By: Freya Preimesberger
According to a new study recently published, laboratory mice infected by one kind of food poisoning-causing bacteria are under a kind of mind control that prevents loss of appetite. The suppression of anorexia, or lack of appetite, serves to keep the host organism alive longer and promote transmission of the bacteria. Interestingly, researchers believe that this mechanism may be useful for increasing appetite in the elderly or severely ill. The study was published on January 26 in the journal Cell by researchers at the Salk Institute for Biological Studies in San Diego, California.
During infection, many animals exhibit “sickness behaviors,” which include anorexia, decreased sociability, increased sleeping and fever. Many of these adaptive behaviors increase the likelihood of surviving and recovering from illness. Sleeping conserves energy needed for fighting off disease and higher body temperature, which is associated with better prognosis, may boost immune cells. However, how lack of appetite affects recovery is more complex and depends greatly on the disease; not eating may starve microbes of necessary nutrients, or it may weaken the host’s defense mechanisms and induce behavioral changes in the microbes that increase virulence. According to the study, it is important to understand how eating behaviors affect disease outcome when forming treatment plans for patients.
In the study, researchers looked at the relationship between anorexia and the bacteria Salmonella, which causes food poisoning in humans and illness in mice. The bacteria has a range of mechanisms with which to manipulate its host during infection, making it a good candidate for study. The researchers found that the protein small leucine-rich proteoglycan, or SIrp, was associated with how lethal the bacteria was. Mice infected by Salmonella that did not contain SIrp died at higher rates. Additionally, these mice ate about 20 percent less food than other mice. The study suggests that SIrp plays a role in mice’s appetite. In mice infected with Salmonella, the vagus nerve, which extends from the brainstem to the stomach, sends signals to the hypothalamus in the brain to reduce appetite. When the protein SIrP is present in Salmonella, it causes inflammation in the small intestine and blocks this communication from occurring, helping the mice to retain their normal appetites.
This mechanism helps the mice to survive, as well as increase the probability that the bacteria will be transmitted to other mice. Salmonella is more likely to spread from the intestines to other organs in infected mice that don’t eat, killing mice exhibiting anorexia at higher rates. Researchers suggested that a lack of food in the small intestines makes organs outside of the gut more appealing to bacteria in their search for nutrients. SIrP blocking an anorexia-inducing signal makes it more likely that the bacteria will stay in the guts of the mice. In turn, survival of the host increases the probability of pathogen transmission. Mice that live longer will defecate more, and their feces spread bacteria and disease to other mice.
The SIrP mechanism creates trade-offs between bacterial transmission and virulence, according to the study, and the bacteria modify their host’s eating behavior to their advantage. The study describes it as one example of how convoluted adaptive strategies can complicate finding out why a physiological response occurs. The paper examines the use of anorexia from the perspective of the pathogens, rather from the host. While reduced eating proved harmful to the individual host, it prevented the disease from spreading to other individuals.
Many people suffering from illness lose their appetites, resulting in them using up their own fat and muscle stores in a process called wasting. The researchers recommend investigating whether the mechanism found in the study could be used to increase appetite in the chronically ill, or possibly in physiologically treating anorexia nervosa patients. Eating may also be useful in aiding recovery from other types of bacterial infections.
Salmonella is a gram-negative bacteria that causes a form of food poisoning called salmonellosis. It results in approximately 1.2 million cases of infection and 450 deaths each year in the United States, with 95 percent of cases being foodborne. Symptoms usually last four to seven days and include diarrhea, fever and cramps. Although infections typically resolve on their own, some people may suffer from bowel disturbances for several months or develop reactive arthritis. Infections with Salmonella may become invasive and spread to the bloodstream, bones or other organs. Invasive infections only occur in a small portion of all cases of Salmonella infection and is fatal in rare cases. Certain populations, such as children, the elderly and those with compromised immune systems, are more prone to serious infections. In serious cases, physicians will prescribe antibiotics to fight off infection, but treatment can be complicated by antibiotic-resistant microbes. According to the Center for Disease Control, 5 percent of Salmonella species are resistant to multiple antibiotics.
No vaccine for Salmonella-related food poisoning exists, but you can take steps to avoid being infected by the bacteria. Infection is usually due to consuming food that contains feces from infected people or animals, but can also be caused by not washing your hands after touching contaminated animals. Animal products, such as beef, poultry, fish and eggs, should be cooked thoroughly to their “safe” internal temperature; using a food thermometer may be helpful. Produce should be washed and raw milk, which has not undergone pasteurization, should not be consumed. Perishable foods should be refrigerated within two hours of preparing or opening, and they should be thawed and stored properly. During food preparation, care should be taken to ensure that cross-contamination does not occur between uncooked meat and other products. Food surfaces should be washed thoroughly after preparing animal products and between uses. During grocery shopping or when storing food in the refrigerator, animal products should be separated from all other food products to prevent cross-contamination. Hands should be washed with soap and water for at least 20 seconds after contact with animal products. Handwashing is also important after touching reptiles and other animals, which often carry Salmonella. For more information on Salmonella and foodborne disease, call 1-800-CDC-INFO.