Combine beneficial bacteria, bacteriophages to fight microbes on produce.

Before fresh fruit is cut, it's important that the outer skin be kept free of foodborne pathogens. That's because microbes on the surface of a peel or rind could piggyback onto a cutting knife and be dragged into the fruit's flesh.

USDA-ARS scientists and colleagues tested a combination of bacteria and viruses that effectively controlled L. monocytogenes on pieces of fresh-cut honeydew melon during exposure tests. L. monocytogenes tolerates environmental stress, multiplies at low temperatures and survives refrigeration. It can cause serious infections. Federal agencies have established a zero tolerance for L. monocytogenes in processed, fresh-cut fruits and vegetables.

To test bacteria-fighting potential, the researchers treated honeydew melon pieces with different protective solutions: an oxidative bacterium known as Gluconobacter asaii, a mixture of six L. monocytogenes-specific bacteriophages or a combination of both. The G. asaii are naturally present on the surface of pome fruits, such as apples and pears, and are nontoxic to humans. Bacteriophages are viruses that, while nontoxic to humans, kill specific human pathogens.

After artificially contaminating the test honeydew pieces with L. monocytogenes, the team found that the combination of phages and G. asaii bacteria was the most effective treatment. This treatment reduced L. monocytogenes populations by more than 99.9%. A synergistic effect seems to have occurred between the phage component and Gluconobacter asaii, which maximized their effectiveness in controlling populations of L. monocytogenes throughout the entire storage period of seven days at 10 C.

The phage component had an immediate inhibitory effect, while G. asaii controlled the pathogen for a longer period of time. 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. 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 specific types of infections.

Further information. William Conway, USDA-ARS Produce Quality and Safety Laboratory, Building 002, Room 223, 10300 Baltimore Ave., Beltsville, MD 20705; phone: [phone]; fax: [phone]; email: [e-mail].