Fructophilic metabolism of mannitol-producing Fructobacillus strains under different carbon sources and electron acceptors.

Aims: Fructobacillus requires electron acceptors (fructose, pyruvate, or oxygen) for glucose metabolism. They metabolize fructose as an energy source by using fructokinase (FK) and glucose-6-phosphate isomerase (GPI), and as an electron acceptor, reducing it to the low-calorie sugar mannitol by using mannitol 2-dehydrogenase (MDH) enzyme. We aimed to study the physiological and biochemical traits of Fructobacillus sp. CRL 2054 and F. tropaeoli CRL 2034, belonging to two different genomic clades, to further improve their mannitol production.

Methods and results: Microbial growth and mannitol production under different carbon sources, saccharide concentrations, and electron acceptors were assayed. The activities and gene expression of the enzymes involved in carbohydrate metabolism and mannitol production by Fructobacillus, along with their genomic context, were determined. Both strains grew up to 40% (w v-1) carbohydrates, while mannitol production decreased with 20% (w v-1) of sugars. Pyruvate competed with fructose as an electron acceptor, reducing mannitol production and MDH activity in both strains. The mdh gene was upregulated while fk and gpi genes were downregulated under glucose, regulating the use of fructose as electron acceptor.

Conclusions: Both Fructobacillus strains were highly osmotolerant. Mannitol production decreased with sugar concentrations above 15% (w v-1), suggesting its use as a compatible solute against osmotic stress. Pyruvate competed with fructose as an electron acceptor, reducing mannitol production, while fructose was reduced to mannitol in the presence of glucose by regulating genes involved in fructose metabolism.