The invention features compositions and methods for delivering small interfering (siRNAs), e.g., shRNAs, to host cells using non-pathogenic strains of Salmonella bacteria containing nucleic acid expression constructs encoding shRNAs. In this process, shRNA expressed by the Salmonella silences or knocks down genes of interest (target genes) inside target cells. The nucleic acid expression constructs of the invention include an RNA polymerase (e.g., a T7 polymerase), an RNA polymerase promoter (e.g., a T7 polymerase promoter), and an RNA polymerase terminator (e.g., a T7 polymerase terminator). The Salmonella bacteria can also include, on the same or different nucleic acid construct, an endosomal release factor (e.g., HlyA).
