I was listening to a program on NPR yesterday that was discussing a current study of whether or not probiotics can affect brain chemistry related to depression and anxiety. They spoke about some of the preliminary results which show a correlation between the probiotic they are studying and the improvement of depression and anxiety. They think it may be related to the gaba receptors and possibly and increase in gaba production. Many times the gut is referred to as the second brain because of all the neural pathways it contains so it is not that far fetched that it could affect brain chemistry. Of course these results are preliminary but they have been reproduced in two separate labs. Here is a link to what I have found on the internet related to these studies.
Probiotic Bacteria May Help Treat Depression | Gut-Brain Link | Mental Health & Psychiatric Disorders | LiveScience