Gandini Chiara, Tarraran Loredana, Kalemasi Denis, Pessione Enrica, Mazzoli Roberto
Department of Life Sciences and Systems Biology, Structural and Functional Biochemistry, Laboratory of Proteomics and Metabolic Engineering of Prokaryotes, University of Turin, Torino, Italy.
Biotechnol Bioeng. 2017 Dec;114(12):2807-2817. doi: 10.1002/bit.26400. Epub 2017 Sep 4.
Lactic acid bacteria (LAB) are among the most interesting organisms for industrial processes with a long history of application as food starters and biocontrol agents, and an underexploited potential for biorefineries converting biomass into high-value compounds. Lactic acid (LA), their main fermentation product, is among the most requested chemicals owing to its broad range of applications. Notably, LA polymers, that is, polylactides, have high potential as biodegradable substitutes of fossil-derived plastics. However, LA production by LAB fermentation is currently too expensive for polylactide to be cost-competitive with traditional plastics. LAB have complex nutritional requirements and cannot ferment inexpensive substrates such as cellulose. Metabolic engineering could help reduce such nutritional requirements and enable LAB to directly ferment low-cost polysaccharides. Here, we engineered a Lactococcus lactis strain which constitutively secretes a β-glucosidase and an endoglucanase. The recombinant strain can grow on cellooligosaccharides up to at least cellooctaose and efficiently metabolizes them to L-LA in single-step fermentation. This is the first report of a LAB able to directly metabolize cellooligosaccharides longer that cellohexaose and a significant step toward cost-sustainable consolidated bioprocessing of cellulose into optically pure LA.
乳酸菌（LAB）是工业生产中最具吸引力的微生物之一，作为食品发酵剂和生物防治剂有着悠久的应用历史，并且在将生物质转化为高价值化合物的生物精炼方面具有尚未充分开发的潜力。乳酸（LA）是它们的主要发酵产物，由于其广泛的应用而成为最受欢迎的化学品之一。值得注意的是，LA聚合物，即聚乳酸，作为化石衍生塑料的可生物降解替代品具有很高的潜力。然而，通过LAB发酵生产LA目前成本过高，以至于聚乳酸无法与传统塑料在成本上竞争。LAB有复杂的营养需求，无法发酵纤维素等廉价底物。代谢工程有助于减少此类营养需求，并使LAB能够直接发酵低成本的多糖。在此，我们构建了一种组成型分泌β-葡萄糖苷酶和内切葡聚糖酶的乳酸乳球菌菌株。该重组菌株能够在至少纤维八糖的低聚纤维糖类上生长，并在单步发酵中将它们高效代谢为L-LA。这是关于LAB能够直接代谢长于纤维六糖的低聚纤维糖类的首次报道，也是朝着将纤维素成本可持续地整合生物加工成光学纯LA迈出的重要一步。
