Methylglyoxal content in Southern African honey : antibacterial and cellular effects

Abstract

The clinical benefits of honey based wound dressings have been established. Methylglyoxal (MGO) containing honey, such as Manuka honey does have numerous beneficial effects, however, concern has been expressed related to the safety of this honey in patients with diabetes. In addition Manuka honey is unique to New Zealand and its use in South Africa is limited due to the cost of importation. Southern Africa has a well described floral biodiversity and the possibility exists that an indigenous honey type with similar or greater wound healing properties than Manuka honey can be found. The aims of this study were to develop a rapid and inexpensive colorimetric method for the quantification MGO in southern Africa honey. Secondly to evaluate the antibacterial and cellular effects of MGO in vitro and to determine whether MGO levels that effectively kills bacteria is cytotoxic to several cell lines. The MGO levels as found in southern Africa honey will be extrapolated to the findings of this study related to the antibacterial activity and cellular effects. Using a newly developed colorimetric method for the quantification of MGO, MGO levels in a selection of southern Africa honey was found to be from 692.59 1261.23 mM MGO which is slightly higher than that found in UMF15 honey. It was determined that MGO had antibacterial effects on both Gram positive and negative bacteria. The IC50 range of MGO on B. subtilis and P. aeruginosa (Gram positive) was between 0.2 0.4 mM MGO and its MIC was at 0.4 1.2 mM. For S. aureus and E. coli (Gram negative) the IC50 range was from 0.2 1.0 mM MGO and the MIC was at 0.8 1.2 mM MGO. With scanning electron microscopy (SEM) it was observed that MGO prevented the formation of fimbriae and flagellae and at higher concentrations caused lysis. Three cell lines were exposed to 0.0003 324.7294 mM MGO for 48 hours. A hormetic effect was observed for all cell lines from a MGO concentration of 0.0003 3.2473 mM while at concentrations >32.473 mM toxicity was observed. At the bacterial MIC concentrations from 0.4 1.2 mM a hormetic effect was observed in RAW 264.7 mouse macrophages and Caco-2 colon carcinoma cells while the SC-1 fibroblast cell line exhibited normal growth. The MGO levels found in southern Africa honey was generally not cytotoxic. With SEM morphological changes were only observed at 324.7294 mM. At this concentration the RAW 264.7 mouse macrophages showed signs of activation and differentiation. The Caco-2 cells became more compacted and spheroid in shape while the SC-1 cells were more spindle-shaped and grew extensions and filopodia at higher concentrations and several large cells, probably myofibroblasts, were observed. In conclusion at the concentration range where MGO was found to effectively kill Gram positive and negative bacteria, MGO was not cytotoxic to cells in vitro. Southern Africa honey contains MGO and levels were generally not cytotoxic. In an endeavour to develop local honey based wound healing products the findings of this study provide important preliminary information on the MGO levels in southern Africa honey. The antibacterial activity of MGO was confirmed and this is the first study to describe the hormetic effects of MGO in several cell lines.

In conclusion at the concentration range where MGO was found to effectively kill Gram positive and negative bacteria, MGO was not cytotoxic to cells in vitro. Southern Africa honey contains MGO and levels were generally not cytotoxic. In an endeavour to develop local honey based wound healing products the findings of this study provide important preliminary information on the MGO levels in southern Africa honey. The antibacterial activity of MGO was confirmed and this is the first study to describe the hormetic effects of MGO in several cell lines.