There are some “stranger than fiction” accounts of how microbes can impact animal behavior. For example, back in 2007, Vyas and colleagues reported their observations on behavior of rats infected with the protozoan parasite, Toxoplasma gondii. It turns out that although this parasite can infect rats, it needs to replicate in cats. And it seems to have figured out a way to increase the odds of getting into the cat gut. Researchers found that when a rat is infected with this parasite, it loses its natural aversion to cat urine. Instead of avoiding areas where cats abound, these rats actually move towards them. This increases the likelihood that the rat will be eaten by the cat and that Toxoplasma will reach the cat gut, where it can replicate. In short, a colonizing microbe changed the behavior of its host.
When researchers showed several years back that probiotics could help reduced abdominal pain in IBS subjects, one hypothesis is that the probiotic somehow reduces the perception of pain by the host. Researchers are uncovering how gut microbes may be important to brain function.
Two researchers, John Cryan and Ted Dinan, at the University College Cork in Ireland recently published an excellent review, “Mind-altering microorganisms: the impact of the gut microbiota on brain and behaviour.” Part of this paper summarizes the current state of knowledge of how probiotics mediate brain activity in both animal models and human studies. Some evidence from human interventional trials supports a role of probiotics in reducing anxiety, decreasing stress responses and improving mood in individuals with IBS and with chronic fatigue. The authors state, “…the gut–brain axis provides a bidirectional homeostatic route of communication that uses neural, hormonal and immunological routes, and that dysfunction of this axis can have pathophysiological consequences.” Mechanisms for how colonizing or probiotic microbes might affect function of the central nervous system are summarized.
Another gut-brain researcher, Emeran Mayer, along with his colleagues at UCLA, showed that consuming probiotic yogurt (containing a live Bifidobacterium animalis subsp. lactis strain, along with yogurt cultures) for 4 weeks decreased connectivity of an amygdala-centered network with the insula, dorsal striatum and lateral prefrontal cortex. These effects were observed through functional MRI imaging while subjects conducted an Emotional Reactivity Task (viewing negative facial images). This study (Tillisch et al. 2013) was a three-arm, double-blind, placebo-controlled trial of healthy women, and was reported at Digestive Disease Weekly meeting in May of 2012. It was the first study to use brain imaging to demonstrate probiotic-induced changes in brain function.
As researchers clarify the role that gut microbes play in modifying brain function and behavior, the next step is figuring out how microbes can be manipulated – by probiotics and other strategies – to correct microbe-associated aberrancies of the central nervous system.
Stay tuned! This is going to get interesting.
Vyas A, Kim SK, Giacomini N, Boothroyd JC, Sapolsky RM. Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors. Proc Natl Acad Sci U S A. 2007 Apr 10;104(15):6442-7.
Cryan JF, Dinan TG. Mind-altering microorganisms: the impact of the gut microbiota on brain and behaviour. Nat Rev Neurosci. 2012 Oct;13(10):701-12.
Tillisch K, Labus J, Kilpatrick L, Jiang Z, Stains J, Ebrat B, Guyonnet D, Legrain-Raspaud S, Trotin B, Naliboff B, Mayer EA. 2013. Consumption of fermented milk product with probiotic modulates brain activity. Gastroenterology. 144(7):1394-401.