Can gut bacteria make zombies?



L. rhamnosus: Not a zombie-
producing bacteria...yet.



If T. gondii weren't enough to worry about, now there is evidence that bacteria in our gut can influence brain function.  Lactobacillus and other probiotic organisms have long been speculated to have beneficial in vivo effects, and are most commonly known for improving the health of the digestive tract.  L. acidophilus is probably the most widely known, since it is used to make yogurt, but there are many other types of lactobacilli with alleged health benefits ranging from lowered cholesterol to improved mood.  Some of these benefits are speculative, and for many years any benefit (such as improved gut health) was thought to be due to local effects or secreted chemicals.  However, the impact of these bacterial colonies may be much more far-reaching.

A Proceeding of the National Academy of Sciences (PNAS) paper published by Heujtz et al. last January (abstract) showed that microbial colonization in the mouse gut led to the activation of signaling pathways involved with motor control and emotional response.  This was the latest in a growing body of literature that suggests gut bacteria could influence how we think and act.  Now, in the August edition of PNAS, Bravo and colleagues take this one step further (abstract).  They show that Lactobacillus rhamnosus can directly influence the expression of GABA receptors in the brain.  GABA is the primary neurotransmitter for regulating many physiological and psychological activities in humans.  For example, caffeine inhibits GABA and results in an overall increase in neurotransmitter activity.  In contrast, alcohol and sedatives tend to increase GABA activity, leading to reduced neurotransmitter activity. Bravo et al. showed that feeding mice L. rhamnosus reduced GABA expression in some areas of the brain, while increasing it in others.  The overall effect was to make the mice more calm.  Here in the Dark Lab, we would test this by subjecting the rats to endless episodes of Jersey Shore, Barney and Friends, and The Jonas Brothers, and then asking how long it takes before they fall into convulsions.  Bravo measured stress-induced hyperthermia (rise in core body temperature from stress) after a battery of different tests, including  forced swimming and mazes (less barbaric than listening to the Barney jingle, but presumably effective at producing stress) and showed that the L. rhamnosus-fed animals exhibited less stress during these activities.

The final point of the paper was what I found most interesting.  Some of the animals had the vagus nerve cut prior to the start of the experiment.  This nerve is a direct link between the gut and the brain and is responsible for transmitting signals about hunger and satiation.  In these animals,there was absolutely no effect from consuming L. rhamnosus.  No changes in GABA expression and no behavior changes versus the control animals.  This means that the probiotic bacteria that colonize in the gut might actually use this nerve to signal directly to the brain.  Although these bacteria appear to provide a health benefit, I can certainly imagine other strains that are more nefarious.  Yeah, I'm talking about zombies again.  This phenomenon is not all that different from what is seen in T. gondii or the zombie ants... microbial agents that manipulate the brain function of the host.  If a probiotic strain can mimic the effect of caffeine-overstimulation or, even worse, caffeine-deprivation, then that would be a whole new kind of scary.  Anyone who has seen me before that first cup of coffee knows what I'm talking about... it's bad enough to give a zombie nightmares!