Honey, a complex substance made by a variety of species of bees, is often celebrated for its sweetness and health benefits. In fact, humans have been using honey, sometimes called liquid gold, for generations to soothe cold symptoms and promote wound healing, citing its anti-inflammatory, antioxidant and antimicrobial properties. Is there scientific merit to these claims, and, if so, what gives honey its medicinal properties?
While the public may see honey as a common grocery store item, it is also home to a diverse population of microorganisms, which essentially create a local microbiome within the viscous substance. The presence of these microbes poses several important questions when considering human consumption, health and well-being. For example, is honey safe to eat when it is raw, pasteurized or if the consumer is especially young in age? Delve into the microbiology of honey, uncover the factors that influence its microbial composition and explore the implications on human health.
The Composition of Honey and Its Relationship to Microbes
Honey is primarily composed of sugars, water and various organic compounds produced by honeybees. The most well-known honeybee species, Apis mellifera, has been cultivated by western farmers to mass produce honey for sale. However, there are 8 other recognized species of bees that produce honey, including Apis cerana and Apis nigrocincta.
In an intricate biological process, honeybees harness external sources of flower nectar and honeydew to enzymatically convert maltose and sucrose into the main sugars in honey: fructose and glucose. As such an appealing source of sugar, honey can be home to a variety of microorganisms, including Bacillus, Klebsiella, Saccharomyces and Aspergillus. These microbes are introduced through the bee's digestive tract and the environment (air, soil, dust, plants). Microbial diversity in honey can therefore vary significantly based on factors, including the microbiome of the honeybee, the floral source of the nectar, geographical location of the hive and conditions of the environment. Some microbes can be beneficial. For example, some bacteria produce compounds, like lactic acid, which contribute to honey's acidity and preservative qualities. While other microbes can be undesirable. Yeasts, such as Saccharomyces and Candida, may be found in batches of honey that have high moisture content and can lead to spoilage.
The honeybee gut microbiome plays a major role in the processing and chemical composition of honey. Two major groups of lactic acid bacteria, namely Lactobacillus and Bifidobacterium, are conserved in honeybee guts globally. These bacteria contribute to the acidity, probiotic nature and anti-microbial peptides found in honey. Other beneficial populations for honeybee health found in bee guts include Enterobacter, Klebsiella, Citrobacter and Serratia. If these beneficial populations are disturbed, it can alter the health of the honeybee, leaving it susceptible to infections, which can ultimately lead to hive collapse. For example, Aspergillus fumigatus, a well-known opportunistic fungal pathogen that can infect immunocompromised human populations, can also affect honeybee larvae when brought in from contaminated nectar and cause stonebrood disease.
What Are the Antimicrobial Properties of Honey?
H2O2, Low pH and the Osmotic Effect
Despite the presence of these microbes at low levels, the natural composition of honey creates an environment that is inhospitable to microbial growth. Worker honeybees produce an enzyme called glucose oxidase in a secretory organ in their heads called the hypopharyngeal gland. Glucose oxidase converts glucose into hydrogen peroxide (H2O2) and water. During the honey making process, the enzyme is regurgitated, along with the nectar that the worker bees collect and bring back to the hive. H2O2 then remains trapped within the honey itself, creating a relatively low pH and acting as a natural preservative. This low pH, along with the high sugar and low moisture content of honey (osmotic effect), creates a natural barrier against most harmful bacterial growth.
Still, the acidity of different honey sources can change according to seasonal flora and the resources that hives use to produce it. Environmental sources of the nectar collected by bees will also influence the microbial composition of the honey. Different flowers produce nectar with varying chemical compositions. As a result, there are 300 unique honey flavors recognized globally, based on regional floral populations. Nectar sources for honeybees in the U.S. include, California avocado blossoms; Buckwheat flowers found in New York and parts of the Midwest and citrus flower blossoms found throughout Florida, parts of Texas and Southern California. One well known type of mono floral honey (a honey generated with nectar and pollen from a single flower source) is Manuka honey, which attributes its nectar source to manuka flowers from Leptospermum scoparium, a bush that is native to Australia and New Zealand.
Manuka Honey and Methylglyoxal (MGO)
Manuka honey has been touted for its anti-inflammatory and antibacterial properties, due, in part, to the presence of a compound known as Methylglyoxal (MGO), which is made from dihydroxyacetone (DHA) in the nectar of manuka flowers. Increased concentration of MGO has been positively correlated with stronger antimicrobial effects. However, research shows that there are other properties in Manuka honey, including H2O2 and the osmotic effect, which make the whole product a more effective antimicrobial than any of its individual parts.
An article published in ASM's mSystems journal, evaluated the antimicrobial effects of manuka honey in its whole form against Pseudomonas aeruginosa, compared to artificial honey and MGO alone and found that the non-peroxide antimicrobial activity (NPA) of Manuka honey is not as strong as the antimicrobial effect demonstrated when the honey is in its whole form. More specifically, transcriptomic analysis found that Manuka honey-treated P. aeruginosa upregulated genes involved in antibiotic resistance and susceptibility and phage/transposons compared to groups controls. These findings suggest that manuka honey can affect transcriptional upregulation of different gene activation pathways in P. aeruginosa. More importantly, findings from these and other studies indicate there are multiple aspects of honey itself that contribute to its antimicrobial and potential medicinal properties.
Practically speaking, there are no definitive studies on the impact of Manuka honey consumption on treating human health diseases, like diabetes, gut health, or cancer, and there have been limited studies on the impact of Manuka honey to ameliorate wound healing and fight off infections in humans. Still, the use of Manuka honey was thought to reduce throat pain in chemotherapy treated lung cancer patients in one clinical trial, but the treatment was not observed to be superior to current best supportive care. The study ultimately called for further research focused on understanding the potential for honey as a treatment relief alternative instead of opioids in clinical use.
Can Microbes in Honey Cause Disease or Illness?
Raw Versus Pasteurized Honey
Honey that is minimally processed tends to retain more of its natural microbial diversity. This unpasteurized form is also referred to as "raw honey." Raw honey is filtered to remove pieces of hard honeycomb from the hive itself, but the honey will remain cloudy holding the many microbes and potential antioxidants generated by the honeybees in the enzymatic digestion process. Raw honey is more risky for people with pollen allergies to consume. Since there is significantly less processing, pollen can still be found in raw honey and may even cause anaphylactic shock in extreme cases. The presence of glandular proteins that bees utilize to construct beeswax to structure their hives is another allergy concern.
In contrast, pasteurized honey often has a reduced microbial load, due to the high temperatures used during processing. Pasteurized honey will undergo significant heat shock processing and filtration, which may remove some of the microbes considered beneficial for human gut health. The way honey is stored can significantly impact its microbial composition as well, as exposure to moisture, heat and air can promote microbial growth and spoilage. For example, one study of the biological activity of chestnut and Rhododendron honeys from Anatolia in Turkey tested antimicrobial activity against 8 microorganisms, (Staphylococcus aureus, Bacillus subtilis, Enterococcus faecalis, Helicobacter pylori, Escherichia coli, Enterobacter cloacae, Pseudomonas aeruginosa, Candida tropicalis and Candida albicans), and observed moderate inhibition against only a few microorganisms, e.g., H. pylori and S. aureus, C. albicans and C. tropicalis.
Botulism
Regardless of manner in which the honey has been processed and stored, there are couple of health concerns that should be considered when consuming honey. Chief among these is the fact that honey, both in raw and pasteurized forms, can contain small amounts of the bacteria Clostridium botulinum, which can produce botulinum toxin, a neurotoxin that acts as an acetylcholine inhibitor at neuromuscular junctions and can cause paralysis. If left untreated, botulism can be fatal. C. botulinum is naturally found (mainly in spore form) in soil, rocks and marine environments. Therefore, introduction of C. botulinum into the hive by dust or pollen is considered rare, but not impossible.
If caught early, botulism can be treated in adults via administration of botulinum antitoxin or gastrointestinal decontamination (i.e, gastric lavage or induced emesis). However, if the condition progresses to paralysis, it cannot be undone. The result of exposure to C. botulinum in children under 1 year of age can be severe and progress at a faster rate compared to adults. In children under 6 months of age, the digestive and mucosal immune systems are still developing. Thus, C. botulinum bacteria can colonize the infant gut environment causing intestinal immobility and muscle paralysis. It is recommended by several health institutions that children under the age of 12 months do not consume honey (raw or pasteurized) to prevent potential onset of infant botulism.
Future Outlook on Honey
The microbiology of honey reveals a fascinating world of microorganisms that coexist within this natural sweetener. From beneficial probiotics to potential expanded antimicrobial properties, honey is not only a culinary delight, but also a remarkable substance with promising health benefits. As we continue to explore the depths of honey's microbiology, we may uncover even more secrets that this golden elixir has to offer.
From probiotics to prevent deformed wing virus to the first honeybee vaccine to treat American foulbrood, research and conservation efforts pertaining to bee health are all the buzz. Learn more about using microbes to promote bee health and prevent colony collapse in our next article.
