The Antibacterial Activity of Honey
University of Wales
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Honey is an old remedy recently rediscovered as a possible alternative to modern antibiotics in wound management but its mode of action is not fully understood. The antibacterial activity of honey can be divided into hydrogen peroxide and non-hydrogen peroxide-derived activity. This later type of activity is characteristic of honeys from Australasia (e.g. manuka honey) and preferred for wound management, although historically local honeys have been used. The main aim of this study was to investigate the mechanisms of antibacterial action of manuka honey. The stability of antibacterial action of manuka honey under different conditions was determined, it was observed that manuka honey lost its antibacterial activity when pH was increased and that it remained the same with heating. Storage seemed to increase the potency of manuka honey. The effects of honey on Staph. aureus and Pseud. aeruginosa, were investigated using MIC/MBC detenninations, time-kill studies, commitment to death, resistance training, electron microscopy, effects on respiration rates, leakage of intracellular material, and for Staph. aureus the proteome of treated and non-treated cells were compared. It was observed that the effect of manuka honey on Gram-positive and Gram-negative cells is different. Gram-positive bacteria had a lower MIC than Gram-negative bacteria, but the time-kill experiments and the commitment-to-deaths howed that Gram negative were inhibited more rapidly. Clinical strains of both bacteria showed different time-kill profiles to type strains. The methodology used for MIC determination was found to affect to the results obtained. No honey-resistant ram-positive bacteria were recovered, but Gramnegative bacteria were found to be able to become phenotypically resistant to manuka honey. Electron microscopy showed that honey inflicted physical damages in both types of cells, and in Gram-positive bacteria led to an increase in the proportion of population of cells with a complete septum. Gram-positive cells incubated in honey increased their endogenousre spiration rate whilst this was decreased in Gram-negative, major leakage was observed in Gram-negative bacteria whilst only minor leakage was observed in Gram-positive bacteria, which is consistent with the amount of damage observed with electron microscopy. The proteome analysis of Staph aureus, revealed a general down regulation of protein synthesis. Thirty Portuguese honeys were assayed for their antibacteriaal ativity and honeys derived from Lavandulas toechas(lavender) were found to possess non-peroxide activity. A selection of manuka honeys was screened for antimicrobial producing bacteria. In total 106 bacteria were recovered (85% were identified as Bacillus sp. ) and of those, 76 were capable of inhibiting the growth of at least one strain of bacteria tested, meaning that some of the antibacterial activity in manuka honey could be due to the presence of antimicrobial agents of bacterial origin. The antibacterial activity of manuka honey has previously been claimed to be due to hydrogen peroxide production and not to a non-peroxide source of activity. A study of free radical production and antioxidant potential demonstrated that manuka honey did not produce any hydroxyl radicals via the Fenton reaction. Thus hydrogen peroxide could not be present. It was also observed that even free radical-producing honeys were able to quench radical production in vitro. In conclusion this study has demonstrated that the non-peroxide activity of manuka honey is not exclusive to Australasia honeys, that it is not derived from hydrogen peroxide generation and may have a microbial origin. Furthermore the action of manuka honey on Gram-negative bacteria seems to be more physical than in Gram positive where it appears to interfere with the cell physiology, perhaps by stopping the cell cycle before cytokinesis.
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