Yale researchers now have insights into how the bacterium that is the leading cause of food-borne illness in the United States enters cells of the gut and avoids detection and destruction.
The scientists presented their findings at the annual meeting of the American Society for Cell Biology in San Diego in December.
Scientists are just beginning to answer basic questions about how Campylobacter jejuni (campylobacter) causes infection. "Campylobacteriosis" is one of the most common causes of diarrhea worldwide, and the U.S. Centers for Disease Control and Prevention estimates that it strikes 2.4 million Americans a year. Most sufferers recover after a few unpleasant days, but the condition can be life-threatening to those with compromised immune systems, such as individuals with AIDS. In addition, a rare but serious complication of campylobacter infection can lead to the autoimmune disorder Guillain-Barré paralysis.
One source of campylobacter is chicken; in fact, a recent analysis by Consumer Reports showed that 80% of fresh, marketed chicken nationwide harbored the bacterium.
Robert Watson, a graduate student in the Section of Microbial Pathogenesis at the Yale School of Medicine, has worked out a better way to study the bacteria and reports that it takes an uncommon path as it infects cells.
Since the intestinal lining cells that campylobacter infects do not normally take up bacteria -- or any particles as large as bacteria -- Watson and his adviser, Jorge Galán, the Lucille P. Markey Professor of Microbiology and Cell Biology, set out to investigate the path of infection through cells.
They found that campylobacter apparently enters into the endocytic pathway that cells use to recycle molecules from their surface. It then quickly diverts its path, creating its own intracellular network of campylobacter-filled vacuoles, or cellular pockets, that make their way toward the nucleus, and finally locate near the cell's transportation hub, the Golgi apparatus.
"It's been known for over two decades that campylobacter can enter intestinal epithelial cells -- but until now no one could show how it was taken up or where it localized. That suggested it had evolved a special mechanism for uptake," says Watson. "Campylobacter seems to have found a special access to these cells and established its own intracellular niche."
Usually, material entering the cell moves to compartments called lysosomes, where an acidic mix of enzymes breaks it down. By monitoring markers for this entry pathway, Watson and Galán could watch as the microbe infected a host cell, briefly associated with the early marker protein EEA-1, and then with the late marker Lamp-1.
"Although the marker proteins indicated that campylobacter trafficked to conventional lysosomes, information from traceable dyes indicated something different," says Watson. While the dyes passed through the endocytic pathway and localized with other material in lysosomes, surprisingly, the dyes did not enter the vacuoles containing campylobacter -- these bacteria had left the conventional pathway.
Watson and Galán also looked at the roles of two Rab GTPases, proteins involved in the maturation of the recycling compartments. These and other experiments gave additional evidence that campylobacter leaves the normal endocytic pathway early and that the separated campylobacter vacuoles move near to the nucleus where they become closely associated with the Golgi apparatus.
"Seeing the path these bacteria follow gives us new perspective for understanding infection and devising ways to combat it," says Galán.
As the next step in understanding campylobacter, Watson and Galán are continuing and expanding the work to include studies in special strains of mice that are infected by and harbor the bacteria but do not show the acute symptoms of infection.
The Ellison Medical Foundation funded this research.
-- By Janet Rettig Emanuel
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