Method of immunization using biologically contained bacterial cells

A method of immunizing a vertebrate animal comprising oral or parental administration of an isolated bacterial host cell, where said bacterial host cell is transformed with a recombinant replicon, said replicon being selected from the group consisting of

• (I) a replicon comprising a first gene encoding a first gene product which has a host cell killing function, wherein the first gene is under the control of an invertible promoter, thereby leading to the stochastic expression of the first gene product within the host cell which results in the killing of the transformed host cell,
• (II) a replicon comprising a first gene encoding a first gene product which directly or indirectly activates the transcription of a second gene, the second gene encoding a host cell killing function, wherein the first gene is under the control of an invertible promoter, thereby leading to the stochastic expression of the first gene product within the host cell and, in turn, causing the transcription of the second gene and the expression of the host cell killing function, which results in the killing of the transformed host cell, and
• (III) a replicon comprising a first gene encoding a antisense RNA which directly or indirectly inhibits the expression of a host cell killing function, wherein the first gene is inserted between directly repeated nucleotide sequences of a sufficient size such that recombination between the repeats will occur at a certain frequency and result in the stochastic excision of the first gene, thereby leading to the expression of the host cell killing function within the host cell, which results in the killing of the transformed host cell, and
• (IV) a replicon comprising a first gene encoding a protein which directly or indirectly inhibits the transcription of a second gene, the second gene encoding a host cell killing function, wherein the first one is inserted between directly repeated nucleotide sequences of a sufficient size such that recombination between the repeats will occur at a certain frequency and result in the stochastic excision of the first thereby leading to the expression of the host cell killing function within the host cell, which results in the killing of the transformed host cell,
• and said replicon (I)-(IV) further comprising a gene encoding an antigen comprising one or more epitopes from a pathogenic agent, wherein the antigen is expressed and transported to the outer surface of the bacterial host cell.