Honey as an antimicrobial agent against multi-drug resistant Gram negative bacterial rods

Abstract

Honey has been shown to have therapeutic properties, which include immunomodulatory and antibacterial activity in vitro and anti-inflammatory, antipyretic and wound healing properties in vivo. A complex mix of factors such as acidity, osmolality and hydrogen peroxide content contribute to antibacterial activity. Unusually manuka honey has been shown to contain methylglyoxal which is derived from nectar collected from the blossom of manuka trees and this confers high antibacterial activity. Manuka honey is used in licensed wound dressings in the UK. Its ability to inhibit staphylococci has been reported, but its efficacy with Gram negative bacteria is less well documented. Since these bacteria are difficult to control and commonly infect military wounds and burns, there is a need to investigate their susceptibility to manuka honey. The main aim of this study is to assess the antimicrobial potential of manuka honey against multi-drug resistant (MDR) Gram negative rods with the potential to infect wounds. Eighty five clinical isolates were tested in this study (30 MDR Acinetobacter and 55 extended spectrum beta-lactamases [ESBL] producing members of the Enterobacteriaceae). The minimum inhibitory concentration (MIC) of manuka honey for each isolate was determined by agar incorporation and broth dilution methods, as well as the minimum bactericidal concentration (MBC). The kinetics of inhibition of selected isolates with high MIC values was monitored by total viable counts. Also, ultrastructural changes in cell morphology were studied before and after exposure to manuka honey using scanning (SEM) and transmission electron microscopy (TEM). Electron micrographs were examined for structural changes, such as altered shape, surface abnormalities and evidence of cell division. Eight Omani honeys were assayed for their antibacterial activity using bioassay, MIC and MBC methods. Omani honeys were also analysed for their chemical and physical properties such as pH, protein, water and sugar contents, hydroxymetheylfurfural (HMF), colour and antioxidant properties. Pollen analysis was also used for identifying the flora origin of honey. All Omani honeys were found to possess peroxide activity nonetheless it exhibited a bactericidal mode of activity against all MDR and ESBLs tested. In addition honey analysis revealed unadulterated and natural honey. A study of anti-radical activity and phenolic contents demonstrated that Omani honey could be used to promote a rapid wound healing and aid its antibacterial activity. The proximity of MIC and MBC values indicates that manuka honey had a bactericidal mode of action against these isolates and this was confirmed by the time to kill curves. The SEM and TEM of images of representative isolates after treatment with manuka honey showed some physical membrane damage, septa formation and irregular shape; whereas non honey treated cells (control) did not appear to be obvious damage. In conclusion manuka honey possesses strong antibacterial activity against the antibiotic-resistant wound pathogens tested here and further investigation into cellular target sites is needed. Both manuka and selected Omani honeys have clinical potential to inhibit pathogens that commonly colonise wounds.