COLLEGE STATION—Ever the cagey foe, Salmonella has been around for millions of years and has managed to beat thousands of attempts to eradicate it. An amazingly “smart” and resilient germ but always totally ruthless, Salmonella and its more than 2,500 different strains are always looking for new ways to survive, but researchers from the University of California-Davis and Texas A&M University believe they have opened doors for new ways to fight it.
Garry Adams, professor in Texas A&M’s College of Veterinary Medicine & Biomedical Sciences, along with University of California-Davis colleagues including Andreas Baumler and Sebastian Winter, among others, are examining ways that Salmonella exploits a host organism and how it gains entry into humans and animals and finds ways to continue its survival and its transmission into other hosts, much the way it has done for millions of years.
Their work is published in Nature magazine, and the project is funded by the National Institute of Allergy & Infectious Disease, part of the National Institutes of Health.
When people eat an undercooked burger or egg, it can contain thousands of Salmonella bacteria. But to live, its cells must compete with beneficial bacteria that are also living in the same area since the humans host’s immune responses often kill the intruders.
“But this process irritates and inflames the host intestinal tissue and causes it to produce highly reactive oxygen radicals that are meant to kill bacteria,” Adams explains.
“These radicals form a chemical in our intestines called tetrathionate (pronounced tetra-thyo-nate) and this allows Salmonella to ‘breathe’ within the intestinal area.”
Baumler, professor of medical microbiology and immunology at the UC Davis School of Medicine and principal investigator of the study, says, “The human body normally has 10 times more microbes than human cells that help protect us against infection from disease-causing bacteria. We have discovered Salmonella’s cunning trick that allows it to quickly take over and outgrow the beneficial microbes in our intestine.”
Stimulating the host to produce tetrathionate enables Salmonella to “breathe” in the intestine, said Sebastian E. Winter, who is a member of Baumler’s laboratory and lead author of the article. “This gives Salmonella a tremendous advantage over the gut bacteria that must grow by fermentation.”
In short, Adams points out, since Salmonella is able to use the host to breathe in the intestine, it can quickly spread to cause disease in the infected host and spread through diarrhea fluids to its next victim.
Adams says the presence of tetrathionate was discovered in the 1920s, but since the substance was not originally found in the intestine, it was assumed the process had no connection with food poisoning.
Adams says Salmonella is incredibly tough—it cannot be destroyed by freezing, and it can survive for weeks outside of a living body and for months in polluted water. But fortunately, he says, using a food thermometer and properly cooking to 172 degrees destroys the pathogen.
Salmonella is frequently in the news as a source of food poisoning outbreaks, usually from eating poorly cooked or unhygienically prepared eggs or meat. Salmonella was the cause of a recall of about half a billion eggs in August and sickened more than 1,500 people. In that case, the ovaries of the hens were contaminated, so the inside of the eggs carried the bacteria and were not safe to eat unless thoroughly cooked, note the UC Davis researchers.
Reptiles such as turtles, lizards and snakes also carry the bacteria on their skin, sometimes causing illness in people who keep them as pets.
Salmonella infection, known as salmonellosis, causes diarrhea, fever, vomiting and abdominal cramps. Although most people recover after several days, it can be fatal, especially in the elderly, infants and people with impaired immune systems.
Since Salmonella affects both humans and animals, it is termed a zoonotic disease. The Center for Disease Control (CDC) estimates that 1.4 million salmonella cases occur annually, with about 600 deaths caused by salmonella infections in the U.S. every year, accounting for 31 percent of all food-related deaths.
“Now that we know how it manages to survive, perhaps we can develop better strategies to fight it,” Adams notes. “It lives off its hosts amazingly well, despite a tough war between it and the host it wants to invade. So the more we know how it lives and thrives, the better chance we have to control it.”
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