Germ-Fighting Germs

Plants and animals arent the only things that get sick. Even pathogenic microbes can succumb to infections. Federal plant pathologists are now looking to capitalize on that phenomenon as a strategy to fight off food poisoning.

Though nature seals most fruits and vegetables in germ-resistant peels and rinds, once those outer barriers are breached–such as when you slice a cucumber or peel an avocado–those foods become sitting ducks for any poisonous bacteria on your hands, utensils, or cutting board.

So, Britta Leverentz and William Conway of the Agricultural Research Services Produce Quality and Safety Laboratory in Beltsville, Md., have started exploring the idea of spraying fresh-cut produce with phages. These are viruses that infect and kill only bacteria.

Indeed, phages are part of natures way of controlling bacteria. Just a milliliter of lake or river water typically holds millions of these viruses. Most are fairly specialized in their toxicity–attacking only certain strains of Salmonella, for example, or perhaps Listeria. Thats good, Leverentz adds, since you wouldnt want any of the phages delivered in the kitchen to also wipe out beneficial microbes, such as some lactobacilli.

Data look promising. . .

As other viruses do, phages enter susceptible cells and insert their own genes. The host cell then replicates those viruses like mad–until it eventually explodes. This releases a new generation of phages. This process repeats itself until the viruses run out of healthy hosts.

In other words, the phage infection is self-limiting. Once the bacterial hosts are gone, the phages will also die, Leverentz explains.

So far, Leverentz and Conway have focused on identifying limitations to their strategy. For instance, phage disinfection doesnt work well on acidic foods, such as apples or, presumably, tomatoes. However, the scientists note, for items with a more neutral pH, such as melon and cucumber slices, phage disinfection outperforms such commercially important sanitizers as chlorine.

In one experiment, the ARS researchers inoculated melon slices with a million Salmonella and then spritzed on a flavorfree solution containing 100 million Salmonella-targeting phages. After waiting 5 days, the scientists counted the surviving bacteria.

Salmonella that had spent the 5 days incubating with the viruses at 40 to 50F–refrigeration temperatures–died in large numbers. Final populations were less than a thousandth of the starting count–or on the order of about 1,000 germs. Thats about 10 times more germ-killing potency than whats achieved with commercially available products today, Leverentz says. Bacteria that had been incubated with the phages at room temperature, about 68F, however, were reduced only to a hundredth of their starting numbers–which means perhaps 10,000 bacteria survived.

By contrast, when the same test was conducted with an acidic fruit–apple slices–the phages provided no significant drop in Salmonella.

In January, the U.S. Department of Agriculture applied for a patent to sanitize plant products with phages. Under a Cooperative Research and Development Agreement–or CRADA–with USDA, the Baltimore-based company Intralytix is supplying the phages for the trials. That company has been gaining renown for pioneering exploitation of phages to quash a host of food poisoning agents in other venues, such as meats (SN: 6/3/00, p. 358).

USDAs new research on phages to disinfect fruits and vegetables already shows great promise, Leverentz says. As part of its CRADA, Intralytix will get first rights to commercialize the technology for this application.

. . .fine tuning the delivery strategy

Phages tend to be fairly specialized in their choice of host. Those that wallop Listeria, for instance, may do nothing to Salmonella or Campylobacter. In fact, one type of phage wont even work against all Salmonella. Thats why the scientists are cooking up recipes for food-protecting cocktails of different phages.

However, this narrow host specificity of each phage means that to employ them effectively, users will have to have a pretty good idea ahead of time as to which germs will likely be present in a food to be sprayed.

Moreover, Leverentz says, theres always the possibility that various pathogens will become resistant to a particular phage. And thats one reason that we use a cocktail of different ones, she says–because its unlikely the bugs will quickly develop resistance to more than one phage. Another strategy, she says, is to vary the recipe of the phage cocktail–to add another phage or take one out, in order to limit the development of resistance.

However, the safest approach, she says, may be to pair this germ-killing strategy with another so that one technique can mop up the survivors left by the other. One possible tool: bacteriocins–natural, bacteria-derived, narrow-spectrum antibiotics. Indeed, efforts are already underway to incorporate these into food wraps (see Germ-killing plastic wrap at Germ-killing plastic wrap).

Leverentz says that her team hopes to identify a line of products that might be used wherever plant-based foods are prepared–in a manufacturing plant, a restaurant, or even the home kitchen. Indeed, the lack of any flavor in phage mists might even give them an advantage over alternatives–such as vinegar mists or chlorine baths–for sanitizing salad greens.

But thats not the end of the story. Leverentz says that her preliminary tests indicate phages may even find use one day in sanitizing counters, cutting boards, and kitchen utensils.

And about those acidic foods? Her team is working on identifying hardier strains of phages that might quash them.