This chapter explores the effect of probiotic treatment on the negative symptoms of schizophrenia in a single male patient. Schizophrenia is a complex multi-factorial mental health problem characterised by severe and intrusive psychic symptoms and debilitating social and emotional symptoms. Probiotics are live bacteria based microorganisms that are non-pathogenic in their nature and when taken in sufficient amount influence the flora in the gut to produce health and immunological improvements. However, pathways linking immunological processes and the development of schizophrenia symptoms and the functioning of the hypothalamic-pituitary-adrenal within the stress vulnerability model of schizophrenia symptom development, provide the framework and targets for the setting of future study hypotheses regarding the testing of efficacy of probiotics. The term microbial endocrinology has been used to describe the study of the action of the biochemicals that influence the functioning of the immune system, gastro intestinal tract and the central and autonomic nervous systems simultaneously.