Propolis or bee glue is a resinous mixture that honey bees collect from tree buds, sap flows, or other botanical sources. It is used as a sealant for unwanted open spaces in the hive. Propolis is used for small gaps (approximately 6 millimeters (0.24 in) or less), while larger spaces are usually filled with beeswax. Its color varies depending on its botanical source, the most common being dark brown. Propolis is sticky at and above room temperature, 20 °C (68 °F). At lower temperatures, it becomes hard and very brittle.
For centuries, beekeepers assumed that bees sealed the beehive with propolis to protect the colony from the elements, such as rain and cold winter drafts. However, 20th-century research has revealed that bees not only survive, but also thrive, with increased ventilation during the winter months throughout most temperate regions of the world.
Propolis is now believed to:
- reinforce the structural stability of the hive;
- reduce vibration;
- make the hive more defensible by sealing alternative entrances;
- prevent diseases and parasites from entering the hive, and to inhibit fungal and bacterial growth;
- prevent putrefaction within the hive. Bees usually carry waste out of and away from the hive. However, if a small lizard or mouse, for example, finds its way into the hive and dies there, bees may be unable to carry it out through the hive entrance. In that case, they would attempt instead to seal the carcass in propolis, essentially mummifying it and making it odorless and harmless.
The composition of propolis varies from hive to hive, from district to district, and from season to season. Normally, it is dark brown in color, but it can be found in green, red, black, and white hues, depending on the sources of resin found in the particular hive area. Honey bees are opportunists, gathering what they need from available sources, and detailed analyses show that the chemical composition of propolis varies considerably from region to region, along with the vegetation. In northern temperate climates, for example, bees collect resins from trees, such as poplars and conifers (the biological role of resin in trees is to seal wounds and defend against bacteria, fungi and insects). "Typical" northern temperate propolis has approximately 50 constituents, primarily resins and vegetable balsams (50%), waxes (30%), essential oils (10%), and pollen (5%). Propolis also contains persistent lipophilic acaricides, a natural pesticide that deters mite infestations.
In neotropical regions, in addition to a large variety of trees, bees may also gather resin from flowers in the genera Clusia and Dalechampia, which are the only known plant genera that produce floral resins to attract pollinators. Clusia resin contains polyprenylated benzophenones. In some areas of Chile, propolis contains viscidone, a terpene from Baccharis shrubs, and in Brazil, naphthoquinone epoxide has recently been isolated from red propolis, and prenylated acids such as 4-hydroxy-3,5-diprenyl cinnamic acid have been documented. An analysis of propolis from Henan, China found sinapinic acid, isoferulic acid, caffeic acid, and chrysin, with the first three compounds demonstrating antibacterial properties. Also, Brazilian red propolis, largely derived from Dalbergia ecastaphyllum plant resin, has high relative percentages of the isoflavonoids 3-hydroxy-8,9-dimethoxypterocarpan and medicarpin. Other flavonoids commonly present include galangin and pinocembrin. Caffeic acid phenethyl ester (CAPE) is also a component of some varieties of propolis from New Zealand.
Occasionally, worker bees will even gather various caulking compounds of human manufacture, when the usual sources are more difficult to obtain. The properties of the propolis depend on the exact sources used by each individual hive; therefore any potential medicinal properties that may be present in one hive's propolis may be absent from another's, or from another sample in the same hive.
The National Institutes of Health rates propolis as "possibly effective" for treating cold sores, genital herpes, and post-surgery mouth pain. Currently, there is "insufficient evidence" to rate the effectiveness of propolis in treating other conditions.
Propolis is the focus of a large number of research projects. Some preliminary research findings (published in the biomedical literature), together with their limitations, are described below.
Preliminary scientific studies show some types of propolis have in vitro antibacterial and antifungal activity with active constituents including flavonoids like galangin and hydroxycinnamic acids like caffeic acid. In the absence of any in vivo or clinical studies however, it is not clear if this antimicrobial activity has any therapeutic relevance.
Preliminary in vivo studies with rats suggest propolis may be effective in treating the inflammatory component of skin burns. Also, a clinical trial has shown Brazilian propolis skin cream to be superior to silver sulfadiazine for the treatment of partial thickness burn wounds. Recent studies have raised concerns about the efficacy of silver sulfadiazine, however, and suggest that it may actually delay wound healing. Further clinical research is needed.
Propolis has been reported to exhibit both immunosuppressive and immunostimulant effects. Further research is needed to establish if there is a practical application for these seemingly opposing pharmacological effects.
Propolis has been the subject of recent dentistry research, and there is some in vivo and clinical evidence that propolis might protect against dental caries and other forms of oral disease, due to its antimicrobial properties. Propolis is also being investigated for its efficacy in the treatment of canker sores and in reducing the inflammation associated with canal debridement and endodontic procedures.
One in vivo study has shown that propolis reduced the chances of cataracts in rat pups. Again however, in the absence of any clinical studies, it is not clear if this activity has any therapeutic relevance.
In in vitro tests, propolis induces cell cycle arrest, apoptosis and reduces expression of growth and transcription factors, including NF-κB. Notably, caffeic acid phenethyl ester down-regulates the mdr-1 gene, considered responsible for the resistance of cancer cells to chemotherapeutic agents. In in vivo studies with mice, propolis inhibits 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced tumorigenesis. Once again, in the absence of any clinical studies, it is not clear if this activity has any therapeutic relevance.
In musical instruments
Propolis is used by most stringed instrument makers (violin, viola, cello and bass) to enhance the appearance of the wood grain. It is a component of some varnishes and its distinctive smell is easily recognizable in a luthier's shop. Propolis was purportedly used by Antonio Stradivari in the varnish of his instruments.
Propolis is used by some chewing gum manufacturers to make propolis gum.
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