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.”
