The gut-brain axis refers to the two-way communication that goes on between the brain or CNS and the gut ENS. It involves the autonomic nervous system as well as endocrine, metabolic, immune and other systems. It allows the brain and emotions to affect intestinal behaviour, while the gut can influence mood, cognition and mental health.^13,14,15

The gut microbiota is a significant regulator in this relationship, which is why the concept of the ‘gut-brain axis’ is now being supplanted with term ‘microbiota-gut-brain axis’. Gut microbes can interact with the brain via hormonal and immune pathways, as well as by directly secreting neurotransmitters such as serotonin, dopamine or gamma-aminobutyric acid (GABA).^13,16

In the other direction, CNS-based cortisol secretion and the sympathetic nervous effects on gut motility, secretions and mucin production, can influence the microbiota’s habitat and composition.¹⁶

An example of the interaction between microbes and the brain is Campylobacter jejuni. Animal studies show that if introduced into the gut in low levels (with no observable gastric effects such as diarrhoea), the bacterium can promote anxiety-like behaviour without an accompanying immune response.¹³