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Phage Eyed as Agents To
Control Foodborne Pathogens
David Holzman, ASM News, October, 2003; David Holzman writes
from Lexington, Mass.
A bacteriophage and the bacteriocin nisin effectively can
control Listeria monocytogenes on contaminated melons and
apples, at least under defined lab conditions, according to
Britta Leverentz of the U.S. Department of Agriculture (USDA)
and her collaborators, who report their findings in the August
2003 Applied and Environmental Microbiology (69:4519-4526). In
another report in that same issue, P. A. Barrow of Compton
Laboratory, Newbury, Berkshire, United Kingdom, and
collaborators report that phage also can be effective in
reducing the numbers of other foodborne pathogens, including
Salmonella enterica serovar Enteritidis and Campylobacter jejuni,
found on chicken skin (Appl. Environ. Microbiol. 69:5032-5036).
L. monocytogenes, a foodborne pathogen, is fatal in
approximately 20% of cases, and USDA policy prescribes zero
tolerance for it on ready-to-eat foods. Although L.
monocytogenes is commonly associated with meat, salad vegetables
are also implicated in outbreaks caused by this pathogen. It is
especially hazardous because it can grow on foods stored at 4°C.
C. jejuni infections usually are self-limiting, and rarely fatal
in healthy individuals, but this pathogen is considered the
leading cause of bacterial diarrhea in the United States.
Salmonella accounts for an estimated 800,000 to 4 million cases
of food poisoning annually, and leads to hundreds of fatalities.
Because S. enteritidis infections are attributed primarily to
contaminated eggs, the Food and Drug Administration requires
shell eggs that are not treated to kill Salmonella to be stored
at 7.2°C or lower. Phage have several advantages over other
means for decontaminating foods that carry microbial pathogens—a
chief one being their specificity in killing a single bacterial
species without upsetting the balance of those that remain, says
Alexander Sulakvelidze of the University of Maryland, Baltimore,
and Intralytix, Inc., a biotechnology company, who is
collaborating with Leverentz at USDA. Phage use also avoids
adding chemicals that might have other health effects (see ASM
News, November, 1998, pages 620 and 622). Phage treatments
reduce contamination by specific bacteria on fresh-cut fruit to
an extent that is “greater than those achieved with aqueous
chemical sanitizers or by washing with water,” report Leverentz
and her collaborators. “The phage-nisin combination was more
effective than the nisin treatment alone, especially after
longer storage periods.” Treatments with phage alone or in
combination with nisin reduce bacterial numbers on cut honeydew
by more than four orders of magnitude after seven days of
storage at 10°C. Treating cut apples reduces bacterial numbers
by about two orders of magnitude. Adding nisin to the mix
reduces the likelihood of bacteria developing resistance to such
treatments, according to Leverentz and her collaborators. Nisin,
which is produced by lactic acid bacteria, is an “approved
preservative in 50 countries and the only purified bacteriocin
that is commercially available.” Moreover, although bacteria may
eventually develop resistance to a particular phage, phage
variants are so abundant that alternatives surely could be
found. Researchers foresee a wide variety of additional
applications in and outside the food industry for phage. They
could be used to clean contaminated processing plants,
Sulakvelidze says. “There are places where you cannot apply
chemical sanitizers because it might get onto the food.” Another
possible application is for degrading biofilms on anything from
simple inanimate surfaces to those found within the lungs of
cystic fibrosis patients, he adds. “There have been limited
studies showing that phage can degrade biofilms.” One means for
doing so involves a phage-encoded exopolysaccharide-degrading
enzyme that might be used by itself or in combination with
phage, he points out. “I think citrus canker is a very good
candidate,” says Joshua Lederberg of Rockefeller University in
New York, N.Y., referring to yet another potential application
for phage—in this case, to treat a rampant disease affecting
citrus trees in Florida. “Quite a few pounds of gentimycin are
being sprayed into the environment as an alternative.”
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