Growth behavior of probiotic microorganisms on levan- and inulin-based fructans

Modulation of the gut microbiota by prebiotics is an effective tool to improve host health. Here, the prebiotic properties of inulin- and levan-based fructans were investigated using microorganisms obtained from strain collections and isolated from probiotic pharmaceuticals and yogurts. Utilizing the recently characterized endo-levanase LevB2286, levan-fructooligosaccharides (L-FOS) were produced and applied for dedicated growth studies. Real-time growth experiments in 48-well format revealed that 8 out of 17 strains isolated from probiotic products or yogurts responded to prebiotic treatment. Lactobacillus paracasei strains, several bifidobacteria, and a Saccharomyces cerevisiae isolate metabolized supplied fructans efficiently. Overall inulin-FOS were consumed more rapidly than L-FOS. However, this effect may be attributed to the lower average degree of polymerization and the presence of GFn-FOS in the I-FOS preparation used. Growth experiments with fractionated L-FOS provided valuable insights into the influence of the degree of polymerization on fermentability by probiotic bacteria

5-Keto-D-Fructose, a Natural Diketone and Potential Sugar Substitute, Significantly Reduces the Viability of Prokaryotic and Eukaryotic Cells

5-Keto-D-fructose (5-KF) is a natural diketone occurring in micromolar concentrations in honey, white wine, and vinegar. The oxidation of D-fructose to 5-KF is catalyzed by the membrane-bound fructose dehydrogenase complex found in several acetic acid bacteria. Since 5-KF has a sweetening power comparable to fructose and is presumably calorie-free, there is great interest in making the diketone commercially available as a new sugar substitute. Based on a genetically modified variant of the acetic acid bacterium Gluconobacter oxydans 621H, an efficient process for the microbial production of 5-KF was recently developed. However, data on the toxicology of the compound are completely lacking to date. Therefore, this study aimed to investigate the effect of 5-KF on the viability of prokaryotic and eukaryotic cells. It was found that the compound significantly inhibited the growth of the gram-positive and gram-negative model organisms Bacillus subtilis and Escherichia coli in a concentration-dependent manner. Furthermore, cell viability assays confirmed severe cytotoxicity of 5-KF toward the colon cancer cell line HT-29. Since these effects already occurred at concentrations of 5 mM, the use of 5-KF in the food sector should be avoided. The studies performed revealed that in the presence of amines, 5-KF promoted a strong Maillard reaction. The inherent reactivity of 5-KF as well as the Maillard products formed could be the trigger for the observed inhibition of prokaryotic and eukaryotic cells