Research: Salmonella Promote Host Survival During Infection
Loss of appetite is a common response to gastrointestinal infections, but not too much is understood about the function of this response in our ability to fight off infection. Where at times eating less may encourage faster recovery, sometimes loss of appetite can be harmful—an example of this would be when HIV-positive patients or cancer patients develop wasting.
Recently, researchers from the Salk Institute made a surprising discovery regarding the link between appetite and infection; when it comes to Salmonella Typhimurium, the bacteria will actually block the body’s appetite loss response, thus encouraging survival of the host so that it can increase its own transmission to other hosts. Their findings, published in the journal Cell, could have a number of implications when it comes to treating infectious diseases.
According to Janelle Ayres, PhD, assistant professor at Salk Institute’s Nomis Foundation Laboratories for Immunobiology and Microbial Pathogenesis, the main goal of the study was to understand the role of sickness-induced behaviors—such as anorexia, loss of appetite, disruption in sleep—in how the body fights off infection.
To do this, Dr. Ayres and her team infected mice with the bacteria Salmonella Typhimurium. They did this because appetite loss is a common response in mice who are infected with the bacteria; as the bacteria becomes more virulent, spreading from the intestines to other organs in the body, the mice will become sicker. However, by feeding the sick mice, despite their loss of appetite, they found that the mice were able to survive longer. In a video explaining the research, Dr. Ayres said, “If we fed [the] animals, so that we could override the anorexic response, the animals actually survived the infection. If we nutritionally-restricted [the] animals during infection, they actually fared worse.”
However, their survival was not based on a stronger immune response afforded by the nutrients supplied by the food. According to Dr. Ayres, “What was really surprising to us was that it wasn’t the host defense response to the infection that was changing. What we found was the strategy that the Salmonella actually employs during the infection to block this anorexic response, to actually prevent it from causing disease in the host—so, it was promoting survival of the host during the infection.”
This discovery raised a number of questions: Why was the bacteria becoming less virulent—not spreading to other organs? Why was Salmonella working to keep the host healthy? The researchers found that even though the bacteria were becoming less virulent, they were working to increase transmission to multiple new hosts.
In the press release, first author Sheila Rao, a Salk research associate, explained, “What we found was that appetite loss makes the Salmonella more virulent, perhaps because it needs to go beyond the intestines to find nutrients for itself. This increased virulence kills its host too fast, which compromises the bacteria’s ability to spread to new hosts. The tradeoff between transmission and virulence has not been appreciated before—it was previously thought that virulence and transmission were coupled.”