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The conjugational transfer of the plasmid pAMPl between two strains of Lactococcus lactis subsp. lactis was measured in the intestinal tract of four conventional and eight gnotobiotic rats. In the conventional rats both donor and recipient strains were eliminated and transconjugants were not observed. Germfree rats were dosed orally with the recipient strain and the donor strain carrying pAMPl. In faecal samples, transconjugants were detected within the first few days after dosing, and the transconjugants established a stable population at a level of 103-105 less than the recipient strain.

We have developed very efficient suicide functions for biological containment based on the lethal Escherichia coli relF gene. The suicide functions are placed in duplicate within a plasmid and arranged to prevent inactivation by deletion, recombination, and insertional inactivation. The efficiency of this concept was tested in a plasmid containment system that prevents transfer of plasmids to wild-type bacteria. Protection against plasmid transfer was assayed in test tubes and in rat intestine. Protection was efficient and refractory to inactivation by mutation and transposons. The efficiency of the suicide system was also tested in soil and seawater. We show that unprecedented suicide efficiency can be achieved in soil and seawater after suicide induction by IPTG (isopropyl-␤-D-thiogalactopyranoside). More than 7 orders of magnitude reduction in suicide bacteria was achieved.

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