Saturday, August 01, 2015

Microbes for Thought


More and more evidence has begun to accumulate in the favor of the idea that your gut microbiome has a lot to do with your brain health. It has also been the contention that there are at least a set of microorganism that can influence behavioral pattern. Something as complex as schizophrenia can have a microbial component attached to it (May not be evidence in every case). Toxoplasma is another example which has been debated in the literature. Isn't it fascinating to even think that some of the behaviors are influenced by the type of bacterial population sitting in your gut?

Fig 1: Microbiome and eating habits.
The story of microbiome influence on behavior is an old one. Gut bacteria influencing behavior has been predicted almost a century ago. It was believed that psychiatric conditions were due to intoxicating bacteria in the gut (Called as Auto-intoxication). The idea was considered so important that people had intestinal surgeries for psychiatric conditions, though later the practice was stopped. However, with the ability to study the entire microbiome that associations have become more clear. I have often pointed in this blog studies showing the role of microbiome with relation to weight gain phenomenon. Now more recently, an article argues that there is an influence of gut microbiome on eating habits. The idea is that microbes are manipulating through vagus nerve a craving to eat more. As Carlo Maley comments “Bacteria within the gut are manipulative. There is a diversity of interests represented in the microbiome, some aligned with our own dietary goals, and others not.” An illustration of the idea is shown in Fig 1, from the paper.

One more interesting example. There is no second question on the influence of serotonin on behavior. So it doesn't come as a surprise that microbiome can be directly involved in serotonin regulation. In a paper from Hsiao lab, it was shown in a SPF (Specific pathogen free) mouse models that many different bacteria can make serotonin by associating with intestinal cells.

The understanding that microbes can manipulate the behavior has led to the idea of psychobiotics. Psychobiotics have been defined as a live organism that, when ingested in adequate amounts, produces a health benefit in patients suffering from psychiatric illness. It is clear that at least certain strains of bacteria, can produce or involve in production of neurotransmitters and chemicals that may directly influence brain development. For example, L. rhamnosus (JB-1) can influence GABA expression. It has been suggested recently that neurogenesis in hippocampus, a key region of brain involved in process of memory can be influenced by the microbiome.

Often, I have been questioned if there are other examples of such cases were microbe modulates host behavior. There is a growing list that studies are reporting. Of human interest, studies have found suggested that probably mosquito carrying pathogens such as Plasmodium or dengue increase their frequency of blood meals thereby aiding more efficiency to transmission. One more example out of human arena, dubbed as "zombie ants" is well described in small things considered (Link). More interesting cases is listed in this link. Also I recommend you watch the TED talk by Elaine Hsiao

The field is naive and much of the details are lacking. I'm simply curious to know what other microbiomes (other than intestinal) can be influential. Perhaps we need a new field- "Behavioral microbiology". Considering that they type of food you eat determines the type of microbiome you have, "Food for Thought" is arguably a microbiology thing.
Alcock J, Maley CC, & Aktipis CA (2014). Is eating behavior manipulated by the gastrointestinal microbiota? Evolutionary pressures and potential mechanisms. BioEssays : news and reviews in molecular, cellular and developmental biology, 36 (10), 940-9 PMID: 25103109

Yano JM, Yu K, Donaldson GP, Shastri GG, Ann P, Ma L, Nagler CR, Ismagilov RF, Mazmanian SK, & Hsiao EY (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell, 161 (2), 264-76 PMID: 25860609

Ogbonnaya ES, Clarke G, Shanahan F, Dinan TG, Cryan JF, & O'Leary OF (2015). Adult Hippocampal Neurogenesis Is Regulated by the Microbiome. Biological psychiatry, 78 (4) PMID: 25700599

Bravo, J., Forsythe, P., Chew, M., Escaravage, E., Savignac, H., Dinan, T., Bienenstock, J., & Cryan, J. (2011). Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve Proceedings of the National Academy of Sciences, 108 (38), 16050-16055 DOI: 10.1073/pnas.1102999108