The Composition and Biological Activity of Honey: A Focus on Manuka Honey

Abstract

Honey has been used as a food and medical product since the earliest times. It has been used in many cultures for its medicinal properties, as a remedy for burns, cataracts, ulcers and wound healing, because it exerts a soothing effect when initially applied to open wounds. Depending on its origin, honey can be classified in different categories among which, monofloral honey seems to be the most promising and interesting as a natural remedy. Manuka honey, a monofloral honey derived from the manuka tree (Leptospermum scoparium), has greatly attracted the attention of researchers for its biological properties, especially its antimicrobial and antioxidant capacities. Our manuscript reviews the chemical composition and the variety of beneficial nutritional and health effects of manuka honey. Firstly, the chemical composition of manuka honey is described, with special attention given to its polyphenolic composition and other bioactive compounds, such as glyoxal and methylglyoxal. Then, the effect of manuka honey in wound treatment is described, as well as its antioxidant activity and other important biological effects.

Full text

Foods 2014, 3, 420-432; doi:10.3390/foods3030420
foods
ISSN 2304-8158
www.mdpi.com/journal/foods
Review
The Composition and Biological Activity of Honey: A Focus on
Manuka Honey
José M. Alvarez-Suarez 1,2,*, Massimiliano Gasparrini 1, Tamara Y. Forbes-Hernández 1,2,
Luca Mazzoni 1 and Francesca Giampieri 3,*
1 Department of Odontostomatologic and Specialized Clinical Sciences, Faculty of Medicine and
Surgery, Polytechnic University of Marche, Avenue Ranieri 65, Ancona 60100, Italy;
E-Mails: m.gasparrini@univpm.it (M.G.); tamara.forbe@gmail.com (T.Y.F.-H.);
l.mazzoni@univpm.it (L.M.)
2 Department of Nutrition and Health, International Iberoamerican University (UNINI), Avenue
Adolfo Ruiz Cortines 112, Torres de Cristal L 101 A-3, Campeche 24040, Me xico
3 Department of Agricultural, Food and Environmental Sciences, Polytechnic University of Marche,
Via Ranieri 65, Ancona 60100, Italy
* Authors to whom correspondence should be addressed; E-Mails: j.m.alvarez@univpm.it (J.M.A.-S.);
f.giampieri@univpm.it (F.G.); Tel.: +39-71-2204-136 (J.M.A.-S.); +39-71-2204-136 (F.G.);
Fax: +30-71-2204-123 (J.M.A.-S.); +30-71-2204-123 (F.G.).
Received: 21 May 2014; in revised form: 9 June 2014 / Accepted: 3 July 2014 /
Published: 21 July 2014
Abstract: Honey has been used as a food and medical product since the earliest times. It
has been used in many cultures for its medicinal properties, as a remedy for burns,
cataracts, ulcers and wound healing, because it exerts a soothing effect when initially
applied to open wounds. Depending on its origin, honey can be classified in different
categories among which, monofloral honey seems to be the most promising and interesting
as a natural remedy. Manuka honey, a monofloral honey derived from the manuka tree
(Leptospermum scoparium), has greatly attracted the attention of researchers for its
biological properties, especially its antimicrobial and antioxidant capacities. Our
manuscript reviews the chemical composition and the variety of beneficial nutritional and
health effects of manuka honey. Firstly, the chemical composition of manuka honey is
described, with special attention given to its polyphenolic composition and other bioactive
compounds, such as glyoxal and methylglyoxal. Then, the effect of manuka honey in
wound treatment is described, as well as its antioxidant activity and other important
biological effects.
OPEN ACCESS

Foods 2014, 3 421
Keywords: manuka honey; polyphenolic composition; wound treatments;
antimicrobial activity
1. Introduction
Honey is a sweet and flavorful natural product, which is consumed for its high nutritive value and
for its effects on human health, with antioxidant, bacteriostatic, anti-inflammatory and antimicrobial
properties, as well as wound and sunburn healing effects [1]. Honey is produced by bees from plant
nectars, plant secretions and excretions of plant-sucking insects. Concerning its nutrient profile, it
represents an interesting source of natural macro- and micro-nutrients, consisting of a saturated
solution of sugars, of which fructose and glucose are the main contributors, but also of a wide range of
minor constituents, especially phenolic compounds [2,3]. The composition of honey is rather variable
and depends primarily on its floral source; seasonal and environmental factors can also influence its
composition and its biological effects. Several studies have shown that the antioxidant potential of
honey is strongly correlated not only with the concentration of total phenolics present, but also with the
color, with dark colored honeys being reported to have higher total phenolic contents and,
consequently, higher antioxidant capacities [3–6].
According to the origin, honey can be classified in different categories as follows: (1) blossom
honey, obtained predominantly from the nectar of flowers (as opposed to honeydew honey);
(2) honeydew honey, produced by bees after they collect “honeydew” (secretions of insects belonging
to the genus, Rhynchota), which pierce plant cells, ingest plant sap and then secrete it again;
(3) monofloral honey, in which the bees forage predominantly on one type of plant and which is
named according to the plant; and (4) multifloral honey (also known as polyfloral) that has several
botanical sources, none of which is predominant, e.g., meadow blossom honey and forest honey.
It is has been suggested that many of the medicinal properties of plants can be transmitted through
honey, so that honey could be used as a vehicle for transporting plant medicinal properties [3]. Within
monofloral honey, manuka honey, a dark honey, has greatly attracted the attention of the international
scientific community for its biological properties, especially for its antimicrobial and antioxidant
capacities. This honey is derived from the manuka tree, Leptospermum scoparium, of the Myrtaceae
family, which grows as a shrub or a small tree throughout New Zealand and eastern Australia [7].
In traditional medicine, different extracts of the manuka tree are used as sedatives and wound-healing
remedies. Moreover, manuka honey itself has long been employed for clearing up infections, including
abscesses, surgical wounds, traumatic wounds, burns and ulcers of different etiology [8]. Currently, the
main bioactive compounds in manuka honey and the mechanisms responsible for their biological
activities are being studied. These studies would support the increased use of manuka honey in skin
medicine, and they can also be the basis for the isolation and purification of compounds for the
development of bio-pharmaceutical products with antimicrobial properties and wound healing
properties; these new findings could represent an added economic value that can favor also the
beekeepers in their productions.

Foods 2014, 3 422
This review focuses on the phytochemical composition of manuka honey and on its biological
effects. An overview of the most abundant phytochemicals is presented, with particular attention to
recent evidence on its antimicrobial activity and its impact on wound treatments, as well as on its
antioxidant capacity.
2. Chemical Composition
Polyphenolic characterization has proven to be suitable for the differentiation of the floral origin of
honeys [9], and therefore, flavonoids could represent a valid botanical marker for honey [10], being
closely related with their antioxidant capacity. The qualitative and quantitative difference in flavonoid
contents of manuka honey determined in diverse studies may represent the result of the different
extraction and detection methods applied, and this limit makes the data available in the literature
difficult to compare. The major compounds identified are represented in Table 1. Several studies have
determined that the major flavonoids in manuka honey are: pinobanksin, pinocembrin and chrysin,
while luteolin, quercetin, 8-methoxykaempferol, isorhamnetin, kaempferol and galangin have been
also identified in minor concentration [11–13].
Regarding phenolic acids and volatile norisoprenoids constituents, Oelschlaegel et al. [13] detected
different profiles in manuka honey attributed to three chemotypes of L. scoparium in New Zealand.
The first group was characterized by high levels of 4-hydroxybenzoic acid, dehydrovomifoliol and
benzoic acid yields, the second one by high concentrations of kojic acid and 2-methoxybenzoic acid
and the third group by high contents of syringic acid, 4-methoxyphenyllactic acid and methyl
syringate. According to the determined average amounts, phenylacetic acid, phenyllactic acid,
4-methoxyphenyllactic acid, leptosin and methyl syringate were the dominating compounds [14,15].
Methyl syringate (MSYR) and leptosin (the novel glycoside of MSYR, methyl syringate
4-O-β-D-gentiobiose) (Figure 1) are the active compounds from manuka honey to which its
myeloperoxidase (MPO)-activity inhibition is ascribed. Although the biological activities and
biosynthetic pathway/origin of the glycoside are still unknown, it may be a good chemical marker for
the purity of manuka honey [7].
Other constituents of interest found in manuka honey are: different 1,2-dicarbonyl compounds, such
as glyoxal (GO), 3-deoxyglucosulose (3-DG) and methylglyoxal (MGO). These compounds are
typically formed during the Maillard reaction or caramelization reactions as degradation products from
reducing carbohydrates, and they have been identified as important contributors to the non-peroxide
antibacterial activity [13,16,17].
From the nutritional point of view, the physiological significance resulting from the uptake of MGO
and other 1,2-dicarbonyl compounds must be a topic of further investigations. MGO and glycation
compounds resulting from the reaction of MGO with amino acid side chains of lysine or arginine,
respectively, have been identified in vivo and are associated with complications of diabetes and some
neurodegenerative diseases, although the role of these compounds in the pathogenesis of different
diseases have not yet been fully understood [16].

Foods 2014, 3 423
Table 1. Most common compounds identified in manuka honey.
Phenolic Acid and Flavonoids
Ref.
Other Compounds
Ref.
Caffeic acid
[12,13]
Phenyllactic acid
[13]
Isoferulic acid
[12]
4-Methoxyphenolactic acid
[13]
p-Coumaric acid
[12]
Kojic acid
[13]
Gallic acid
[13,17]
5-Hydroxymethylfurfural
[13]
4-Hydrobenzoic acid
[13]
2-Methoxybenzoic acid
[13]
Syringin acid
[13]
Phenylacetic acid
[13]
Quercetin
[12,17]
Methyl syringate
[13]
Luteolin
[12,13]
Dehydrovomifoliol
[13]
8-Methoxykaempferol
[12]
Leptosin
[13]
Pinocembrin
[12]
Glyoxal
[13,16]
Isorhamnetin
[12,17]
Methylglyoxal
[13,16]
Kaempferol
[12]
3-Deoxyglucosulose
[13,16]
Chrysin
[12]
-
-
Galangin
[12]
-
-
Pinobanksin
[12]
-
-
Figure 1. Chemical structures of methyl syringate and leptosin.
methyl syringate
leptosin
3. Use of Manuka Honey in Wound Treatments
The importance of honey in the field of wound treatments has been well known since ancient times.
This healing property is related to the antioxidant and antibacterial activity that honey offers,
maintaining a moist wound condition, and to the high viscosity that provides a protective barrier on the
wound, preventing microbial infection. Its immunological activity is relevant also for wound repair,

Foods 2014, 3 424
exerting the same time pro- and anti-inflammatory effects [18–23]. Normal wound healing is a
complex process composed of a series of overlapping events (coagulation, inflammation, cell
proliferation, tissue remodeling) in which the damaged tissue is gradually removed and replaced by
restorative tissues [24]. While normal inflammation resolves within 1–2 days as the neutrophil number
decreases, the accumulation of these cells in the wound site contributes to a disordered network of
regulatory cytokines, leaving the wound in a chronic state of inflammation [25]. In these chronic
wounds, bacterial cells predominantly exist as biofilms, where cells are embedded within a matrix of
polysaccharides and other components that limit the availability of antibiotics for wound healing.
Furthermore, the emergence of bacterial resistance to multiple antibiotics has worsened the problem of
chronic wound biofilm treatment [26].
Current therapeutic products widely used in wound care (silver sulfadiazine (SSD), hydrogel,
hydrocolloid and alginate dressings impregnated with silver) are considered useful for limiting
bacterial infections, even if excessive use of ionic silver has generated some concern regarding the
development of bacterial resistance [27,28]; this situation, in recent years, has stimulated modern
medicine to focus attention on natural products with antimicrobial activity and their use in clinical
practice. The low cost and absence of the antimicrobial resistance risk of natural products, such as
honey, aloe vera or curcumin, are the major arguments for implementing natural products in wound
treatment [29]. Although it is an ancient topical treatment for wounds, honey has been currently
established in conventional medicine as a licensed medical device, either combined into sterile
dressings or sterilized in tubes [30].
The healing time decrease after honey treatment can be explained through a dual effect on the
inflammatory response. Firstly, honey prevents a prolonged inflammatory response suppressing the
production and propagation of inflammatory cells at the wound site; secondly, it stimulates the
production of proinflammatory cytokine, allowing normal healing to occur [31] and stimulating the
proliferation of fibroblasts and epithelial cells [32,33]. The effect of honey and its components on the
production of inflammatory cytokines has been evaluated in primary human monocytes cells [34]. In
these studies, it was shown that manuka honey stimulated the production of inflammatory
cytokines TNF-α, IL-1β or IL-6 via a TLR4-dependent mechanism. For the first time, a 5.8-kDa
component responsible for cytokine induction in human monocytes via TLR4 was isolated from
manuka honey [35].
Microorganisms that colonize a burn wound originate from the patient’s gastrointestinal and
respiratory flora, from endogenous skin or from contaminated external sources (soil, water, air) [36].
The topical application of honey rapidly clears wound infection, promoting the healing process of deep
surgical infected wounds [37–39], also when they do not respond to conventional antibiotic and
antiseptic therapy [37]. Furthermore, in burn wounds, honey application decreases the wound area,
exerts an antibacterial effect and promotes better re-epithelialization compared to hydrofiber silver or
SSD treatment. Moreover, the anti-inflammatory action of honey decreases damage caused by free
radicals that result from inflammation, preventing further necrosis [40].
The antibacterial nature of honey depends on different factors acting singularly or synergistically,
the most salient of which are phenolic compounds, wound pH, H2O2, pH of honey and
osmotic pressure exerted by the honey itself [3]. It has been documented that the pronounced
antibacterial activity of manuka honey directly originates from the MGO it contains (Figure 2A) [16].

Foods 2014, 3 425
This non-peroxide antibacterial activity due to the presence of MGO is called the unique manuka
factor (UMF) [16].
Figure 2. (A) Chemical structures of the methylglyoxal (MGO). (B) Homology model of
defensin-1 from Apis mellifera. The model of the mature protein (residues 44–94) was
obtained using the experimentally-resolved structure of lucifensin from Lucilia sericata
(PDB ID: 2LLD) as a template. Alignment and modeling was performed using the Swiss
Model server [41]; the figure has been obtained through PyMOL Molecular Graphics
System, Version 1.5.0.4, Schrödinger, LLC (Portland, OR, USA).
A
B
Other antimicrobial compounds in honeys include bee defensin-1 (Figure 2B), various phenolic
compounds and complex carbohydrates [1,2]. The combination of these diverse assaults may account
for the inability of bacteria to develop resistance to honey, in contrast to the rapid induction of
resistance observed with conventional single-component antibiotics [42,43]. A few studies have
examined the antimicrobial effect of manuka honey, showing that it is active against a range of
bacteria, including Group A Streptococcus pyogenes, Streptococcus mutans, Proteus mirabilis,
Pseudomonas aeruginosa, Enterobacter cloacae and Staphylococcus aureus [44–47]. A list of
microorganisms that have been found to be sensitive to manuka honeys is shown in Table 2.
Furthermore, no resistant bacteria (Escherichia coli, MRSA, Pseudomonas aeruginosa and
Staphylococcus epidermidis) have been isolated after exposure of wound isolates to sub-inhibitory
concentrations of manuka honey [42,43]. This seems to be very likely due, at least in part, to
differences in the levels of the principle antibacterial components in the honey, MGO and hydrogen
peroxide, which varies with the floral and geographic source of nectar, honey storage time and
conditions and any other possible treatment that could affect it. Anti-biofilm activity was highest in the
honey blend that contained the highest level of manuka-derived honey; the effectiveness of the
different manuka-type honeys tested increased with MGO content, although the same level of MGO,
with or without sugar, could not eradicate biofilms. This suggests that additional factors in these
manuka-type honeys are responsible for their potent anti-biofilm activity [48].

Foods 2014, 3 426
Table 2. List of microorganisms that have been found to be sensitive to manuka honeys [49].
Gram Positive Strains
Gram Negative Strains
Streptococcus pyogenes
Stenotrophomonas maltophilia
Coagulase negative staphylococci
Acinetobacter baumannii
Methicillin-resistant Staphylococcus aureus (MRSA)
Salmonella enterica serovar typhi
Streptococcus agalactiae
Pseudomonas aeruginosa
Staphylococcus aureus
Proteus mirabilis
Coagulase-negative Staphylococcus aureus (CONS)
Shigella flexneri
Hemolytic streptococci
Escherichia coli
Enterococcus
Enterobacter cloacae
Streptococcus mutans
Shigella sonnei
Streptococcus sobrinus
Salmonella typhi
Actinomyces viscosus
Klebsiella pneumonia
-
Stenotrophomonas maltophilia
-
Burkholderia cepacia
-
Helicobacter pylori
-
Campylobacter spp.
-
Porphyromonas gingivalis
Manuka honey has been shown to eradicate methicillin-resistant Staphylococcus aureus (MRSA)
from colonized wounds and to inhibit MRSA in vitro by interrupting cell division. Furthermore,
the presence of manuka honey restores MRSA susceptibility to oxacillin; molecular analysis
indicated that it also affects the regulation of the mecR1 gene, possibly accounting for the restored
susceptibility [30]. In another study, a synergistic effect between rifampicin and commercially
available FDA-approved manuka honey (Medihoney, Medihoney Ltd, Slough, United Kingdom) was
demonstrated on clinical S. aureus isolates, including MRSA strains. Unlike with rifampicin alone, in
which resistance was observed after overnight incubation on plates, the combination of Medihoney and
rifampicin maintained the susceptibility of S. aureus to rifampicin [50]. Manuka honey, therefore,
seems to offer real potential in providing novel synergistic combinations with antibiotics for treating
wound infections of multidrug-resistant (MDR) bacteria. It is interesting to note that the antibiotics that
have shown synergy with manuka honey are from different antibiotic classes, which inhibit distinct
targets, such as the 30 S ribosome, RNA polymerase, membranes and penicillin binding proteins. This
finding supports the idea that honey is a complex substance, perhaps with multiple active components
that affect more than one cellular target site [30].
Manuka honey is also known to have a relatively low pH (3.5–4.5), which, besides inhibiting
microbial growth, stimulates the bactericidal actions of macrophages and, in chronic wounds, reduces
protease activity and increases fibroblast activity and oxygenation [51–53]. Growth factors, such as
TGF-β, are known to become physiologically active when subjected to an acid treatment, and the use
of Medihoney demonstrates a further increase in cellular activity. This impact has been reported in the
hDF-based studies and in an in vitro wound healing assay study, where Medihoney supplements
resulted in statistically significant increases in cell proliferation and migration [25].
Finally, manuka honey has been shown to specifically decrease the inflammatory response associated
with ulcerative colitis, an inflammatory intestine disease characterized by an overexpression of

Foods 2014, 3 427
inflammatory cells, in embryonic kidney cell lines. The anti-inflammatory effect by the manuka
honey was strongest in the presence of the Pam3CSK4 ligand, indicating that the honeys act through
the TLR1/TLR2 signaling pathway. The anti-inflammatory activity of manuka honeys is therefore
pathway specific [31].
4. Antioxidant Activity
In addition to antibacterial activity, honeys are known to possess strong antioxidant capacity,
which acts in modulating free radical production, thus protecting cell components from their
harmful action [54,55].
Manuka honey contains a high amount of phenolic compounds [14,15], as well as other phenolic
compounds that have been identified with a potent capacity to reduce free radicals, providing a
relevant antioxidant capacity [56,57]. For its relevant bioactive properties, it has often been used in
different studies as the “gold standard” [8] to test and evaluate the antioxidant capacity of different
kinds of honey from different botanical and geographical origins. Manuka honey, in fact, exhibits the
highest value in terms of phenolic content and antioxidant capacity, for example compared to acacia,
wild carrot and Portobello honeys [58,59], obtained, respectively, from Germany, Algeria,
Saudi Arabia and Scotland. Similar results are obtained with Malaysian monofloral honeys [56] and
Tualang honey, a Malaysian multifloral jungle honey [60]. The scavenger role of manuka honey against
superoxide anion radicals has also been investigated through electronic paramagnetic resonance [54,61];
the results proved that the quenching properties of manuka honey could be attributed to methyl
syringate [62]. Finally, manuka honey seems to exert a protective role against oxidative damage also in
an in vivo model [57], reducing DNA damage, the malondialdehyde level and glutathione peroxidase
activity in the liver of both young and middle-aged groups of rats. These effects could be mediated
through the modulation of antioxidant enzyme activities (such as catalase) and through the high
antioxidant capacity of its relevant total phenolic content. The results obtained suggest a possible use
of manuka honey as an alternative natural supplement to improve the physiological oxidative status.
5. Other Effects
In addition to its antimicrobial and antioxidant activities, recent studies demonstrated that honey
can exert anti-proliferative effects against cancer cells [62–64]. These anticancer properties can
involve different processes: (1) the apoptosis of cancer cells through the depolarization of the
mitochondrial membrane, (2) the inhibition of cyclooxygenase-2 by various constituents (like
flavonoids), (3) the release of cytotoxic H2O2 and (4) the scavenging of ROS and have been
correlated with the phytochemical compounds [65]. Manuka honey has been shown to possess a potent
anti-proliferative effect on murine melanoma (B16.F1), colorectal carcinoma (CT26) and human breast
cancer (MCF-7) cell lines in a time- and dose-dependent manner [8]. The main mechanism by which it
exerts such an anti-proliferative effect is through the activation of mitochondrial apoptotic pathways,
involving the stimulation of the initiator, caspase-9, which determines the activation of the
executioner, caspase-3 [65]. Moreover, it induces apoptosis via the activation of PARP, the induction
of DNA fragmentation and the loss of Bcl-2 expression. In vivo, manuka honey is also effective in:
(1) decreasing the tumor volume and increasing the apoptosis of tumor cells in a mouse melanoma

Foods 2014, 3 428
model; and (2) reducing colonic inflammation in inflammatory bowel disease in rats, restoring lipid
peroxidation and improving antioxidant parameters [65].
Finally, in healthy individuals, manuka honey UMF 20+ has been evaluated for its safety: its
consumption showed: (1) no significant effect on the allergic status of the subjects; (2) no detrimental
effect in relation to advanced glycation end products, which are implicated in a number of serious
diseases, including renal disease, diabetes, neurodegenerative disease and heart disease; and (3) no
change in gut microbiota homeostasis, confirming its safety for healthy individuals [66].
6. Conclusions
Besides its main components, manuka honey contains a large number of other constituents in small
and trace amounts, able to exert numerous nutritional and biological effects, like antimicrobial and
antioxidant activities. The above information shows that in microbiological and clinical tests, manuka
honey offers advantages in controlling bacterial growth and in the treatment of several health
problems. The easiness of administration in wound treatment and the absence of antibiotic resistance,
which instead is found with conventional antibiotics, are important characteristics for the use of this
honey in the treatment of clinical wounds.
Acknowledgments
The authors wish to thank Monica Glebocki for extensively editing the manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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