An investigation of the prebiotic properties of Australian honeys.

Abstract

Non-digestible carbohydrates that promote specific, favourable changes in the composition and functionality of the gut microbiota, and thus contribute to improving health and well-being, are referred to as prebiotics. These favourable changes might be due to a relative increase in the numbers of potentially beneficial bacteria, and/or an increase in the metabolic activity of gut microbiota to produce more beneficial substances, such as short chain fatty acids (SCFA). An established method for quantifying the prebiotic effect of a carbohydrate utilises the Prebiotic Index (PI), which is the ratio of the changes in the populations of the potentially beneficial and potentially harmful bacteria. Honey contains non-digestible oligosaccharides and there is some evidence that certain honeys could induce beneficial changes in the gut, however there is limited information on Australian floral varieties of honey. The aim of this work was to conduct an in-depth investigation of the prebiotic properties of Australian honeys from a variety of floral sources. Three broad approaches were used. Initially, the influences of the monosaccharides in high fructose content Australian honeys on the composition and metabolic activity of gut microbes were tested in microcosms established with human intestinal microbiota. Secondly, the impact of the monosaccharides and non-digestible components (oligosaccharides) found naturally in honey on the growth and metabolic function of the gut microbiota was tested. The non-digestible components were obtained by simulating gastrointestinal conditions using a digestion process that mimicked the upper regions of the gastrointestinal tract. Both whole and digested honeys were assayed in microcosms established using human intestinal microbiota to simulate the lower regions of the gastrointestinal tract. Standard culture-based techniques were used to determine the impact of the honey monosaccharides and oligosaccharides on the numbers of the major bacterial groups of the gut. Molecular profiling of the microcosm microbiota enriched with four of the honeys supported the data from the culture-based methodology. The PI values of the honeys were determined, and the effect of the honeys on SCFA production by the gut microbiota was measured. Finally, the effect of the in vitro fermentation of honey by the gut microbiota on the growth of three enteropathogens and a probiotic strain was determined as an assessment of the resilience of the microcosm to an introduced species.

It was shown that the saccharides present in honey affected the bacterial composition of the gut microbiota. While much of the compositional changes can be attributed to the complex sugars that remain after digestion (oligosaccharides), the simple sugars (monosaccharides) in honey also contributed to modulation of the gut microbiota. The presence of the monosaccharides in honey significantly affected the changes in the microbial composition. Positive changes in the potentially beneficial lactobacilli populations were observed when high fructose content honeys were used, and these effects were attributed to the fructose components of the honeys. The second part of this study focused on the oligosaccharide components of the honey. This was the first study to investigate the impact of simulated digestion of Australian honeys on their prebiotic activity. The oligosaccharide components of all of the tested varieties of Australian honeys exerted favourable effects on the gut ecosystem by promoting the growth of the beneficial bacteria at levels similar to commercial prebiotic, inulin. The honeys suppressed the potentially harmful populations of the gut, and the growth of clostridia specifically was significantly impaired in the presence of the honeys. All honeys had positive PI values, and they also enhanced the production of SCFA, especially the butyric acid, but both of these parameters varied considerably depending on the honey type. Finally, the fermentation of honey by human gut microbiota resulted in the production of compounds with inhibitory activity against three common enteropathogens, namely Salmonella typhimurium, Clostridium difficile and Escherichia coli, and at levels that matched the commercial prebiotic control, inulin. Most of the honeys tested were more effective at inhibiting the growth of C. difficile than the prebiotic control, and inclusion of inulin in the microcosms had little added inhibitory effect on C. difficile growth, suggesting that the honeys allowed enhanced production of the inhibitory substances. These compounds did not inhibit the growth of a commercial beneficial probiotic strain, Lactobacillus fermentum. In summary, the results from this investigation demonstrate that Australian honeys have considerable prebiotic capacity which was comparable to or better than inulin. The prebiotic properties of the honeys were associated with the fructose and the oligosaccharides of the honeys which promoted the beneficial bacteria, inhibited the potential pathogens and elevated butyric acid levels.