In recent years gut microbiota has become the focus of increasing interest. The driving force behind this interest is the ongoing discovery of new dimensions to the connection between gut bacteria and the host – namely us. The following paragraphs outline the latest research on the influence of gut bacteria on the human brain
The intestinal tract, and especially the lower gut, harbors a vast range of bacteria that maintains a symbiotic relationship with the host. The complex bidirectional communication between over a thousand species of bacteria and the human Central Nervous System (CNS) is often referred to as the gut–brain axis. This axis is important in brain development and behavior (1).
After the birth of a child, diverse classes of microbes from the environment colonize the newborn`s gastrointestinal tract, forming the intestinal microbiome ecosystem. Various factors are involved in the development of this complex ecosystem. The infant`s gestational age, mode of delivery, type of nutrition, and early use of antibiotics all modify the composition of this microbiome and may have significant and long-lasting effects (2).
The transmission between gut bacteria and host is performed primarily by the Entric Nervous system (the mesh-like system of neurons that governs the function of the gastrointestinal system). The signal transmission of the gut bacteria assumes different forms. Some probiotic strains excrete neurotransmitters such as Acetylcholine and GABA (Gamma-aminobutyric acid).
Acetylcholine is found in the central and peripheral nervous systems and performs an important role in cognitive function and especially in memory and learning. GABA is a central neurotransmitter that regulates many bodily and psychological processes. Dysfunction of this system has been found to be implicated with anxiety and depression (3). Another form of interaction between microbiota and the brain is through the ability of some microbiota to synthesize serotonin (5-HT), a neurotransmitter that has an important role in regulating a number of brain functions including mood. Indeed, some anti-depressant drugs address mood improvement by elevating the levels of serotonin in the brain (4).
Mice, like humans, are a social species and so naturaly seek stable social situations. A study performed with bacteria-free mice indicated significant social development impairment expressed in social avoidance and reduced preference for novel social situations (5). studies performed in animals that have been exposed to germ-free conditions on one hand and to probiotics, antibiotics and bacterial infections on the other, suggest that gut microbiotia has a role in regulating mood, cognition, pain and obesity (6).
An early mention of probiotics as an adjuvant treatment for depression was in 2005 when Logan AC and Katzman M explored the possibility of administering Probiotics in cases of Major depressive disorder (7) . More recent discoveries regarding the connection between gut and brain have led to the emerging realm of Psychobiotics which further explores the treatment of psychiatric patients by administration of probiotics.
It seems that there is much more to learn about the fascinating interaction between the human mind and the bacteria lining our gut. While we are far from fathoming this intreaging connection, it is clear that administering probiotic bacteria may hold great potential when addressing a wide variety of health conditions.