Explanation: A. tumefaciens and its close relatives have been used for over 20 years to generate transgenic plants. More recently, it has been shown that A. tumefaciens can transfer DNA to cultured human cells.

Explanation: The protoplast fusion technique can be regarded as a highly artificial form of bactofection, but the amount of human intervention required distinguishes the technique.

Explanation: In the case of Salmonella species, lysis occurs in the phagocytic vesicle, while for other species lysis occurs after the bacterium has escaped from the vesicle.

Explanation: A. tumefaciens transfers DNA to mammalian cells without invading them. Transfer occurs by attachment to the outside of the cell followed by conjugation.

Explanation: In A. tumefaciens, transfer occurs by attachment to the outside of the cell followed by conjugation. It is a process of transfer of DNA through a conduit called a pilus, which is assembled by the bacterial cell.

Explanation: An important principle in the use of live bacteria as invasive gene-transfer vehicles is that they must be attenuated.

Explanation: The gene transfer system exploits the natural ability of the bacteria to infect and subvert the activity of eukaryotic cells. Without attenuation, the bacteria would multiply and destroy the host cells.

Explanation: Attenuation is achieved in several ways. One way is to use auxotrophic mutants that are bacterial strains that are unable to manufacture essential molecules.

Explanation: AroA mutants are unable to synthesize aromatic amino acids and Salmonella typhimurium and Shigella flexneri strains carrying this mutation have been used for gene transfer.

Explanation: The bacteria can be engineered so that they undergo inducible autolysis. Attenuation can be achieved by induced suicide, introducing an autolysin-encoding gene that is activated once the bacterium is inside the host cell.

Explanation: There are no auxotrophic strains of Listeria monocytogenes, so attenuation is achieved by induced suicide. Autolysin-encoding gene is introduced.

Explanation: Bacteria mediated gene transfer has been used not only as general transfection method for the introduction of DNA into cultured cells but also as a high efficiency method for gene transfer in vivo.

Explanation: Virus particles have a natural ability to adsorb to the surface of cells and gain entry, and this can be exploited to deliver recombinant DNA into animal cells.

Explanation: If the transgene is added to the genome, or if it replaces one or more genes that are non-essential for the infection cycle in the expression host being used, the vector is described as helper-independent because it can propagate independently.

Explanation: If the transgene replaces an essential viral gene, this renders the vector helper-dependent so that missing functions must be supplied in trans.