The invention relates to selection of bacterial strain for use in treating a disease related to serotonin deficiency by culturing bacteria of a lactic acid producing bacterial strain, under aerobic conditions, in a culture medium comprising tryptophan. Any serotonin produced by the bacteria of the lactic acid producing bacterial strain in the culture medium is detected and the lactic acid producing bacterial strain is selected as effective in treating serotonin deficiency and/or a disease related to serotonin deficiency in a subject if serotonin is detected in the culture medium.