Advances in Protection Against Listeria

Fresh-cut fruits come with particular quality and shelf-life challenges. For example, food-contaminating microbes on a peel or rind’s surface can piggyback onto a cutting knife and be dragged into the fruit’s flesh.

Now an ARS-led research team has found that applying a combination of antagonistic microorganisms and bacteriophages can be effective in reducing Listeria monocytogenes on fresh-cut honeydew melon pieces. L. monocytogenes is a foodborne human pathogen that tolerates environmental stress, multiplies at low temperatures, and survives refrigeration.

Widely distributed in nature, Listeria can cause a serious, even fatal, infection. That’s why the U.S. Food and Drug Administration has established zero tolerance for L. monocytogenes in ready-to-eat foods, including processed fresh-cut fruits and vegetables.

William Conway, with the ARS Produce Quality and Safety Laboratory in Beltsville, Maryland, and Wojciech Janisiewicz, with ARS’s Appalachian Fruit Research Station in Kearneysville, West Virginia, led the study.

They artificially contaminated honeydew melon pieces that had been treated with three different solutions: an oxidative bacterium known as Gluconobacter asaii, a mixture of six bacteriophages, or a combination of both. G. asaii bacteria are naturally present on the surface of pome fruits, such as apples and pears. Bacteriophages—or phages—are viruses that, while nontoxic to humans, sicken certain human bacterial pathogens and eventually kill them.

The team found that the combination of phage and G. asaii was the most effective of the three treatments. It reduced L. monocytogenes populations by more than 99.999 percent.

“The phage component had an immediate inhibitory effect, while G. asaii controlled the pathogen for a longer period,” says Janisiewicz.

“As a beneficial bacterium, G. asaii’s mechanism of action may be that it competes for space and nutrients on fruit and vegetable surfaces where fungi or bacteria would otherwise thrive,” says Conway.

Phages invade bacteria, multiply, and eventually damage bacterial walls, releasing many new phages capable of invading more bacteria. Because the phages only attack specific bacterial species and strains, they can be applied to prevent those kinds of infections.—By Rosalie Marion Bliss, Agricultural Research Service Information Staff.

William S. Conway is with the USDA-ARS Produce Quality and Safety Laboratory, 10300 Baltimore Ave., Bldg. 002, Beltsville, MD 20705-2350; phone (301) 504-6980, fax (301) 504-5107.

"Advances in Protection Against Listeria" was published in the March 2007 issue of Agricultural Research magazine.