Prior posts have noted that abuse and misuse of antibiotics in people and animals has lead to an increase in antibiotic resistant organisms, to the detriment of human health. Prior posts have also noted the importance of bacteria to human health and welfare (and that there are 10 bacteria in the human body for each cell). Although the bacteria are considered important to the digestion of food and the production of important factors that the body cannot produce itself, there has been little consideration given to the importance of friendly bacteria to the immune function.
Using a common animal model, researchers found that mice given antibiotics are not able to fight the flu as well as mice that have not been given the drugs; the researchers have found that antibiotics quash the immune system's infection-fighting power by killing off so-called "friendly" bacteria living in the intestines. These friendly (or "commensal") bacteria help defend against viruses by keeping the immune system primed for viral invaders.
As the researchers noted, it has been assumed that friendly bacteria in the intestines could help stop disease-causing bacteria from becoming established in the gut by outcompeting them for nutrients and location. Previous experiments have given indications that gut microbes could influence how well the immune system works, but researchers believed that the effect was mainly confined to the digestive system. Such a view may have been too narrow.
The lungs are normally sterile, so researchers were surprised that killing bacteria as far away as the colon would have any effect on how well the lungs could fight viruses. The researchers treated mice for a month with four antibiotics commonly given to people with bacterial infections, then infected the rodents with the flu. Antibiotic treatment impaired the mice's ability to make an important flu-fighting molecule known as interleukin-1 beta or IL-1 beta; IL-1 beta is necessary to combat influenza and other viruses. However, it is important to note that the antibiotic-treated mice did not have a weakened immune systems; the antibiotic-treated mice were able to fight herpes, for example, because the immune system fights off herpes and some other viruses using a different molecular weapon. In contrast, intestinal gut bacteria are constantly priming the immune system to make IL-1 beta, keeping the immune system vigilant against the flu and other viruses. The researchers were not able as of yet to specify which bacteria in the intestinal tract are responsible for the virus-defense mechanism, though they were able to identify some that were not responsible for the response.
Some Lactobacillus bacteria are known as "friendly gut bacteria" and may play a role in virus defense. Mice treated with an antibiotic called neomycin, which wipes out most types of Lactobacillus bacteria while leaving Sphingomonas bacteria alone, have a hard time fighting the flu. Not surprisingly the researchers are hoping they can identify which bacteria are responsible for the beneficial effect, and then they might be able to make probiotics that will boost virus-fighting capabilities.
The study can be found at http://www.pnas.org/content/108/13/5354.