Lactobionic acid (LBA) is a specialty aldonic acid that has been broadly employed in various industries, including food, cosmetics, and pharmaceuticals. In this study, we isolated a new excellent LBA-producing bacterium from soil, designated as strain KRICT-1. A phylogenetic analysis revealed that this strain was identified as Enterobacter cloacae. Previously, we successfully produced LBA using Pseudomonas taetrolens, an excellent LBA-producing bacterium. In a flask culture at 30 °C, E. cloacae KRICT-1 showed the same LBA production concentration (209.3 g/L) and yield (100%) from 200 g/L lactose, compared with those obtained from P. taetrolens cultivated at 25 °C, the growth temperature of which showed the highest LBA production ability. E. cloacae KRICT-1 exhibited an approximately 1.08-fold higher LBA productivity (8.37 g/L/h) than that (7.75 g/L/h) of P. taetrolens, indicating that E. cloacae KRICT-1 was a more efficient LBA-producing bacterium than P. taetrolens. The LBA productivity of E. cloacae KRICT-1 further improved to 8.72 g/L/h at 35 °C. Furthermore, the LBA productivity (9.97 g/L/h) improved at 35 °C in a 5 L fermenter. After whole-genome sequencing of E. cloacae KRICT-1, we found that two quinoprotein glucose dehydrogenases (GDH1 and GDH2) from this strain could produce LBA from lactose by recombinantly expressing these genes in Escherichia coli. Our present results clearly demonstrated that E. cloacae KRICT-1 can be a new promising host for the industrial production of LBA.

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Lactobionic acid (LBA) is an emerging chemical that has been widely utilized in food, cosmetic, and pharmaceutical industries. We sought to produce LBA using Escherichia coli. LBA can be produced from lactose in E. coli, which is innately unable to produce LBA, by coexpressing a heterologous quinoprotein glucose dehydrogenase (GDH) and a pyrroloquinoline quinone (PQQ) synthesis gene cluster. Using a recombinant E. coli strain, we successfully produced LBA without additional supplementation of PQQ, and changing the type of heterologous GDH improved the LBA production titer and productivity. To further enhance LBA production, culture conditions, such as growth temperature and isopropyl-β-d-1-thiogalactopyranoside concentration, were optimized. Using optimized culture conditions, batch fermentation of the recombinant E. coli strain was performed using a 5 L bioreactor. After fermentation, this strain produced an LBA titer of 209.3 g/L, a yield of 100%, and a productivity of 1.45 g/L/h. To our best knowledge, this is the first study to produce LBA using heterologous GDH in an E. coli strain without any additional cofactors. Our results provide a simple method to produce LBA from lactose in a naturally non-LBA-producing bacterium and lay the groundwork for highly efficient LBA production in E. coli, which is one of the most versatile metabolite-producing bacterial hosts.