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
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