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Turning the Phage on
Produce Pathogens
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Cultures from fruit tissue
contaminated with a Salmonella
strain and stored for 5 days at
5° C. The cultures on the
left were treated with phages.
(K9514-1)
|
Even bacteria have their nemesis.
Tiny viruses, called phages, infect and kill bacteria naturally, including the
foodborne pathogens that sometimes make humans so sick, they wish they were
dead.
So why not put these phages to work on fresh-cut fruit, thought
ARS plant pathologists Britta Leverentz
and William S. Conway at the Produce Quality and Safety Laboratory in
Beltsville, Maryland. |
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Cultures from fruit tissue
contaminated with a Salmonella
strain and stored for 5 days at
15° C. Cultures on the left
were treated with phages.
(K9514-3)
|
Since phages home in on a
bacterium's surface proteins, they are very selective about their hosts. Phages
specific for Salmonella, for instance, would leave beneficial bacteria
free to multiply on fresh-cut produce and crowd out potential pathogens,
Leverentz explains.
What's more, these tiny viruses are natural, safe, and ubiquitous. A small
dropperful of fresh water from a stream or lake, for example, contains an
average 250 million phages. Before antibiotics, phages were used to treat human
infections in the United States and are still used therapeutically in other
parts of the world. |
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Cultures from fruit tissue
contaminated with a Salmonella strain
and stored for 5 days at 20° C.
Cultures on the left were
treated with phages.
(K9514-4)
|
Phages are already under study to
control pathogens in poultry, meat, and eggs. Leverentz and Conway are the
first to investigate their potential to reduce pathogens on fruits and
vegetables—both whole and fresh-cut. They are working under a cooperative
research and development agreement with Intralytix in Baltimore, Maryland,
which is providing known phages for Salmonella Enteritidis. A patent
application has been filed on the use of phages with produce.
Human infections with S. Enteritidis usually come from products
containing eggs. But during the last 10 years, various Salmonella
strains, including S. Enteritidis serotypes, have been associated with
contaminated produce. |
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Electron micrograph of a Salmonella
phage isolated from the Inner
Harbor in Baltimore, Maryland.
(K9514-5)
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The peel or rind of intact fruits
provides a physical and chemical barrier to microbes, says Leverentz. However,
the growing market for fresh-cut produce could become another channel for
pathogens to reach humans.
Bacteria multiply rapidly on cut surfaces—especially if those surfaces are
not too acidic and have warmed up to room temperature.
Leverentz chose fresh-cut melons, with low acidity, and apples, with higher
acidity, to test a cocktail of four anti-Salmonella phages under
conditions likely to occur during normal food processing and storage. She
inoculated each slice with 1 million bacteria and 100 million phages.
The results on the melons were very encouraging, she says. The phage cocktail
consistently reduced Salmonella more than a thousandfold on melon chunks
stored at 40°F and 50°F and more than a hundredfold on fruit stored
at 68°F, or room temperature.
But on apples, the phages made no detectable dent in pathogen numbers.
Apparently the fruit's higher acidity reduced phage survival, says Leverentz.
But she and colleagues are already looking for acid-tolerant phages or a way to
buffer the viruses against acid.
Conway says the industry is seeking alternatives to chlorine and other
sanitizers now in use. Bacteria are developing resistance to chlorine, which
can also be irritating to humans. Solutions are often too dilute or too
overused to reduce bacteria more than 10- to 100-fold, he says. That's a far
cry from the industry's goal of 100,000-fold reductions.
Conway suspects the phages may have to be combined with other treatments, such
as natural bacteria-killing compounds, to reach that goal for commercial
use.—By Judy
McBride, Agricultural Research Service Information Staff.
This research is part of Food Safety, an ARS National Program (#108)
described on the World Wide Web at http://www.nps.ars.usda.gov.
Britta Leverentz and
William S. Conway are with the
USDA-ARS Produce
Quality and Safety Laboratory, 10300 Baltimore Blvd., Bldg. 002,
Beltsville, MD 20705-2350; phone (301) 504-6128, fax (301) 504-5107. |
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"Turning the Phage on Produce Pathogens"
was published in the July 2001 issue
of Agricultural Research magazine.
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