Construction of a dextran-free Leuconostoc citreum mutant by targeted disruption of the dextransucrase gene

Aims: Leuconostoc citreum is an important lactic acid bacterium in fermented foods, but dextran production often causes undesired ropiness. To prevent this side effect, a dextran-free mutant needs to be created. Methods and Results: Homologous recombination of the dextransucrase gene (dsrC) was conducted using a segregationally unstable plasmid, pCBM32-DSUDs. A mutant was obtained on sucrose agar medium, and a site-specific insertional inactivation in the gene was confirmed. When cultured in sucrose medium, the mutant strain produced no dextransucrase or dextran. Additionally, it showed a longer lag phase (9 h) than the wild-type strain (3 h), providing new insights into the role of dextransucrase in carbohydrate metabolism of Leuconostoc. Conclusions: In this study, a dextransucrase knockout mutant was constructed. It was found that Leuc. citreum dextransucrase not only synthesizes dextran for cell protection but also provides fructose as an important carbon source for cell growth. Significance and Impact of the Study: This knockout mutation was generated for the first time in Leuc. citreum. The dextran-free mutant has the potential to be used for various industrial purposes, including as a starter culture for production of nonviscous fermented foods and as a dextran-free host for production of recombinant proteins.