The invention relates to novel bacteria of the species Lactobacillus plantarum and to compositions comprising Lactobacillus plantarum, and methods and uses of said compositions for preventing and/or treating mental illness, symptoms affecting mental health and/or conditions associated with chronic stress, in a mammal. The invention can be applied in restoring, maintaining, and/or promoting mental (including cognitive) health and has potential applications, inter alia, in the area of dietary and food supplements, medicaments and pharmaceuticals.