The special viruses called bacteriophages are meant to kill strains of the Listeria monocytogenes bacterium, the Food and Drug Administration said in declaring it safe to use on ready-to-eat meats prior to their packaging.

The viruses are the first to win FDA approval for use as a food additive, said Andrew Zajac, of the regulatory agency's office of food additive safety.

The bacterium the viruses target can cause a serious infection called listeriosis, primarily in pregnant women, newborns and adults with weakened immune systems. In the United States, an estimated 2,500 people become seriously ill with listeriosis each year, according to the Centers for Disease Control and Prevention. Of those, 500 die.

Luncheon meats are particularly vulnerable to Listeria since once purchased, they typically aren't cooked or reheated, which can kill harmful bacteria like Listeria, Zajac said.

The preparation of bacteriophages—the name is Greek for "bacteria-eater''—attacks only strains of the Listeria bacterium and not human or plant cells, the FDA said.

"As long as it used in accordance with the regulations, we have concluded it's safe,'' Zajac said. People normally come into contact with phages through food, water and the environment, and they are found in our digestive tracts, the FDA said.

Consumers won't be aware that meat and poultry products have been treated with the spray, Zajac added. The Department of Agriculture will regulate the actual use of the product.

The viruses are grown in a preparation of the very bacteria they kill, and then purified. The FDA had concerns that the virus preparation potentially could contain toxic residues associated with the bacteria. However, testing did not reveal the presence of such residues, which in small quantities likely wouldn't cause health problems anyway, the FDA said.

"The FDA is applying one of the toughest food-safety standards which they have to find this is safe,'' said Caroline Smith DeWaal, director of food safety for the Center for Science in the Public Interest, a consumer advocacy group. "They couldn't approve this product if they had questions about its safety.''

Intralytix, based in Baltimore, first petitioned the FDA in 2002 to allow the viruses to be used as a food additive. It has since licensed the product to a multinational company, which intends to market it worldwide, said Intralytix president Vazzana. He declined to name the company but said he expected it to announce its plans within weeks or months.

Intralytix also plans to seek FDA approval for another bacteriophage product to kill E. coli bacteria on beef before it is ground, Vazzana said.

Scientists have long studied bacteriophages as a bacteria-fighting alternative to antibiotics.