Antimicrobial Effects of Honey

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Honey has seen a revival recently in the Western medical field, as it has shown inhibitory activity against a range of detrimental and antibiotic-resistant microbes of infected wounds ¹ (Nassar et al. 2012). Honey may be the first recorded medicine, having been documented in the Smith Papyrus of Egypt, which dates to between 2200-2600 BC ² (Zumla 1989). Since ancient times, honey has been renowned for its wound-healing properties ³ (Kwakman et al. 2010). With the advent of antibiotics, clinical application of honey was neglected in modern Western medicine, although it is still used in many cultures ³ (Kwakman et al. 2010). The overwhelming use of antibiotics has resulted in widespread resistance and the development of new antibiotics is lagging behind, therefore alternative antimicrobial strategies are necessary ³ (Kwakman et al. 2010).

Honey has demonstrated potent in vitro activity against antibiotic-resistant bacteria and it has been successfully applied as treatment of chronic wound infections not responding to antibiotic therapy (Kwakman et al. 2010). Furthermore, honey has received attention as an important tool against strains of bacteria such as Methicillin-resistant Staphylococcus aureus, which have become resistant to current antibiotics (Cooper 1999). There is no such resistance build-up involving honey, making it attractive as a treatment for wound infections (Cooper 1999). Honey possesses several antimicrobial propertis and can act via various mechanisms of action. There are many different types of honey from around the world, made from different floral sources and these different types of honey can have variable antimicrobial effects and mechanisms of action. The antimicroial potency and medical applications of honey are tremendous as it has demonstrated inhibitory effects against a number of pathogenic bacteria.

Mechanisms of Action

Honey prevents microbial growth through the use of hydrogen peroxide (H₂O₂), methylglyoxal (MGO), bee defensin-1, flavonoids, and a relatively low pH (~3.3) (Kwakman et al. 2010, Nassar et al. 2012). Not all of the factors listed are present in all types of honey, and these compounds must be tested for and considered for clinical applications (Kwakman et al. 2010).

The high osmolarity of honey can also contribute to the inhibition of growth, although this is true of sugar solutions as well (Kwakman et al. 2010, Nassar et al. 2012). Honey is 70 to 80% sugar and this high percentage causes hypertonic conditions that may lead to lysis of microbial cell walls (Cushnie and Lamb 2011).

Hydrogen peroxide is produced by the Apis mellifera (honeybee) glucose oxidase enzyme on dilution of honey, and is produced in low but effective concentrations (Bang 2003, Kwakman et al. 2010). Due to the slow release of H₂O₂, there is much less cytotoxic damage to the patient’s cells, providing a better method than applying H₂O₂ directly to wounds (Bang 2003).

Methylglyoxal is a compound found in manuka honey that was reported to have an antibacterial property (Cushnie and Lamb 2011). MGO can be converted into its inactive form, S-lactoylglutathione, by glyoxalase I and this was done in an experiment to test the effects of MGO on honey’s bactericidal activity (unprocessed Revamil honey was used in this experiment) (Kwakman et al. 2010). Neutralization of MGO or H₂O₂ alone did not alter bactericidal activity of RS honey, but simultaneous neutralization of MGO and H2O2 in 10% honey reduced the killing of B. subtilis by 4-logs (Kwakman et al. 2010). At higher concentrations of honey, the bactericidal activity was not affected by neutralization of MGO and H2O2 (Kwakman et al. 2010).

Bee defensin-1 is found in honey and is the only cationic bactericidal compound currently identified (Kwakman et al. 2010). In honey dilutions of 20% and greater, when H₂O₂ and MGO were neutralized bactericidal activity was retained but when bee defensin-1 was also neutralized, the bactericidal activity was strongly reduced at 20% but was not affected at 30 and 40% (Kwakman et al. 2010). Bee defensin-1 was previously isolated from royal jelly, the major food source for bee queen larvae and was identified in honeybeen hemolymph (Kwakman et al. 2010). This peptide is secreted by the hypopharyngeal gland of worker bees into collected nectar along with carbohydrate-metabolizing enzymes and bee defensin-1 presumably contributes to protection of royal jelly and honey against microbial spoilage (Kwakman et al. 2010).

Flavonoids are a group of pigments produced by plants and their presence was suggested to contribute to the antimicrobial properties of honey (Nassar et al. 2012). Actions of flavonoids include direct antibacterial activity, synergism with antibiotics, and suppression of bacterial virulence (Cushnie and Lamb 2011). The direct antibacterial activity of flavonoids may be attributable to several tested mechanisms: cytoplasmic membrane damage (caused by perforation and/or a reduction in membrane fluidity possibly by generating hydrogen peroxide), inhibition of nucleic acid synthesis (caused by topoisomerase inhibition and/or dihydrofolate reductase inhibition), inhibition of energy metabolism (caused by NADH-cytochrome c reducatse inhibition and ATP synthase inhibition), inhibition of cell wall synthesis (caused by D-alanine-D-alanine ligase inhibition) and inhibition of cell membrane synthesis (caused by inhibition of several enzymes) (Cushnie and Lamb 2011). There are 14 classes of flavonoids in total, categorized by their chemical nature and structure (Cushnie and Lamb 2011). Because most studies on the mechanism of action of flavonoids were conducted on only one or two types of flavonoids, it remains unclear as to whether flavonoids have multiple mechanisms of action or flavonoids have a single mechanism that has yet to be convincingly determined (Cushnie and Lamb 2011).

Honey has a low pH primarily due to the conversion of glucose into hydrogen peroxide and gluconic acid by glucose oxidase (Kwakman et al.2010). This low pH might also contribute to the bactericidal activity of honey, demonstrated by the titration of the pH of 10-40% honey from 3.4-3.5 to 7.0 combined with neutralization of other bactericidal factors (H₂O₂, MGO and bee defensin-1) reduced the bactericidal activity of honey to the same level of a honey-equivalent sugar solution (Kwakman et al. 2010).

Effectiveness of Different Types of Honey

The minimum inhibitory concentration (MIC) of manuka honey on Stayphylococcus aureus was between 2 and 3% (v/v) ([(volume of solute)/(volume of solution)] x 100%) (Cooper et al. 1999). Manuka honey is from Leptospermum trees. The MIC of honey from a mixed pasture source was between 3 and 4% on S. aureus (Cooper et al. 1999). These honeys prevent growth of S. aureus even when diluted by body fluids a further seven-fold to fourteen-fold beyond the point where their osmolarity ceased to be completely inhibitory (Cooper et al. 1999). Pasture honey acts by releasing H₂O₂, while Manuka honey’s action also has a phytochemical component (Cooper et al. 1999). In this study by Cooper et al. (1999) the antibacterial activity of manuka and pasture honeys on S. aureus were determined by an agar well diffusion bioassay using phenol as a reference standard antiseptic both in the presence of catalase and not in the presence of catalase, to detect any non-peroxide antibacterial activity; the MIC of each honey was determined by an agar incorporation technique.

In a study on the effect of honey on Streptococcus mutans, natural honey bought from a local grocery store in Jeddah, Saudi Arabia was compared to artificial honey composed of 40.5% fructose, 33.5% glucose, 7.5% maltose and 1.5% sucrose dissolved in deionized water (Nassar et al. 2012). Different natural and artificial honey concentrations were obtained using serial dilutions with tryptic soy broth (TSB) and at 12.5%, natural honey supported less bacterial growth and biofilm formation an artificial honey with the same amount of sugars, suggesting that sugar content is not the only antibacterial factor (Nassar et al. 2012). Natural honey was able to decrease the maximum velocity of S. mutans growth compared to artificial honey (Nassar et al. 2012). Overall, natural honey demonstrated more inhibition of bacterial growth, viability, and biofilm formation than artificial honey (Nassar et al. 2012).

Unprocessed Revamil source honey was effective at killing several different strains of bacteria at 10-20% (v/v), while greater than 40% (v/v) of a honey-equivalent sugar solution was required for similar activity (Kwakman et al. 2010).

One brand of commercial honey obtained from Saudi Arabia called Black Forest honey, Langaneza, Germany was tested and found to inhibit eight different types of microbes at concentrations between 10 to 100% (Masaudi and Albureikan 2013). Growth of all microbes was reduced at 10% and completely inhibited at 20% for Methicillin-Sensitive S. aureus, Methicillin-Resistant S. aureus, and E. coli, and at 50% for P. aeruginosa and C. albicans, and at 100% for S. pyogenes, Vancomycin-sensitive enterococci and Vancomycin-resistant enterococci (Masaudi and Albureikan 2013).

Microbes Inhibited by Honey

Coagulase-positive Staphylococcus aureus has been shown to be sensitive to both pasture and manuka honeys (Cooper et al. 1999). In this study there was a lack of significant variance in the sensitivity of a large number of clinical isolates collected from a wide range of wounds, which indicates that there is no mechanism of resistance to the antibacterial activity of honey (Cooper et al. 1999).

Streptococcus mutans growth, viability, and biofilm formation were inhibited by natural honey at concentrations between 25 and 12.5% (Nassar et al. 2012). Bacterial growth and biofilm formation were determined using a microplate spectrophotometer on wells inoculated with S. mutans containing varying concentrations of natural and artificial honey and biofilms were fixed using formaldehyde solution, followed by crystal violet, and then isopropanol, after that the wells were aspirated and their absorbances were read (Nassar et al. 2012). The number of colony-forming units (CFU) for varying concentrations of honey was determined using an automated colony counter and compared to values from the tryptic soy broth (TSB) control culture to determine the effect of honey on S. mutans viability (Nassar et al. 2012).

Unprocessed Revamil source honey effectively killed Bacillus subtilis, methicillin-resistant Staphylococcus aureus, extended-spectrum β-lactamase producing Escheria coli, ciprofloxacin-resistant Pseudomonas aeruginosa, and vancomycin-resistant Enterococcus faecium (Kwakman et al. 2010). The activity of honey against E. coli and P. aeruginosa was markedly reduced when either H₂O₂ or MGO was neutralized (Kwakman et al. 2010).

The relationship between the presence of honey and bacterial growth was tested on the following bacteria on nutrient-agar and honey-nutrient agar plates: Vibrio cholerae, enteropathogenic E. coli, Salmonella typhi, Shigella boydii, Klebsiella pneumoniae, P. mirabilis, Psuedomonas aeruginosa and Serratia marcescens (Jeddar et al. 1985). Staphylococcus aureus, Streptococcus pyogenes, Streptococcus faecalis, and Listeria monocytogenes were tested on blood agar and honey-blood agar plates (Jeddar et al. 1985). Finally, chocolate-agar and honey-chocolate agar plates were used to test the growth of Haemophilus influenzae (Jeddar et al. 1985). There was good growth of all bacteria on their respective control plates and all intestinal bacterial pathogens tested failed to grow in honey at concentrations of 40% and above (Jeddar et al. 1985). Furthermore, the growth of V. cholerae, S. pyogenes, and H. influenzae were inhibited in honey at concentrations as low as 20% and the growth of all bacteria tested was inhibited at honey concentrations of 50% (Jeddar et al. 1985).

Langaneza Black Forest honey inhibited the growth of S. pyogenes, E. coli, P. aeruginosa, C. albicans, Methicillin-sensitive and Methicillin-resistant S. aureus, and Vancomycin-sensitive and Vancomycin-resistant enterococci (Masaudi and Albureikan 2013).

Further Reading

Evidence Supporting the Use of Honey as a Wound Dressing Molan (2006) reviewed 17 randomized controlled trials involving a total of 1,965 participants, 5 clinical trials of other forms involving 97 participants treated with honey, and 16 trials on a total of 533 wounds on experimental animals all with findings that demonstrate the effectiveness of honey in assisting wound healing. This review found that honey has antibacterial activity capable of rapidly clearing infection and protecting wounds from becoming infected, while providing a moist healing environment without the risk of bacterial growth (Molan 2006). This review also reports that honey produces anti-inflammatory effects to reduce edema and exudate and prevent or minimize hypertrophic scarring (Molan 2006). Honey also stimulates the growth of granulation tissue and epithelial tissue so that healing is hastened (Molan 2006).

References

1. Nassar, H. M., Li, M., & Gregory, R. L. (2012). Effect of honey on Streptococcus mutans growth and biofilm formation. Applied and environmental microbiology, 78(2), 536-540.

2. Zumla, A., & Lulat, A. (1989). Honey--a remedy rediscovered. Journal of the Royal Society of Medicine, 82(7), 384.

3. Kwakman, P. H., Te Velde, A. A., de Boer, L., Speijer, D., Vandenbroucke-Grauls, C. M., & Zaat, S. A. (2010). How honey kills bacteria. The FASEB Journal, 24(7), 2576-2582.

Edited by Celina Hayashi, a student of Nora Sullivan in BIOL168L (Microbiology) in The Keck Science Department of the Claremont Colleges Spring 2014.