Propolis: A Sticky Antimicrobial Secretion Helping Bees Fight Diseases
Bees amaze us with their incredible social organization. Even more fascinating is how bees maintain colony health. Bees have evolved a mutualistic relationship with plants that fulfill their nutritional requirements but also support their immune system function. As social insects, honey bees rely on their individual and social immune responses to fight pathogens inside and outside of their bodies1. An individual’s immune response is often characterized by an increased production of antimicrobial peptides, which represents an energetic cost to the individual bee since the immune system requires a significant amount of energy to maintain. Conversely,
social immunity consists of behavioral defense mechanisms performed by bees, and requires a lower expenditure of a bee’s energy reserves. Examples of social immunity in honey bees include hygienic behavior and foraging for resins to construct a propolis envelope inside the nest. The presence of a propolis envelope in the bee nest is an important component of the immune defenses of individual bees and to colony health2–4.
When nesting in standard beekeeping equipment with solid and smooth inner walls, bees do not deposit propolis in a continuous envelope as they do when nesting in a natural tree cavity. However, when the inner walls of man-made boxes are modified to create a rough, tree cavity-like pattern, bees will deposit propolis in the cracks and construct a propolis envelope (Figure 1).
Figure 1. Modified man-made beekeeping equipment. a) Propolis traps
stapled to inside walls of a hive to encourage bees to construct a propolis
envelope. b) View of the propolis envelope when traps were removed. Bees will
deposit propolis within most of the gaps of each propolis trap (brown lines on
the box are the deposited propolis).
In my PhD research, I wanted to explore how the presence of a propolis envelope inside Langstroth boxes benefits colony health. Through my experiments, I found that bees from healthy colonies that were allowed to construct a propolis envelope inside of the hive had significantly lower baseline immune function (i.e., less energetically costly) over the summer and fall months compared to bees from healthy colonies without a propolis envelope3. Additionally, bees from colonies with a propolis envelope had higher protein levels (e.g., vitellogenin) coming out of the winter, which may lead to higher brood production in the spring3.
What happens when the colony is not healthy? My second research study demonstrated the benefits of the propolis envelope after field colonies were challenged with Paenibacillus larvae, the causative agent of American foulbrood (AFB) disease. First, I found that in challenged colonies with a propolis envelope, the food provided to young larvae (the life-stage susceptible to AFB infection) had higher antimicrobial activity compared to challenged colonies without a propolis envelope2. Two months after infection, colonies with a propolis envelope had 52% fewer cells infected with AFB compared to colonies without a propolis envelope2 (Figure 2). It is not
clear if antimicrobial compounds from propolis come in direct contact with brood jelly or if nurse bees from propolis envelope colonies have the ability to synthesize higher levels of their own antimicrobial compounds into brood jelly and potentially decrease colony-level AFB infection more rapidly and efficiently compared to bees in challenged colonies without a propolis envelope. Either way, these results confirm the existence of a natural defense mechanism in honey bees against AFB by feeding larvae food with a higher antimicrobial activity5,6, and more importantly, this mechanism of defense against AFB was only observed when challenged colonies had a propolis envelope.
Figure 2. American foulbrood infection level. Severity scores (0 = 0 cells
containing sign of AFB; 1 = 1-5 cells; 2 = 6-25 cells; and 3 = ≥ 26 cells per
comb) average ± S.E (N= 10 colonies) were compared between treatments
using two tailed t-test with frames as random variables (α = 0.05).
From these two studies we learned that resin collection in honey bees seems to be both constitutive (i.e., collected regardless of physiological demand or pathogen level), and inducible (i.e., a conditional response to infection). My next study was to determine how honey bees use plant resins as a form of self medication in response to pathogens. My results indicated that bees will self-medicate in response to a fungal infection from Ascosphaera apis, the causative agent of Chalkbrood, but not in response to a bacterial infection from P. larvae4 (Figure 3).
Figure 3. Resin foraging activity ± S.E (N= 10 colonies per treatment)
measured as the difference of resin foragers between after and before colony
inoculation. Significant difference between treatment groups was determined
by ANOVA followed by Tukey-HSD test. Treatment groups designated with
different letters (A, B) are significantly different
These studies provided new insights into the complex way that behavioral defenses benefit colony health and contribute to reduce pathogen infection, as well as helped emphasize the importance of resins as pharmacological agents in the ecology and evolution of plant-animal interactions.
What’s next? There has been a growing interest in studying the natural defense mechanisms of honey bees (e.g., social immunity). Examples of on-going research include: bee self medication with respect to specific plant sources of resin, the effects of propolis on the abundance and diversity of the honey bee gut microbiota, the mode of action of propolis on modulating honey bee immune response and pathogen defense, quantifiable benefit of propolis to colony health and productivity in commercial beekeeping operations, and breeding programs for honey bee natural defenses (including propolis deposition in the nest). Research focusing on honey bees’ natural ability to fight pathogens and parasites will provide tools to reduce the need for human intervention and promote sustainable beekeeping.
References:
1. Evans, J. D. & Spivak, M. Socialized medicine: Individual and communal
disease barriers in honey bees. J. Invertebr. Pathol. 103, S62–S72 (2010).
2. Borba, R. S. & Spivak, M. Propolis envelope in Apis mellifera colonies
supports honey bees against the pathogen, Paenibacillus larvae. Sci. Rep. 7,
1–6 (2017).
3. Borba, R. S., Klyczek, K. K., Mogen, K. L. & Spivak, M. Seasonal benefits of
a natural propolis envelope to honey bee immunity and colony health. J. Exp.
Biol. (2015). doi:10.1242/jeb.127324
4. Borba, R. Constitutive and therapeutic benefits of plant resins and a propolis
envelope to honey bee, Apis mellifera L., immunity and health. (University of
Minnesota, 2015). doi:3734812
5. Rose, R. I. & Briggs, J. D. Resistance to American foulbrood in honey bees IX.
Effects of honey-bee larval food on the growth and viability of Bacillus larvae. J.
Invertebr. Pathol. 13, 74–80 (1969).
6. Thompson, V. C. & Rothenbuhler, W. C. American Foulbrood
in Honey Bees. II. Differential Protection of Larvae by Adults of
Different Genetic Lines. J. Econ. Entomol. 50, 731–737 (1957).