Dr.Mercola suggests the following things in order to optimize your gut flora and thus ensure your health and  mental well-being:

“Regularly eating traditionally fermented and cultured foods is the easiest, most effective and least expensive way to make a significant impact on your gut microbiome.

Healthy choices include lassi (an Indian yogurt drink, traditionally enjoyed before dinner), fermented grass-fed organic milk such as kefir, natto (fermented soy) and various pickled fermentations of cabbage, turnips, eggplant, cucumbers, onions, squash, and carrots.

Although I’m not a major proponent of taking many supplements, probiotics are an exception if you don’t eat fermented foods on a regular basis. It’s also important to avoid things known to disrupt or kill your microbiome, and this includes: antibiotics, unless absolutely necessary, conventionally-raised meats and other animal products, processed foods, chlorinated and/or fluoridated water, and antibacterial soap and products containing triclosan.