"The brain and gut have a strong, two-way communication system referred to as the gut-brain axis. Each are essential to the other. Studies have found that brain development is abnormal in the absence of the gut microbiome. However, a recent review stated that research hasn't yet figured out which exact gut bacterium are crucial for brain development.
Further investigation is uncovering how interlinked the gut is to the brain, however, including our mood and mental health, says Katerina Johnson, a researcher of the microbiome-gut-brain axis at the University of Oxford." Source: BBC: Microbes & Me; The Life Inside All of Us