Salmonella attacked on two fronts

By LIDIA WASOWICZ, UPI Senior Science Writer  |  Oct. 31, 2001 at 8:58 PM
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Researchers working with Salmonella bacteria have uncovered details of a delivery system that transports damaging chemicals into cells, causing disease.

The discovery may lead to new drug designs for a variety of disorders, including fatal food poisoning and bubonic plague, they said.

A second team working with a Salmonella strain that causes gastrointestinal misery has devised a gene-based method that cuts the time required for bacterial detection from two weeks to two hours.

In the first study, reported in the British journal Nature, C. Erec Stebbins and Jorge Galán of the Yale School of Medicine in New Haven, Conn., and their colleagues noted broad potential application from their discovery since numerous disease-causing bacteria use the same means of transporting their damaging cargo.

This so-called type III protein-secretion system involves customized "chaperone" molecules, which hook up with the toxic chemicals, called virulence effector proteins, ensuring their effective movement.

Gleaning a peek into one such interaction, Stebbins, Galán and company shed light on one of the first steps taken by bacteria on the attack -- the injection of the offending proteins into the cells of the organism under siege.

"Many bacterial pathogens use a complex secretion system to inject proteins, known as effectors, into target host cells," said molecular biologists Craig Smith and Scott Hultgren of the Washington University School of Medicine in St. Louis, who analyzed the significance and ramifications of the findings in an accompanying News and Views article.

"This process is part of a molecular subversion tactic that allows bacteria to persist and cause disease and requires specific molecular chaperones. Our understanding of how these chaperones work is boosted by" the new report on a system used by Salmonella.

The bacteria can cause a variety of disorders, from typhoid fever, which infects 17 million and kills 600,000 people around the world each year, to food poisoning, which annually strikes 1.4 million Americans -- 1,000 of them fatally.

Potential applications could extend to a variety of diseases, including bubonic plague and a host of potentially fatal gastrointestinal conditions, investigators told UPI.

"Our study sheds light on a fundamental aspect of the mechanism of action" used by Salmonella and numerous other bacteria to disarm and take over unsuspecting cells, Galán, Lucille P. Markey Professor of Microbiology and chairman of the Section of Microbial Pathogenesis at Yale, told UPI.

The mechanism involves a needle-like surface appendage produced by Salmonella and other bacteria to gain entry into a cell. Once inside, the bacterial proteins wreak havoc with the cell's normal machinery, manipulating it to their own advantage. The chaperone molecule is needed to guide the protein directly to its intended target. The new findings elucidate this role.

The crystal structure reveals how the Salmonella-secreted protein, called SptP, links up with its chaperone, which, as its name implies, has the task of preventing its charge from making unacceptable liaisons and encouraging it to encounter and interact with acceptable partners, Smith and Hultgren explained. The kidney-bean-shaped chaperone, called SicP, latches onto SptP, like two Lego pieces snapping together, to direct its precise movement in the cell invasion.

Knowing the pathway taken by the proteins on their health-destroying way could help scientists design specific drugs aimed at putting up roadblocks to obstruct progression of the disease, scientists told UPI.

"Although it is too early to speculate, it is likely that other ... organisms that have this system will use a similar mechanism to inject the effector proteins into their hosts," Smith told UPI. "Thus, it may be possible to design drugs or other therapies to help combat diseases caused by Salmonella (typhoid fever, food poisoning) as well as other diseases such as bubonic plague (Yersinia) and Chlamydia," the most common sexually transmitted disease.

Other related pathogens include Shigella, of which there are 30 types that can lead to bloody diarrhea, fever, cramps, chills and vomiting, and Escherichia coli, which since first coming to infamy in 1982 during an outbreak of tainted hamburger meat has triggered hundreds of thousands of cases of painful, bloody diarrhea, fatal kidney failure and food poisoning, many of them among the very young and the very old.

The next step is to use the information from the new study to design compounds that can specifically interfere with the chaperone function, Galán said, noting that may still be years away.

"Our ultimate objective is to understand the (transport) system so that we can develop drugs to inhibit its function and interfere with disease," Galán said, noting the findings may also pave the way for creation of new vaccines.

In the second study, reported in the journal Applied & Environmental Microbiology, biomedical scientists from the Lawrence Livermore National Laboratory describe a rapid new detection technique for Salmonella.

Development of the DNA-based system culminates a year of research by Peter Agron, Gary Andersen and colleagues from Livermore and the California Animal Health and Food Safety laboratories in Davis and San Bernardino, Calif.

First, the team identified several potential unique strands of DNA in the Salmonella enteritidis strain, familiar to anyone who has suffered the unpleasant consequences of consuming tainted raw eggs, whether in a Caesar salad or eggnog. Next, the scientists compared the DNA sections with the genetic blueprints of other closely related Salmonella microbes to ensure they were unique to the enteritidis strain.

"It is difficult to distinguish this pathogenic Salmonella from all of the other Salmonella strains that do not cause disease and are not a problem" since neither chickens nor eggs infested with the bacteria show any symptoms, Andersen said.

For the past six months, the system has been undergoing tests that compare it to the slower, more traditional microbiological methods of bacterial detection, said Richard Walker, professor of clinical microbiology at UC Davis and the Davis lab.

"So far, the laboratory's DNA signatures look very good; they're promising," he said. "The primers (DNA copies) are very specific and provide tools that can speed up testing. They're very useful in screening out negative samples."

If the technique stands up to further examination, it could eventually be used to test the health and safety of animals and products, scientists said.

The key advantage of the approach rests in its speed -- two hours versus two days to two weeks needed for the traditional microbiological methods. Currently, a sample is incubated in a broth overnight, grown in a lab dish for another night and then inspected for any suspicious colonies, which may require additional analysis.

Agron and Andersen designed copies of short unique DNA regions of S. enteritidis that can be used in a specialized method called the polymerase chain reaction, in which the primers seek out their own kind. If any are present in a sample, the primers make contact, creating billions of copies -- and a positive test result. If not, no binding and a negative reading result.

The speedy precision of the DNA signature system could help to significantly reduce the number of cases of S. enteritidis and to track the source of the bacterial contamination, whether it be feed, water, manure pile or processing equipment, Andersen said.

"This technology could be used widely in the future," he said. "I think the day will come when virtually every poultry farm is analyzed for the presence of Salmonella enteritidis."

The study was funded in part by the California Egg Commission.

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