That honey has antibacterial properties has been established for over a century, but in many cultures honey has been used as a medicine since ancient times. The effectiveness of honey in many of its medicinal uses is probably due to its antibacterial activity. It is now less established that honey inhibits a broad spectrum of bacterial and fungal species.
Honey is a super-saturated solution of sugars and the strong interaction of these sugar molecules with the water leaves very few water molecules available for microorganisms. This "free" water is measured as the water activity (aw): mean values for honey have been reported from 0.562 to 0.62. Many species of bacteria have their growth completely inhibited if the aw is in the range 0.94-0.99. These values correspond to solutions of a typical honey (aw of 0.6 undiluted) of concentrations from 12% down to 2% (v/v). Hence Honey has the ability inhibit the growth of bacteria through osmotic effects, although this is obviously dependent on the type of bacteria.
The acidity of honey is also an effective inhibitor of many species of bacteria. The minimum pH values for growth of some common wound-infecting species is: Escherichia coli, 4.3; Salmonella sp., 4.0; Pseudomonas aeruginosa, 4.4; Streptococcus pyogenes, 4.5. Hence, since honey characteristically has a pH in the range 3.2-4.5, the acidity of hone is a significant factor in its antibacterial activity.
Hydrogen Peroxide Activity
The major antibacterial activity in honey has been found to be due to hydrogen peroxide produced enzymically in the honey. The glucose oxidase enzyme is secreted from the hypopharyngeal gland of the bee into the nectar to assist in the formation of honey from the nectar. The hydrogen peroxide and acidity of honey are produced by the reaction:
glucose + H2O+ O2 --> gluconic acid + H2O2
and promote the preservation of the honey. However the acidity produced in the action of the enzyme drops the pH to a point which is too low for the enzyme to work any more. On dilution of honey the activity increases by a factor of 2,500 - 50,000, thus giving a "slow-release" antiseptic at a level which is antibacterial but not tissue-damaging.