Answer by Ashish Vora:

Well most obviously if you’re stressed out your androgen levels are high. These hormones can significantly change cell signalling pathways and the resulting metabolites given off in the extra-cellular fluid. For example acid production in the gut. This change in the pH will of course change in the microbial population and give a case of the poo poos in the short or long term. This is just a very basic example. Any change in the human body will cause a corresponding change in the microbiome. (Exposure to a social stressor alters the structure of the intestina…)

Brain–Gut–Microbe Communication in Health and Disease

This is OK, only interesting if you’re a pure microbiologist. If I may suggest, you should ask the question in reverse to make it really interesting.

How does the microbiome affect human behaviour?

There are multiple studies now showing that a change in the microbiome in your gut can influence mood, stress, anxiety and depression. (The microbiome: stress, health and disease.)


Further, if you’re stressed out during pregnanacy, this will change the microbiome of the vagina which, in  turn can cause neurological disorders like schizophrenia and autism in the offspring. (Maternal stress and the vaginal microbiome: impacts on brain development)

This is only the beginning of us trying to figure out whos is in control, us or the microbes? Who is ‘us’ considering there are more bacteria inside humans than mammalian cells?

How does stress affect the gut flora?