Researchers have engineering a nonpathogenic (rather than pathogenic) bacterium to deliver DNA. They have created an Escherichia coli strain that is capable of invading mammalian cells, self-destructing, and delivering foreign DNA, they report in Nature Biotechnology. --Jerry 09/01/98

It sounds counter-intuitive to give someone who is sick a bacterial infection. But the delivery of DNA vaccines by attenuated pathogenic bacteria is exactly what several biomedical researchers are currently advocating. Tamed versions of pathogenic bacteria such as Salmonella and Shigella have been used to deliver DNA vaccines to mammalian cells very effectively, without deleterious effects. But the danger always exists that these tamed pathogens will back-mutate to their more virulent forms and thus harm the patient. In this issue, Catherine Grillot-Courvalin and colleagues have turned this idea on its head by engineering a nonpathogenic (rather than pathogenic) bacterium to deliver DNA. They have created an Escherichia coli strain that is capable of invading mammalian cells, self-destructing, and delivering foreign DNA.

Starting with an E. coli strain containing a stretch of DNA from the intracellular pathogen Shigella flexneri that allows entry into mammalian cells and subsequent lysis, the Pasteur Institute researchers introduced the inv gene from Yersinia pseudotuberculosis and the listeriolysin O gene from Listeria monocytogenes either singly or in combination---genes that encode proteins that facilitate invasion of host cells and escape from cell vesicles (encapsulating membranes) into the host cytoplasm. When they infected HeLa cells with E. coli carrying the green fluorescent protein (GFP) or beta-galactosidase genes, they found that the bacteria entered the cells, lysed, and delivered the plasmid DNA to the nucleus, resulting in the production of GFP and beta -galactosidase. Suprisingly, only certain cell lines showed increased GFP expression in the presence of listeriolysin. The authors suggest that these engineered nonpathogenic bacteria may have safety advantages over attenuated pathogenic bacteria as DNA delivery vectors.