Acidophilus (lactobacillus acidophilus) is a beneficial, probiotic bacteria which is found in the stomach and intestines. Some medications, such as antibiotics, kill acidophilus and can cause an imbalance of gut bacteria.
Human studies, such as the 2015 study "Clinical and metabolic response to probiotic administration in patients with major depressive disorder", have suggested a correlation between mental health conditions and probiotics.
Acidophilus is available in capsules, and as an ingredient with other beneficial bacteria in Probiotic supplements.