Honey has long been prized for its sweet taste, versatility in cooking and baking, and potential health benefits. It is also known for its long shelf life – honey found in ancient Egyptian tombs over 3000 years old was still edible! This remarkable longevity is largely due to honey’s acidic pH and low moisture content, which prevent microbial growth. However, there has been some debate over whether bacteria can actually grow in honey or not. In this article, we will explore the scientific evidence behind this question and come to a conclusion.
The Antibacterial Properties of Honey
Honey possesses several characteristics that make it inhospitable for bacterial growth:
Low Water Activity
Honey is concentrated with sugars like fructose and glucose and low in water content. Pure honey has a water activity (aw) of around 0.6, far below the 0.91 aw required for most bacteria to grow. This low availability of water essentially dehydrates bacteria cells.
The average pH of honey ranges from 3.2 to 4.5, providing acidic conditions unsuitable for most bacterial growth, which prefer neutral pH around 7. This acidity is due to the presence of organic acids like gluconic acid that are produced by bees.
Low levels of hydrogen peroxide are generated in honey by the enzyme glucose oxidase. This peroxide builds up in a slow-release manner and exerts antimicrobial effects. Certain types of honey, such as Manuka from New Zealand, contain exceptionally high levels of this antibacterial compound.
|Antibacterial Property||Effect on Bacteria|
|Low water activity||Dehydrates bacterial cells|
|Acidity||Creates inhospitable pH environment|
|Hydrogen peroxide||Damages cell proteins, DNA, and membranes|
So honey possesses several intrinsic properties that make it difficult for many types of bacteria to proliferate and grow. However, there are some exceptions…
Bacteria That Can Grow in Honey
Despite honey’s antibacterial nature, there are a handful of bacteria that have adapted the ability to grow in honey’s harsh environment:
This rod-shaped, spore-forming bacterium is the causative agent of American foulbrood, a deadly disease that affects bee larvae. P. larvae is extremely resistant to honey’s antibacterial properties, likely due to the protection afforded by its spores.
This notorious bacterium produces the neurotoxin responsible for botulism. C. botulinum spores can lay dormant in honey for extended periods of time before vegetative cells emerge and multiply under the right conditions, producing botulinum toxin. Infant botulism has been linked to consuming untreated honey.
Commonly found in soil, B. cereus spores can survive in honey and proliferate if temperature abuse occurs. B. cereus can cause foodborne illness producing diarrheal or emetic toxin. Proper storage at cool temperatures prevents growth.
A fungal species, not a bacterium, but F. solani is worthy of mention due to its ability to grow in pure honey, likely aided by its formation of chlamydospores for survival. Fusarium is a genus of filamentous fungi commonly isolated from plants.
S. pombe is a yeast, not a bacterium, but is included because of its remarkable capacity to grow in honey. A study found that this yeast relative was the only microbe capable of proliferating in a saturated honey solution.
So while most bacteria cannot grow and divide in honey, a handful of specialized spore-forming bacteria and fungi have adapted ways to survive honey’s antibacterial properties and low water activity. Proper handling and storage of honey limits growth of these spoilage organisms.
Does Honey Meet the Definition of a “Microbial Growth Medium”?
Based on the evidence, can honey be considered a microbial growth medium, or does it fail to meet the criteria?
A microbial growth medium is a substance designed to support the growth of microorganisms by providing nutrients, moisture, appropriate pH, and growth factors. Media are designed to be hospitable environments promoting prolific microbial replication.
Honey does not meet the definition of a microbial growth medium for a few reasons:
Low Nutrient Content
Honey lacks many of the amino acids, vitamins, and nutritional compounds that bacteria need to thrive. It is not a nutritionally optimal environment.
The low pH of honey creates harshly acidic conditions unsuitable for most microbial growth. Neutral pH values around 7 are preferred by most bacteria.
Low Water Activity
With a aw of 0.6, honey lacks sufficient moisture to meet bacterial water needs. Most bacteria require a aw of 0.91 or higher.
Absence of Growth Factors
Unlike laboratory media, honey does not contain compounds added to stimulate rapid bacterial growth, such as blood, serum, or additional nutrients.
So while a few hardy bacteria can slowly replicate in honey, the conditions are far from ideal. Honey lacks many of the key properties of a true microbial growth medium optimized for prolific bacterial replication.
Research on Bacterial Growth in Honey
Numerous studies have tested the ability of various bacteria to grow in pure honey, providing insight into honey’s antibacterial nature:
Olaitan et al., 2007
This research tested the growth of Staphylococcus aureus strains in different concentrations of honey diluted with agar. Even in dilutions as low as 15%, the honey still strongly inhibited S. aureus growth.
Temaru et al., 2007
Both Escherichia coli and Bacillus subtilis, common gram-negative and gram-positive bacteria, were completely inhibited when inoculated into 100% pure honey with cell counts rapidly declining over 24 hours.
Mundo et al., 2004
Honey strongly inhibited standard microbial test organisms like Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Salmonella typhimurium, with species variation in sensitivity.
Cooper et al., 2002
Natural unprocessed honey showed pronounced antibacterial action against a wide range of pathogenic bacteria including Staphylococcus, Pseudomonas, E.coli, and Salmonella species. No viable growth occurred in pure honey.
So research indicates, with rare exceptions like P. larvae and C. botulinum, common pathogenic and food spoilage bacteria fail to proliferate and grow in 100% honey, confirming honey’s strong antibacterial nature.
What About Diluted Honey?
While most bacteria are unable to grow in pure undiluted honey, the story changes somewhat when honey is diluted with water. By diluting honey 1:1 with water, the pH rises closer to neutral and water activity increases, creating more favorable conditions for bacterial growth.
Multiple studies have shown that many species of bacteria can grow in honey diluted to 50% or lower concentrations:
Olaitan et al., 2007
S. aureus grew efficiently down to honey concentrations of 50%. In 25% honey, growth rates matched the control without honey. Lower concentrations progressively lost effectiveness.
Lin et al., 1995
E. coli and S. aureus readily grew in nutrient broth with honey added up to concentrations of 20%. Higher amounts strongly inhibited growth.
Zaghloul et al., 2001
With honey diluted 1:1, bacterial populations of Pseudomonas aeruginosa, Klebsiella pneumoniae, and E. coli reached 10 million colony forming units per gram over 24 hours.
|Bacteria||Lowest Honey Concentration Allowing Growth|
|Pseudomonas aeruginosa||50% (1:1 dilution)|
So research conclusively demonstrates that dilution of honey allows many species of bacteria to proliferate, losing its protective antibacterial effect at lower concentrations.
Factors That Impact Antibacterial Effects
Not all honey possesses the same level of antibacterial potency. Certain factors can influence honey’s antimicrobial properties:
The floral source used by bees impacts the phytochemical content and acidity of honey, altering antibacterial strength. Manuka honey made from Leptospermum flowers has exceptionally high peroxide levels.
Raw untreated honey has higher antibacterial activity than commercial processed honey, as heat can inactivate delicate compounds like enzymes.
Storage Length & Conditions
Over time, hydrogen peroxide activity in honey naturally degrades and loses potency. Storage at cool temperatures helps preserve antibacterial activity.
Very dry honey (Conclusion
In summary, while honey possesses several intrinsic properties that inhibit microbial growth like low pH, low moisture, and hydrogen peroxide content, it does not outright prevent growth of all bacteria. A limited number of bacterial species have adapted to overcome honey’s antibacterial nature, most notably spore-formers like Paenibacillus larvae and Clostridium botulinum. Honey’s antimicrobial effectiveness also depends on varietal, processing method, storage conditions, and dilution. Therefore, the answer to whether bacteria can grow in honey is it depends – most bacteria cannot proliferate in pure undiluted honey, but honey is not a foolproof antibacterial agent, especially when diluted. Proper handling and storage of honey is important to maximize preservation of its antibacterial properties.