The human gut flora of trillions of bacteria is vital for general health and greatly influences digestion, immune system function, and brain development. Through neuronal, hormonal, and immunological channels, the gut-brain axis (GBA), a bidirectional communication network, links the gut microbiota to the central nervous system (CNS). This relationship has been linked to affective diseases, including depression and anxiety, as well as mental health issues. This review explores the intricate relationship between gut bacteria and mood disorders, focusing on how gut microbiota-host interactions, immune system modulation, and neurotransmitter control support mental health. The function of important microbial metabolites, including short-chain fatty acids (SCFAs), in preserving blood-brain barrier integrity and modulating neuroinflammation is covered in this review. It also examines the bidirectional impact between gut health and mental health, including how dysbiosis could aggravate mood disorders and how depressed states might change the composition of gut bacteria. Furthermore, we discuss how psychotropic drugs affect gut flora and consider other elements such as nutrition and lifestyle that affect gut microbiome composition. Potential paths for treating mood disorders through gut microbiota modification are presented as emerging treatment techniques, including probiotics, nutritional therapies, and precision medicine. The development of new therapeutic approaches for mood disorders depends on the awareness of the GBA. Gut bacteria significantly affect mental health through immune modulation, neurotransmitter generation, and other intricate processes. Future studies should concentrate on large, varied populations to better understand these interactions and to create customized treatments that combine gut microbiota modulation with conventional mental health therapies.