A recombinant Listeria bacterium for use in a method of treatment, wherein said bacterium comprises: (a) a polynucleotide encoding a PrfA* mutant polypeptide wherein the PrfA* mutant polypeptide comprises a G155S mutation; and (b) a recombinant polynucleotide comprising (i) an actA promoter; and (ii) a polynucleotide encoding a heterologous polypeptide, wherein the polynucleotide encoding the heterologous polypeptide is operably linked to the actA promoter, and wherein the heterologous polypeptide is non-bacterial or is an antigen of a heterologous infectious pathogen; and said method is selected from: (i) a method of inducing an immune response in a host to the non-listerial antigen encoded by the heterologous polypeptide; (ii) a method of enhancing the immunogenicity of the non-listerial antigen encoded by the heterologous polypeptide in a host; (iii) a method of preventing or treating a non-listerial infectious or cancerous condition in a host; or (iv) a method for enhancing an immune response in a mammal to the non-listerial antigen encoded by the heterologous polypeptide in a host, wherein the mammal previously had been administered an effective amount of a prime dose of a vaccine that provided the non-listerial antigen, wherein (a) the vaccine does not contain live, metabolically active Listeria that encode the non-listerial antigen; and (b) when the vaccine contains naked DNA encoding the non-listerial antigen. said bacterium comprises: (a) a polynucleotide encoding a PrfA* mutant polypeptide wherein the PrfA* mutant polypeptide comprises a G155S mutation; and (b) a recombinant polynucleotide comprising (i) an actA promoter; and (ii) a polynucleotide encoding a heterologous polypeptide, wherein the polynucleotide encoding the heterologous polypeptide is operably linked to the actA promoter, and wherein the heterologous polypeptide is non-bacterial or is an antigen of a heterologous infectious pathogen; and (a) a polynucleotide encoding a PrfA* mutant polypeptide wherein the PrfA* mutant polypeptide comprises a G155S mutation; and (b) a recombinant polynucleotide comprising (i) an actA promoter; and (ii) a polynucleotide encoding a heterologous polypeptide, wherein the polynucleotide encoding the heterologous polypeptide is operably linked to the actA promoter, and wherein the heterologous polypeptide is non-bacterial or is an antigen of a heterologous infectious pathogen; and (i) an actA promoter; and (ii) a polynucleotide encoding a heterologous polypeptide, wherein the polynucleotide encoding the heterologous polypeptide is operably linked to the actA promoter, and wherein the heterologous polypeptide is non-bacterial or is an antigen of a heterologous infectious pathogen; and said method is selected from: (i) a method of inducing an immune response in a host to the non-listerial antigen encoded by the heterologous polypeptide; (ii) a method of enhancing the immunogenicity of the non-listerial antigen encoded by the heterologous polypeptide in a host; (iii) a method of preventing or treating a non-listerial infectious or cancerous condition in a host; or (iv) a method for enhancing an immune response in a mammal to the non-listerial antigen encoded by the heterologous polypeptide in a host, wherein the mammal previously had been administered an effective amount of a prime dose of a vaccine that provided the non-listerial antigen, wherein (a) the vaccine does not contain live, metabolically active Listeria that encode the non-listerial antigen; and (b) when the vaccine contains naked DNA encoding the non-listerial antigen. (i) a method of inducing an immune response in a host to the non-listerial antigen encoded by the heterologous polypeptide; (ii) a method of enhancing the immunogenicity of the non-listerial antigen encoded by the heterologous polypeptide in a host; (iii) a method of preventing or treating a non-listerial infectious or cancerous condition in a host; or (iv) a method for enhancing an immune response in a mammal to the non-listerial antigen encoded by the heterologous polypeptide in a host, wherein the mammal previously had been administered an effective amount of a prime dose of a vaccine that provided the non-listerial antigen, wherein (a) the vaccine does not contain live, metabolically active Listeria that encode the non-listerial antigen; and (b) when the vaccine contains naked DNA encoding the non-listerial antigen.