Did you know that bacteria may be entirely responsible for exactly how you react to stress, right from the moment that you are born?
When you are born, your brain's stress network isn't fully formed. Amazingly, the bacteria in your gut when you are a small child may decide exactly how your brain’s stress network gets wired up to serve you for the rest of your life.
According to mouse studies, how you react to situations today could be the result of the precise bacteria living in your gut when you were a little baby. If what happens in mice also happens in humans then, if you did not have the ‘right’ bacteria as a baby, your brain’s wiring with regard to stress could be seriously flawed.
We can take newborn mice and rear them in strict laboratory conditions so they grow up without any bugs in their bodies. They can be reared to have absolutely no intestinal bacteria whatsoever. In these mice, the brain’s stress network is extremely flawed. The mice show an excessive and abnormal stress response to perfectly harmless situations.
If we take faecal matter from ‘normal’ mice who have grown up with intestinal bacteria and place this faecal matter into the intestines of these bug-free mice, this makes their stress response normal again. We have even identified the specific bacterial strain within the faecal matter, which is responsible for normalizing the response, Bifidobacterium infantis. 
Bacteria can continue to affect your stress levels and mood even when you become an adult. If your mood is low, then giving your gut some good bacteria may do wonders for your mood. Taking a milk-based probiotic drink for even a short period of time can reduce symptoms of depression. Taking probiotics regularly for just a month can reduce stress and feelings of anxiety.
If you are feeling negative for no reason, you might want to start including a good portion of natural probiotic yoghurt or kefir in your daily routine. A happy gut means a happy mind!
 Sudo, N. et al. Postnatal microbial colonization programs the hypothalamic–pituitary–adrenal system for stress response in mice. J. Physiol. 558, 263–275 (2004).
 Benton, D. et al. (2007) Impact of consuming a milk drink containing a probiotic on mood and cognition. Eur. J. Clin. Nutr. 61, 355–361
 Messaoudi, M. et al. (2011) Beneficial psychological effects of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in healthy human volunteers. Gut Microbes 2, 256–261