Degradation of the low-calorie sugar substitute 5-ketofructose by different bacteria.

There is an increasing public awareness about the danger of dietary sugars with respect to their caloric contribution to the diet and the rise of overweight throughout the world. Therefore, low-calorie sugar substitutes are of high interest to replace sugar in foods and beverages. A promising alternative to natural sugars and artificial sweeteners is the fructose derivative 5-keto-D-fructose (5-KF), which is produced by several Gluconobacter species. A prerequisite before 5-KF can be used as a sweetener is to test whether the compound is degradable by microorganisms and whether it is metabolized by the human microbiota. We identified different environmental bacteria (Tatumella morbirosei, Gluconobacter japonicus LMG 26773, Gluconobacter japonicus LMG 1281, and Clostridium pasteurianum) that were able to grow with 5-KF as a substrate. Furthermore, Gluconobacter oxydans 621H could use 5-KF as a carbon and energy source in the stationary growth phase. The enzymes involved in the utilization of 5-KF were heterologously overproduced in Escherichia coli, purified and characterized. The enzymes were referred to as 5-KF reductases and belong to three unrelated enzymatic classes with highly different amino acid sequences, activities, and structural properties. Furthermore, we could show that 15 members of the most common and abundant intestinal bacteria cannot degrade 5-KF, indicating that this sugar derivative is not a suitable growth substrate for prokaryotes in the human intestine. KEY POINTS: • Some environmental bacteria are able to use 5-KF as an energy and carbon source. • Four 5-KF reductases were identified, belonging to three different protein families. • Many gut bacteria cannot degrade 5-KF.

An alternative pentose phosphate pathway in human gut bacteria for the degradation of C5 sugars in dietary fibers.

The microbial degradation of pentoses in the human gut is a crucial factor for the utilization of plant-based dietary fibers. A vast majority of gut microbes are able to use these C5-sugars as a carbon and energy source. However, the underlying metabolic pathways are not fully understood. Bioinformatic analysis showed that a large number of abundant gut bacteria lack genes encoding a transaldolase as a key enzyme of the pentose phosphate pathway. Among them was the important human gut microbe Prevotella copri, which was able to grow in minimal media containing xylose or hemicelluloses as the sole carbon source. Therefore, we looked for an alternative pathway for pentose conversion in P. copri using bioinformatics, enzyme activity assays, and the detection of intermediates of pentose metabolism. It became evident that the organism converted C5-sugars via the sedoheptulose-1,7-bisphosphate pathway (SBPP) to connect pentose metabolism with glycolysis. To circumvent the transaldolase reaction, P. copri uses the combined catalysis of a pyrophosphate-dependent phosphofructokinase and a fructose-bisphosphate aldolase. Furthermore, we present strong evidence that the SBPP is widely distributed in important gut bacteria, including members of the phyla Bacteroides, Firmicutes, Proteobacteria, Verrucomicrobia, and Lentisphaerae.

Introducing Vibrio natriegens as a Microbial Model Organism for Microgravity Research.

Microbial contamination of human-tended spacecraft is unavoidable, making the study of microbial growth under space conditions essential for the preservation of astronauts' health and equipment integrity. Previous studies suggested that spaceflight conditions, such as microgravity, cause a range of physiological microbial alterations including increased growth yields and decreased antibiotic susceptibility. Because of its fast generation time, Vibrio natriegens could be used as a model organism for a variety of studies where generation time is a critical factor. In this study, V. natriegens was used as a tool to study growth characteristics by determining the viable cell number and antibiotic susceptibility under simulated microgravity using a 2-D clinostat (60 rpm) to establish a test system that resolves changes in microbial growth on a solid surface (agar) under microgravity. The data show that V. natriegens biomass increases significantly after 24 h at 37°C under simulated microgravity. The final cell population after cultivation under simulated microgravity was 60-fold greater than when cultivated under normal terrestrial gravity (1 × g). No change in susceptibility to the antibiotic rifampicin after cultivation under simulated microgravity or normal gravity was detected. These data show that V. natriegens is a new and innovative model organism for microbial microgravity research.