Acetoin production by resting cells of Lactococcus lactis for direct electrochemical synthesis of 2-butanone

2-Butanone and acetoin are important commodity chemicals, while only the latter can be produced biotechnologically from renewable resources in high concentrations. Recently, electrochemical conversion of acetoin to 2-butanone was shown to be feasible directly from microbial cultivation broth supernatants. However, the electrocatalysis in cultivation broth suffered from parasitic reactions due to dissolved media components. Here, we propose enantiopure (3R)-acetoin production in dilute resting cell buffer using genetically modified Lactococcus lactis to cater for the subsequent electrochemical catalysis. In resting cell buffers with a minimal amount of ions and dissolved molecules, the strain achieved product yields as high as 0.49 g(acetoin) g(glucose)(-1) and product titers of 19.4 g L-1. The resting cell broth was directly applied as the electrolyte in an electrochemical flow cell, where the conversion of the produced acetoin to 2-butanone was assessed. The 2-butanone yield of more than 50% outperformed the synthesis from complex media fermentation supernatants and that of previous studies. This interdisciplinary study at the interface of bio- and electrocatalysis shows that resting cells have a high potential to move microbial synthesis towards a cascade of processes, instead of single isolated conversion steps for the valorization of biomass.