Many of our body’s responses to food poisoning and infection are there to help us fight them. Becoming lethargic, for example, may save energy that can then be used by the immune system to attack the pathogen. But another of the common responses is a bit harder to untangle: Why do people have a reduced appetite? A new study has found that it may be down to the bacteria itself altering signals in the brain.
Research suggests that eating less improves the chances of survival in some cases, but this seems counter-intuitive considering it reduces energy intake. One explanation could be that cutting what you eat starves the pathogen itself of energy, weakening it enough to allow the immune system to then overpower it. But in other cases, eating less can cause even more complications for patients, such as those suffering from cancer.
To look into this aspect of infection, researchers decided to investigate how the Salmonella bacteria affects the appetites of mice, and they found something intriguing. When infected, and the mice lost their appetite, the bacteria actually became more virulent, spreading outside of the intestines and into other tissues. But if the appetite suppressed mice were fed extra calories, the bacteria remained in the gut and seemed to prevent further appetite loss in the host, itself.
“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,” explained Sheila Rao, the first author of the study published in Cell. “This increased virulence kills its host too fast, which compromises the bacteria’s ability to spread to new hosts.”
This seems to suggest there is some form of trade-off for the bacteria: Suppress the appetite of the host and spread quickly but potentially kill them, or allow the host to continue eating in which case they will defecate, and increase the bacteria’s own chance in spreading to new hosts. It turns out that Salmonella can shift towards one or the other of these strategies using a particular molecule known as SlrP.
It turns out that this molecule blocks signals in the vagus nerve in the gut, which usually relays information to the brain to tell the body that it should stop eating when infection is detected. The discovery of this fascinating interaction could have profound implications, from being able to manipulate the body to suppress appetite in those suffering from metabolic diseases, to the opposite for those not eating enough, such as those suffering from cancer.
But it could also create a novel new way to fight disease through altering nutrition, rather than using antibiotics, particularly with the steady rise in antibiotic resistance threatening the treatment of common diseases.