Antimicrobial activity of functional food ingredients focusing on manuka honey action against Escherichia coli : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Engineering and Technology at Massey University, Auckland, New Zealand

Abstract

The goal of this research was to identify functional food ingredients/ingredient combinations able to manage the growth of intestinal microorganisms, and to elucidate the mechanisms of action of the ingredient(s). By developing a high-throughput in vitro microbial growth assay, a variety of preselected ingredients were screened against a panel of bacteria. Manuka honey UMF(TM) 20+ and BroccoSprouts(R) were identified as the most effective at managing microbial growth, alone and in combination. Manuka honey was particularly effective at increasing probiotic growth and decreasing pathogen growth. Testing of these two ingredients progressed to an animal feeding trial. Here, contrary to the in vitro results, it was found that no significant in vivo effects were observed. All honeys are known to be antimicrobial by virtue of bee-derived hydrogen peroxide, honey sugar-derived osmotic effects, and the contribution of low pH and the other bioactive compounds present, hence their historical usage as an antiseptic wound dressing. The in vitro antimicrobial effect of manuka honey has currently been the subject of much investigation, primarily focusing on the Unique Manuka Factor (UMF), recently identified as methylglyoxal, a known antimicrobial agent. This work has taken the novel approach of examining the effects of all of the manuka honey antimicrobial constituents together against Escherichia coli, in order to fully establish the contribution of these factors to the observed in vitro antimicrobial effects. For the first time, it has been demonstrated that the in vitro antimicrobial activity of manuka honey is primarily due to a combination of osmotically active sugars and methylglyoxal, both in a dose-dependent manner, in a complex relationship with pH, aeration and other factors. Interestingly, the manuka honey was revealed to prevent the antimicrobial action of peroxide, and that whilst methylglyoxal prevented E. coli growth at the highest honey doses tested, at low concentrations the osmotically active sugars were the dominant growth-limiting factors. Contrary to the literature, it was discovered that methylglyoxal does not kill E. coli, but merely extended the lag phase of the organism. In conjunction with the lack of antimicrobial activity in vivo, this is a landmark discovery in the field of manuka honey research, as it implies that the value of manuka honey lies more towards wound dressing applications and gastric health than as a dietary supplement for intestinal health.