These germs target only other microbes, ones that make people sick
CHICAGO Adding viruses to foods doesn’t sound appetizing, much less healthy. But it’s a stratagem being explored to knock some of the more virulent food poisoning bacteria out of the U.S. food supply. Scientists described data supporting the tactic July 18 at the Institute of Food Technologists’ annual meeting in Chicago.
Every year, some 76 million food poisoning cases occur throughout the United States. Most people suffer major distress but recover fine. Roughly 375,000 Americans do become hospitalized, however, and roughly 5,000 die. Since bacteria play a major role in provoking these gut wrenching illnesses – remember the E.coli O157:H7 outbreak affecting spinach four years ago – the food industry is looking for new ammo to protect its products and reputation.
A whole session of the IFT meeting was devoted to mushrooming interest in bacteriophages – viruses that quash bacteria. Phages are very discriminating. Each seeks out a particular bacterial species and largely ignores the rest.
These viruses also eschew larger beings, like plants, fish, birds and mammals. Indeed, it’s this exquisite selectivity that makes them so appealing to food scientists. Find the right phage, and it will knock out the food poisoner of concern. And nothing else.
Some phages have already been granted federal approval for use on foods; approval for others is still pending, noted Lawrence Goodridge of Colorado State University in Fort Collins.
He described experiments that showed spraying cattle a few hours prior to slaughter could reduce by roughly 90 percent the pathogens present on the hide of those animals over the next 90 minutes. (In these tests, the bacteria had been added in known amounts prior to treatment.) University of Florida experiments on tomatoes suffering from a blight in the field due to a test inoculation with Xanthomonas not only slowed the spread of lesions to other plants, Goodridge said, but also improved crop yields by preventing vigor-robbing low-level disease.
Encouraging but no panacea
The Food and Drug Administration has approved phages for use in foods – but only against Listeria monocytogenes, notes food microbiologist Ipek Goktepe of North Carolina Agricultural and Technical State University in Greensboro. And these products find use primarily on meat and deli products, she added. But that’s not bad, since Listeria is one of the most common food-poisoning agents. And a particularly recalcitrant one since it happily grows at refrigerated temperatures.
Goktepe reported new data showing that Listeria phages aren’t uniformly effective in protecting every contaminated food to which they’re applied. Food producers would like to see at least a “4 log” reduction in bacteria – that is, a reduction to one ten-thousandth of the starting population of bugs. In some of Goktepe’s tests, she may see a three log reduction or less.
One recent success: An E. coli phage that targets O157:H7 strains killed a huge share of these bacterial cells that had been growing on loose lettuce and spinach leaves. When the phages were applied in a moist mist, Goktepe says, “We achieved a 3 to 7 log reduction – and that’s a lot. We were not expecting that,” she says. “Usually a 4 log reduction is considered very significant.”
She cautions that the big bacterial drop occurred under fairly ideal conditions, such as at 3 °C, a good refrigerator's temperature. Raise the leafy greens’ temperature to 10 °C (about 50 °F) and the phage delivered only a 2 to 5 log drop in E. coli numbers.
Some phages prey on Salmonella. But to date, Goktepe says, most phage studies haven’t yielded much success in quashing populations of these bacteria. So viral protection from this major pathogen remains a challenge. This, as in many instances, may reflect trouble matching the right virus to the bacterium. Select the wrong phage and the virus will die of hunger as the food-poisoning agent prospers.
Such observations suggest, Goktepe says, that because farmers or food manufacturers are unlikely to be able to predict which strain of bacterium stands poised to afflict their crops or product, effective treatment may require the development of viral cocktails containing many phages. Indeed, her lab is actually interested in developing a supercocktail mixed from phages specific to a medley of pathogens – from E. coli to Salmonella.