Lactococcus lactis strains are promising cell factories and delivery vehicles of plasmid DNA and recombinant protein for therapeutic applications. However, the limited yields of recombinant molecules obtained with these bacteria limits their wide applicability. Genome engineering of this host may solve the problem. However, the current genome editing toolbox available for L. lactis is either too laborious or incapable of large edits, limiting the scope of strain editing experiments. In this work, the basis for a one-plasmid CRISPR-Cas9 based genome editing plasmid was developed and tested. The new plasmid (pTCas9dO) adapted from the pKCcas9dO plasmid was used to delete 657 bp of the lactococcal nuclease nth of L. lactis subsp. lactis LMG19460, with the aim of improving yield and quality of plasmid DNA replicated in this strain. Although deletion mutants were successfully generated, plasmid curing was unsuccessful. Thus, further modifications are required before the plasmid is truly applicable for genome editing experiments. Unexpectedly, the generated deletion mutants generated a roughly 40% decrease in plasmid yield alongside with a decrease in the quality of produced pDNA.

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