It sounds like a plot from a horror film – some invisible critters occupying your gut controlling your mind. Yet, science is proving that the bacteria living in your intestines can not only affect your digestion but also your mood and emotions.

A study published on 22 August in the journal Frontiers in Psychiatry investigated potential links between the gut micro-flora and depression. The researchers say there is quite a lot of evidence of abundant communication going on between the gut and the brain. Changes in the composition of bacteria living in the intestines appear to be contributing to the development of various mental health disorders including depression.

This communication between the gut and the brain is happening on the level of hormones and all other sorts of chemicals. The researchers call it the gut-brain axis.

“The brain and the gastrointestinal tract are bi-directionally linked through the central nervous system, endocrine system, and immune system, and perturbations to any of these systems can have repercussions across the others, in turn potentially influencing a person’s overall wellbeing,” the researchers said.

The microscopic inhabitants of the gut can influence, which genes will get expressed. They can also directly affect the central nervous system and the immune system. Evidence exists that out of whack gut microflora can cause changes in human behaviour (crazy, isn’t it?), as well as affect cognitive processes. Essentially, the bacteria determine what type of person you are and how you behave.

“The pathophysiology of neuropsychiatric disorders, such as depression, schizophrenia, anxiety, and autism, appears to involve gut microbiome changes,” the researchers said in the paper.

What is the lesson to be taken from all of this?

“Our habits, including our diet, are important factors modulating the microbiome-gut-brain axis,” said Juan M. Lima-Ojeda, lead author of the review and a physician and researcher at the University of Regensburg, Germany. “So, an appropriate diet is important for adequate mental health.”

Eating junk clearly doesn’t affect only your weight and metabolism but it can apparently also mess with your mind.

The researchers also found evidence that disruption to the gut microbiome in early life can lead to an increased risk of developing depression in adulthood. So be careful, what do you feed your kids. It could turn them into crazy monsters later on. Antibiotics also negatively affect the gut microbiome.

study published in the journal Microbiome looked at how exactly do the gut-inhabiting critters cause anxiety.

The team from the University College Cork, Ireland, studied mice that lived in a totally microbe-free environment. The researchers observed changes in biological molecules called microRNAs in the brains of these mice and compared them to the regular microbe-inhabited mice. The critter-free mice were at the same time showing abnormal anxiety, were less sociable and showed impaired cognition. They also appeared more depressed than their critter-inhabited counterparts.

“Gut microbes seem to influence miRNAs in the amygdala and the prefrontal cortex,” said Gerard Clarke, the corresponding author of the paper.

“This is important because these miRNAs may affect physiological processes that are fundamental to the functioning of the central nervous system and in brain regions, such as the amygdala and prefrontal cortex, which are heavily implicated in anxiety and depression.”

miRNAs are short sequences of nucleotides (the building blocks of DNA and RNA), which can control how genes are expressed. miRNA dysregulation or dysfunction is believed to be an underlying factor contributing to stress-related psychiatric disorders, neurodegenerative diseases and neurodevelopmental abnormalities. miRNA changes in the brain have been implicated in anxiety-like behaviors.

“It may be possible to modulate miRNAs in the brain for the treatment of psychiatric disorders but research in this area has faced several challenges, for example, finding safe and biologically stable compounds that are able to cross the blood-brain barrier and then act at the desired location in the brain,” Clarke said.

“Our study suggests that some of the hurdles that stand in the way of exploiting the therapeutic potential of miRNAs could be cleared by instead targeting the gut microbiome.”

The team also looked at rats whose gut microbiome had been damaged by the use of antibiotics. They found that the miRNAs of these rats were affected in a similar way as those of the germ-free mice. This, the researchers said, suggests that even if a healthy microbiota is present in early life, subsequent changes in adulthood can impact miRNAs in the brain relevant to anxiety-like behaviours.

The findings suggest that a healthy microbiome is necessary for appropriate regulation of miRNAs in these brain regions. Previous research demonstrated that manipulation of the gut microbiome affects anxiety-like behaviors but this is the first time that the gut microbiome has been linked to miRNAs in both the amygdala and prefrontal cortex, according to the authors.