".....  flagellin may act as a basic signal to alert the immune system to the
presence of bacteria....."

 Report:
Salmonella's dirty tricks
Investigator: Andrew Gewirtz

Monday Dec 11th, 2000

by Mari N. Jensen

Salmonella typhimurium bacteria cause diarrhea using a novel mechanism: the
bacterial protein flagellin finds its way through the first layer of the
intestinal cells lining the gut, where the body's response mechanisms kicks in
to try and rid the foreigner. The finding, due to be published early next year
and reported here today, is the first report of any bacteria actively
translocating a protein across an entire cell layer.
Although researchers knew that S. typhimurium bacteria bind to cells in the
inner lining of the intestine, no one knew how the bacteria induce the
gut-cleansing diarrhea all too familiar to anyone who has ever had Salmonella
cooking.net">food poisoning.

"The benefit to the bacteria is that [the diarrhea] aids their spread," says
biochemist Andrew Gewirtz of Emory University in Atlanta. "The benefit to the
host is that, most of the time, it clears the infection."

The body's basic response was already known: when S. typhimurium bacteria bind
to surface of intestinal epithelial cells, there's an inflammatory response,
mediated by the cytokine interleukin-8 (IL-8), which causes diarrhea.

When the team looked closer, they found that only certain cells had bacteria
bound to them, yet all the cells had NK-kappa beta, the factor that induces
secretion of the diarrhea-causing IL-8. Somehow, the bacteria seemed to cause a
generalized response from all the cells in the inner lining, rather than act on
just the cells to which they bound. So the scientists hypothesized the bacteria
elicited the secretion of a substance that stimulated all the cells. That was
indeed the case, but it doesn't happen quite the way the researchers expected.
"We thought we would find something made by the host. Instead we found
something made by the bacteria that's translocated across the epithelium," says
Gerwirtz.

The researchers used a polarized tissue culture system, one in which they had
access to both sides of the cells, to test their hypothesis. When S.
typhimurium bound to the side of the cells exposed in the gut, a factor
appearing on the body, or basolateral, side of the cells was sufficient to
stimulate IL-8 secretion.

When they analyzed fluid in the body-side of the cell, they found the presence
of flagellin. Many bacteria, including the ubiquitous Escherichia coli and many
types of "good" bacteria, produce and secrete flagellin, a protein that makes
up bacterial flagella. As the gut is full of bacteria, intestinal epithelial
cells are constantly exposed to flagellin to no ill effect.

Next, the team tested how their culture system responded to S. typhimurium
missing the flagellin gene. They cells didn't secrete IL-8.

When they injected flagellin from other bacteria into the far-gut side, IL-8
was secreted. Yet putting E. coli on the gut-side did nothing.

The team concluded that the bacterial protein flagellin induces nausea by
traveling all the way through the first layer of endothelial cells in the gut
to enter the body's inner domain.

Their next step is to figure out how the flagellin gets across the epithelium
and what flagellin does once it gets there. And no one yet knows how this type
of salmonella bacteria allows flagellin to break across the barrier when
flagellin secreted from other bacteria do not.

The work may also shed light on Crohn's disease, a chronic disease marked by
inflammation of the intestines and diarrhea, Gerwitz says. It may be that in
such patients, the intestinal epithelium is "leaky" and does not keep out the
gut contents, thereby exposing inner cell layers to flagellin. Another
possibility is that the epithelium is interacting in an inappropriate manner
with E. coli, a bacteria commonly present in the gut, he adds.

"It's a very intriguing result," says microbiologist Jorge Galan of Yale
University School of Medicine. Since flagellin is conserved in many
microorganisms, Galan adds that flagellin may act as a basic signal to alert
the immune system to the presence of bacteria.