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Background  A goal for the food industry has always been to improve strains of  Lactococcus lactis  and stabilize beneficial traits. Genetic engineering is used extensively for manipulating this lactic acid bacterium, while electropolation is the most widely used technique for introducing foreign DNA into cells. The efficiency of electrotransformation depends on the level of electropermealization and pretreatment with chemicals which alter cell wall permeability, resulting in improved transformation efficiencies is rather common practice in bacteria as in yeasts and fungi. In the present study, treatment with lithium acetate (LiAc) and dithiothreitol (DTT) in various combinations was applied to  L. lactis  spp.  lactis  cells of the early-log phase prior to electroporation with plasmid pTRKH3 (a 7.8 kb shuttle vector, suitable for cloning into  L. lactis ). Two strains of  L. lactis  spp.  lactis  were used,  L. lactis  spp.  lactis  LM0230 and ATCC 11454. To the best of our knowledge these agents have never been used before with  L. lactis  or other bacteria.    Results  Electrotransformation efficiencies of up to 10 5  transformants per μg DNA have been reported in the literature for  L. lactis  spp. lactis  LM0230. We report here that treatment with LiAc and DDT before electroporation increased transformation efficiency to 225 ± 52.5 × 10 7  transformants per μg DNA, while with untreated cells or treated with LiAc alone transformation efficiency approximated 1.2 ± 0.5 × 10 5  transformants per μg DNA. Results of the same trend were obtained with  L. lactis  ATCC 11454, although transformation efficiency of this strain was significantly lower. No difference was found in the survival rate of pretreated cells after electroporation. Transformation efficiency was found to vary directly with cell density and that of 10 10  cells/ml resulted in the highest efficiencies. Following electrotransformation of pretreated cells with LiAc and DDT, pTRKH3 stability was examined. Both host-vector systems proved to be reproducible and highly efficient.    Conclusion  This investigation sought to improve still further transformation efficiencies and to provide a reliable high efficiency transformation system for  L. lactis  spp.  lactis . The applied methodology, tested in two well-known strains, allows the production of large numbers of transformants and the construction of large recombinant libraries.

High efficiency electrotransformation of Lactococcus lactis spp. lactis cells pretreated with lithium acetate and dithiothreitol