in the honey bee gut are diverse and functional. One of their functions is to support digestion.


 

About the Author


Vera Strogolova is a scientist fascinated by microbes. As a co-founder and CTO of Strong Microbials, Vera is charged with the development, research, and improvement of probiotics for honey bees.

 

1Zheng, H; Powell, J E; Steele, M I; Dietrich, C; Moran, N A (2017) Honeybee gut microbiota promotes host weight gain via bacterial metabolism and hormonal signaling. PNAS 114 (18): 4775-4780


2Human, H; Brodschneider, R; Dietemann, V; Dively, G; Ellis, J; Forsgren, E; Fries, I; Hatjina, F; Hu, F-L; Jaffé, R; Jensen, A B; Köhler, A; Magyar, J; Özkýrým, A; Pirk, C W W; Rose, R; Strauss, U; Tanner, G; Tarpy, D R; Van Der Steen, J J M; Vaudo, A; Vejsnæs, F; Wilde, J; Williams, G R; Zheng, H-Q (2013) Miscellaneous standard methods for Apis mellifera research. In V Dietemann; J D Ellis; P Neumann (Eds) The COLOSS BEEBOOK, Volume I: standard methods for Apis mellifera research. Journal of Apicultural Research 52(4): http://dx.doi.org/10.3896/IBRA.1.52.4.10


3Anderson, K; Carroll, M J; Sheehan, T; Mott, B M; Maes, P; Corby-Harris, V (2015) Hive-stored pollen of honey bees: many lines of evidence are consistent with pollen preservation, not nutrient conversion. Molecular Ecology 23:5904-5917


4Christensen, S M; Munkres, I; Vannette, R L (2021) Nectar bacteria stimulate pollen germination and bursting to enhance microbial fitness. Current Biology 31:1-8


5Callegari, M., Crotti, E., Fusi, M. et al. Compartmentalization of bacterial and fungal microbiomes