Honey’s magical power: four ways to slay microbes

You’ve heard the rumors, right? Honey never goes bad, it’s been found in Egyptian tombs ready to eat, and it’s the only cure for some antibiotic-resistant bacteria. You may wonder if any of this is true, and if so, why. Let’s look at some of the details. the beginning of recorded history, honey has been known to have antiseptic properties. Frequent mention of honey in medicine was made by the Egyptians, Assyrians, Greeks, and Romans. But then, as now, users were well aware that some honeys were better healers than others, so a method of grading honey was developed. Beginning in 1937 the underlying cause of honey’s healing power was given the name “inhibine” and a number was assigned to different honey types to indicate how strong the inhibine was by measuring how well it killed specific bacteria like staph.

Related to honey’s antiseptic properties is its extraordinary shelf life. Depending on its source and how it was handled, honey may remain edible for many years. According the National Honey Board, “Honey stored in sealed containers can remain stable for decades and even centuries.”Nevertheless, the why of it can be difficult to understand.

A review of the literature reveals four distinct reasons for the medicinal action and stability of honey. Three of them are directly related to things honey bees do to the nectar they collect. The fourth comes from the plants themselves.

The four factors that affect the antiseptic strength of honey are the osmotic concentration, acidity, amount of hydrogen peroxide, and the presence of specialized plant compounds. The healing power of any one sample of honey is simply the sum of all the factors,3 so we will look at each one separately.

Osmotic concentration

The osmotic concentration of a solution refers to the number of particles dissolved in a unit of liquid. If you’ve ever made sugar syrup, you know that one part of sugar dissolves easily in one part of water. But two parts of sugar in one part of water begins to get tricky. After stirring forever, you may give up and use heat to force the sugar into solution.

But honey is approximately four parts of sugar dissolved in one part of water. We call this a supersaturated solution because the liquid is holding more particles than it could under normal circumstances. But a supersaturated sugar solution is unstable; it may suddenly crystallize or it may absorb water from the surrounding environment.

When a substance absorbs water from its surroundings, we say it is hygroscopic. For example, if you leave a jar of honey uncovered on the counter, it absorbs moisture from the atmosphere. Likewise, if you put honey on a bacterium, it will suck the water right out of the cell, killing it by dehydration. This hygroscopic action is one key to honey’s long shelf life and its ability to heal wounds—it simply dehydrates any microbe it touches.

But the osmotic concentration of honey changes as water is absorbed. Once the honey absorbs enough water to reach equilibrium, it no longer absorbs more. That jar of honey that you left uncovered will eventually absorb so much water from the air that it is no longer supersaturated. At that point, a microbe such as a yeast spore can land on it and germinate, causing the honey to ferment.

You get a similar result when you extract honey frames that contain many uncapped cells. Because uncapped cells contain excess water, they can lower the osmotic concentration of the entire batch, leading to fermentation. The inverse relationship between osmotic concentration and the amount of water in the honey means this mode of microbial suppression is temporary.

The next two modes of microbial suppression, acidity and the presence of hydrogen peroxide, are both due to the action of one enzyme, glucose oxidase.

Acidity

The hydronium ion concentration, or pH, of honey varies from about 3.2 to 4.5. This high acidity is partially due to acids found in the nectar, including acetic, butyric, formic, lactic, and malic. But the major source of acidity in honey is produced by the bees themselves. After collecting nectar in the field, the bees carry it in their bodies where it is mixed with glucose oxidase. In a multistep process, this enzyme oxidizes glucose into gluconolactone and then into hydrogen peroxide and gluconic acid.

The acidity of gluconic acid is enough to weaken, if not kill, many microorganisms. If nothing else, the acidity may slow their growth and reproduction. But similar to osmotic concentration, the acidity of the honey will attenuate with the addition of water. Water from the microbes themselves and moisture from the atmosphere will, in time, lessen the acidity and therefore reduce the honey’s ability to suppress additional microbes.

Hydrogen peroxide

The chief antimicrobial agent in honey is hydrogen peroxide. In fact, in 1963 hydrogen peroxide was found to be the mysterious inhibine. Since then, analyses have shown the presence of hydrogen peroxide in all samples of honey that show antimicrobial action, including manuka.

During the conversion of nectar to honey, honey bees use a number of different enzymes. To begin, the bees secrete invertase into the nectar. The invertase splits the sucrose, a disaccharide, into two monosaccharides, glucose and fructose. Then, in the presence of water and oxygen, the glucose oxidase converts the glucose portion into gluconic acid and hydrogen peroxide, as described above.

At one time, a solution of 3% hydrogen peroxide was popular for treating wounds and infections, but it gradually fell out of favor due to its tendency to cause tissue damage. Experiments have shown that while hydrogen peroxide aids wound healing at low concentrations, it delays healing at high concentrations. But the level of hydrogen peroxide in honey is much lower than the manufactured product and is insufficient to kill pathogens outright. As a wound treatment, honey works by delivering a sustained low-level dose of hydrogen peroxide instead of a single high dose. The sustained low dose works by interrupting cell division and degrading bacterial DNA.

Since nectar comprises water and sugar, it could easily ferment in the hive before the transition to honey is complete. Instead, the presence of glucose oxidase protects it during the curing process. Conversely, without a good supply of water and oxygen, glucose oxidase remains inactive. For that reason, fully ripened honey in a covered container produces no hydrogen peroxide. But once it’s exposed to oxygen and water again, the glucose oxidase is reactivated and resumes production of the protective compounds.

In essence, water and oxygen together act like a switch, turning the glucose oxidase on and off. So if you spread fully-cured honey on a wound, atmospheric oxygen plus exudates from the wound provide the conditions necessary for glucose oxidase to produce both hydrogen peroxide and gluconic acid which, in turn, kill the microbes present in the wound.

Specialized plant compounds

A wide variety of plant compounds are antimicrobial to some degree. These non-peroxide chemicals, which are found naturally in nectar, become concentrated in the honey as the bees remove the water. They include enzymes, flavonoids, organic acids, and proteins. Examples of plants with strong microbial action include heather, viper’s bugloss, lavender, kanuka, kamahi, and of course manuka. While most fully-cured honeys provide some degree of antimicrobial protection, nectar from plants with specialized phytochemicals can be especially effective in wound care.

Production of glucose oxidase in the honey bee

Glucose oxidase is produced by the honey bee worker. For a long time, it was thought that only bees of honey-processing age produced this enzyme, but later research showed that it is also produced by younger bees. For example, nurse bees have been shown to secrete glucose oxidase directly into larval food, which results in antimicrobial protection for the developing brood.

Honey bees secrete a variety of enzymes from several different glands, including the hypopharyngeal, mandibular, head salivary, and thorax salivary glands, but glucose oxidase is produced solely by the hypopharyngeal glands. The amount of production increases from young cleaner bees, to nurse bees, to honey-processing bees, and then gradually decreases in foragers.

Bee health, diet, and glucose oxidase

Several researchers have found a correlation between honey bee health and the amount of glucose oxidase produced. A well-balanced and varied honey bee diet increases glucose oxidase production. In fact, the highest glucose oxidase levels have been found in polyfloral honey, indicating that colonies consuming pollen from diverse sources were able to obtain the nutrition needed to secrete higher amounts. These results suggest that a varied honey bee diet enhances the microbial defenses of the colony.

Glucose oxidase has been found in the food stored by at least nine species of eusocial Hymenoptera, including one ant, one wasp, and seven species of bees in the genera Apis, Bombus, and Trigona. In all cases, no hydrogen peroxide was detected until the honeys were diluted with water.

Based on these findings, it is easy to see how a poor or sparse selection of flowering plants might affect the colony health of not only honey bees but other pollinators as well.

Variation in microbial action

The level of antimicrobial action varies from one floral source to another. With the exception of manuka, variations in antimicrobial action of honey are closely correlated with the amount of hydrogen peroxide in the honey. Consequently, anything that affects the amount of hydrogen peroxide affects the antimicrobial action of the hone

Ironically, one compound that can negatively affect the amount of hydrogen peroxide is catalase, a plant-derived enzyme commonly found in pollen. Catalase reduces hydrogen peroxide to water and oxygen. The amount of catalase in a sample of honey is related to the amount of pollen as well as the source of the pollen.

However, other research has shown that even exceptionally high levels of catalase will not destroy all of the hydrogen peroxide. This finding suggests that either some hydrogen peroxide is inaccessible to the catalase or that the rate of production of hydrogen peroxide exceeds the rate of destruction. In addition to catalase, other phytochemicals, including an array of antioxidants, may be responsible for suppressing the hydrogen peroxide activity in some honeys.

Post-harvest handling and antimicrobial activity

Another reason for low antimicrobial activity in some honey is poor post-harvest handling. Both glucose oxidase and hydrogen peroxide are easily degraded by heat and light. Remember, hydrogen peroxide is sold in a brown bottle for a reason: exposure to light speeds its disintegration into water and oxygen.

In short, honey intended for medicinal use should be treated with care. It should not be heated, even slightly. In addition, some sources recommend it be pressed from the comb instead of processed through a radial extractor. Extraction by centrifugal force can incorporate both oxygen and atmospheric moisture into the honey, which prematurely initiates the production of hydrogen peroxide, and may damage some of the protective phytochemicals. After extraction, honey should be kept at moderate room temperatures and protected from sunlight.

Areas for further research

Research is needed to discover whether honey bees can be bred to produce heightened levels of glucose oxidase. Honey with elevated levels of glucose oxidase could be especially appropriate for medical purposes. In addition, such honey might better protect a honey bee colony against specific pathogens.

Other researchers are looking into whether honey with high levels of glucose oxidase could be used as a food preservative, especially in those items which typically require minimal processing and little heat.

Please note that this article is for informational purposes only and is not a substitute for medical advice. If you have a medical condition, please consult your physician.

This article first appeared in American Bee Journal, Volume 157 No 12, December 2017, pp. 1325-1327.