The human gastrointestinal tract harbours a complex and dynamic population of microorganisms known as the gut microbiota. The genetic content of the microbial communities in our gut outnumbers our genetic content by approximately 100-fold (Ley, Peterson and Gordon 2006).
In a review by Carabotti et al (2015) the authors conclude that strong evidence suggests that gut microbiota has an important role in bidirectional interactions between the gut and the nervous system. It interacts with CNS by regulating brain chemistry and influencing neuro-endocrine systems associated with stress response, anxiety and memory function.
A study by Lurie et al (2015) reported that one course of antibiotics can increase the risk of major depression by 24% and anxiety disorders by 17%. The study found that two courses of antibiotics in a one-year period increased the risk of major depression by 52% and anxiety disorders by 44%.