Microorganisms and Phages Help Reduce Listeria on Fresh-Cut Fruit

Recent research by scientists from the Produce Quality and Safety Laboratory in Beltsville, MD, and the Appalachian Fruit Research Station in Kearneysville, WVboth associated with USDAs Agricultural Research Service (USDA-ARS)have found that applying a combination of antagonistic microorganisms and bacteriophages (or phages) can effectively reduce Listeria monocytogenes on fresh-cut honeydew melon.

The researchers treated honeydew pieces with three different solutions: Gluconobacter asaii (an oxidative bacterium), a mixture of six phages, or a combination of both. They then inoculated the honeydew pieces with L. monocytogenes.

ARS notes that G. asaii bacteria occur naturally on the surface of fruits like apples and pears and that phages are viruses that have an ability to weaken certain bacterial pathogens, and eventually kill them. However, the phages are nontoxic to humans.

Phages invade bacteria, multiply, and eventually damage bacterial walls, releasing many new phages capable of invading more bacteria, notes Rosalie Marion Bliss, public affairs specialist, ARS, Beltsville, MD, in her coverage of the study in the March issue of Agricultural Research magazine (see http://www.ars.usda.gov/is/AR/archive/mar07/listeria0307.htm). Because the phages only attack specific bacterial species and strains, they can be applied to prevent those kinds of infections.

The research team found that a combination of phage and G. asaii was the most effective of the three treatments, reducing L. monocytogenes populations by more than 99.999%.

Wojciech Janisiewicz, research plant pathologist, Appalachian Fruit Research Station, and William Conway, research plant pathologist, Produce Quality and Safety Laboratory, led the study. They note that each component of the treatment had a distinct effect.

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. asaiis 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.

FDA has established zero tolerance for L. monocytogenes in ready-to-eat (RTE) foods, including processed fresh-cut fruits and vegetables, so these findings should prove valuable to manufacturers working with RTE fresh-cut fruits.