Feb. 5, 1999 CHAMPAIGN, Ill. -- A gene that dictates salmonella's ability to live dormantly or cause disease in pigs has been found by researchers at the University of Illinois. In the laboratory, the scientists even fooled the bacteria in one strain into switching back and forth between these two forms.
The finding, says Richard E. Isaacson, a professor of veterinary pathobiology, is the first documentation of a phase-shifting process in salmonella and in any food-borne bacteria that is related to its ability to grow in specific environments. Finding the controlling switch, he said, could pave the way for developing methods to rid the disease-causing form from farm animals.
Salmonella attacks the stomach and intestines in animals and humans, and is a leading cause of gastrointestinal infections. Some 2,000 strains have been identified. Until now, scientists have not been able to explain why salmonella is often found in apparently healthy animals.
"We think that the reason that animals can appear healthy but go on with long-term infection is because the bacteria switches back and forth between these two forms, so that it has just the right combination of the right kind of cells to survive and linger there but not cause disease," Isaacson said.
Isaacson and graduate student Lola Y. Kwan, now at Northwestern University, reported their finding in the December issue of the journal Infection and Immunity. The work -- funded by the U.S. Department of Agriculture -- points to a sugar-containing substance known as O-antigen that sits on the outside of the bacterial cell. When produced inside an animal, salmonella cells become less visible to the immune system and cause illness as levels increase. The other version, normally found in the environment, lacks O-antigen and is easily removed when detected by the immune system once inside.
"What we have identified is an important gene that is involved in the ability of salmonella to cause disease," Isaacson said. "We now know that this gene can be turned on or off depending on where the organism is growing. What we think is really important is that this gene or a master switch controlling this gene is really a key process in how salmonella makes a living in animals."
Isaacson reported salmonella's two genetic phenotypes in 1992. The new work sought to find the differences by comparing mutants with the two forms of the Salmonella typhimurium strain. Using antibiotic-resistant markers and a detection technique called SDS-polyacrylamine gel electrophoresis, they identified the RfaL gene that is required to produce O-antigen.
U. of I. scientists also have been looking on farms for reservoirs of salmonella. Their results suggest that the prevalance of salmonella in swine herds is high, and that the pig is an important reservoir. "It does look like the most likely reservoir is the pig itself," Isaacson said. "It may be that the only real solution is a biotechnological approach that would trick salmonella into turning itself off so it could be naturally cleared from the pigs."
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