Patent ID: 7871604

Claim:
A method for inducing an immune response in a warm-blooded animal comprising administering to the animal a composition comprising a bacterial cell, wherein (a) the bacterial cell comprises an expression gene that encodes an antigen, and an Environmentally Limited Viability System, (b) the antigen is introduced into the animal, (c) the bacterial cell is viable when in the animal and non-viable when outside of the animal, and (d) the Environmentally Limited Viability System comprises an essential gene that is under the control of an environmentally regulatable control sequence, wherein (i) expression of the essential gene in the cell is essential to the viability of the cell, (ii) the essential gene is expressed when the cell is in the animal and is not expressed when the cell is outside of the animal, (iii) the essential gene is essential for metabolism, growth, cell wall integrity or cell membrane integrity of the bacterial cell, and (iv) the essential gene is a copy of a native chromosomal gene wherein the chromosomal copy of said native gene is inoperable.