Construction and application of a food-grade expression system for Lactococcus lactis.

A food-grade host/vector expression system for Lactococcus lactis was constructed using alanine racemase gene (alr) as the complementation marker. We obtained an alanine racemase auxotrophic mutant L. lactis NZ9000Δalr by double-crossover recombination using temperature-sensitive integration plasmid pG(+)host9 and a food-grade vector pALR with entirely lactococcal DNA elements, including lactococcal replicon, nisin-inducible promoter PnisA and the alr gene from Lactobacillus casei BL23 as a complementation marker. By using the new food-grade host/vector system, the green fluorescent protein and capsid protein of porcine circovirus type II were successfully overexpressed under the nisin induction. These results indicate that this food-grade host/vector expression system has application potential as an excellent antigen delivery vehicle, and is also suitable for the use in the manufacture of ingredients for the food industry.