New Salmonella
Finding—
Inter-Bacterial Communication!
Salmonella enteritidis. (K8732-2)
Talking bacteria may sound bizarre,
but microorganisms do have a primitive form of communication--much as some
insects use pheromones to lure a mate or signal an attack. An
Agricultural Research Service scientist
found this happens with Salmonellabacteria, and her discovery could
have big implications for food safety.
Veterinarian Jean Guard-Petter, who is in ARS' Southeast Poultry Research Unit
at Athens, Georgia, found that the food pathogen Salmonella
enteritidis uses acyl-homoserine lactone, or AHL, as its chemical
"call to arms."
"This chemical tells cells to rewrite their genetic programming,"
says Petter. "It enhances their ability to grow as much as a hundredfold
and signals cells to produce molecules that increase virulence—the ability
to invade living things and cause disease."
Over the last 15 years, occurrences of S. enteritidis food poisoning
have increased fourfold in the United States and fortyfold in Europe.
This large bacterial colony of Salmonella
enteritidis grew rapidly (62 millimeters
in diameter in 16 hours) and readily contaminated eggs when given to chickens
by injection but not when given by mouth. (K8732-1)
Petter's findings are a first look
at how a major foodborne pathogen couples heightened growth potential with
virulence factors to maintain itself as a food-safety threat.
In an animal host, the bacteria swim along, putting out low levels of AHL,
Petter explains. But once there are a lot of bacteria concentrated in a
confined space—like the spleen of a chicken—the chemical builds up
and signals an aggressive attack, so the next battlefront—usually a hen's
eggs—can be occupied.
Salmonella enteritidis in eggs can be a big problem. The infected hens
don't appear sick, so farmers won't know anything is wrong. The contaminated
eggs, however, can make people seriously ill.
Fortunately, the new USDA Hazard Analysis and Critical Control Points (HACCP)
inspection system uses sensitive detection techniques to keep pathogen numbers
as low as possible.
Petter was the first to see that AHL is a factor in this bacterium's rapid
growth and spread. It was J. Woodland (Woody) Hastings at Harvard University
who, in the late 1960s, discovered that microorganisms can communicate
chemically. Scientists call the phenomenon quorum-sensing.
Petter made her discovery by using a special plasmid reporter system developed
at the University of Nottingham in England. This reporting system puts genes in
S. enteritidis bacteria that cause them to emit light if they are
producing AHL. The theory was that as the light came on and got brighter,
growth to a high cell density would happen at the same time.
The samples showed this was happening. Now, Petter is working with ARS
microbiologist Amy Charkowski in Albany, California, to understand the chemical
structure of AHL.
Petter theorizes that egg contamination is a cooperative effort by two forms of
S. enteritidis produced by certain strains. The first are "scout
cells" that are ingested by hens and move from the gut to the spleen.
Scout cells can't infect eggs any better than common Salmonella
strains. But they blaze a trail for a second type of the bacterium that is very
good at producing AHL and growing to unexpectedly high levels within the body
of the bird.—By Jill Lee, formerly with ARS.
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/programs/appvs.htm.
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