Hi deserttree, yes the microbes that live in our guts release chemicals that affect our moods! The "second brain" refers to the 100 million nerve cells that line our guts and may cause such familiar feelings as "butterflies in our stomach" when we get nervous.

If you are interested in reading more about "the second brain" and the effect of gut microbes on our moods, here are two great articles from Scientific American and The New York Times.
https://www.scientificamerican.com/article/gut-second-brain/
https://www.nytimes.com/2015/06/28/magazine/can-the-bacteria-in-your-gut-explain-your-mood.html