Effect of probiotic gut microbiota on host health is very interesting for scientists as an example for active interactions among different living organisms. This article answers basic questions about this interaction, its effects on human mental health and its application in healthy and therapeutic nutrition of host. Mechanisms of gut microbiota affecting human mental health and limitations of probiotics in host gut were discussed. Probiotics application in healthy nutrition is only one factor in integrated healthy lifestyle for good physical and mental health of human.