Table 3. Some of the studies with a positive association of gut microbiome to depression.

CRP- C- Reactive Protein; ARNTL- Aryl hydrocarbon Receptor Nuclear Translocator-Like; CPG- 5'-C-phosphate-G-3'; LBP- Lipopolysaccharide-Binding Protein; CD14- Cluster of Differentiation 14; MDD- Major Depressive Disorder

Author/Publication Date	Study Design	Sample size	Main Points
Heym et al. [20], 2019	Observational Study	40	There is a bi-directional link between gut bacterial commensals and the central nervous system; the study involved 40 subjects; fecal and blood samples were collected to assay the microbiota as well as pro-inflammatory molecules; hierarchical regression analyses reveal limited cognitive empathy and poor self-judgment predicted depression; the more diverse gut microbiota led to positive self-judgment and protection from depression; the level of CRP predicts the negative cognitive empathy
Bengesser et al. [22], 2019	Observational Study	32	The gut microbiota contains more genetic material than the cells of our entire body; it has direct effects on the different physiological process and creates a link to the brain through the gut-brain axis; the study intends to establish the correlation between the gut microbiome diversity and the clock gene ARNTL gene methylation in Bipolar Disorder patients; ARNTL CPG position methylation is linked to diversity and evenness; there is a significant difference of bacterial composition between depressed patients and patients on euthymic episodes; gut bacterial microflora diversity is inversely correlated to the ARNTL methylation
Kiecolt-Glaser et al. [23], 2018	Clinical trial	86	Depression and marital stressors potentiate the risk of developing inflammatory conditions; bacterial translocation promotes systemic inflammatory responses; the study involves a secondary analysis of a double-blind experiment assessing lipopolysaccharide-binding protein (LBP) and soluble CD14 (sCD14); there were 43 healthy married subjects within the age range of 24-61 who were asked to talk about relationship conflicts on two occasions; it showed that negative marital behaviors lead to the distress of both partners; the study reveals that aggressive interactions between the couples led to a higher LBP level than the less hostile group; when mood disorder history was added, higher LBP/sCD14 ratio was noted; stressful marriage and history of mood disorders promote inflammatory responses that worsen the condition
Kelly et al. [9], 2016	Observational/ Laboratory Study	67	Multiple pathways were noted linking both gut microbiota and the nervous system; there is a bidirectional relationship affecting behavior and brain functions; there were 34 subjects with major depressive disorder (MDD) and 33 healthy subjects as the control group; blood and fecal samples were collected; fecal microbiota sample came from MDD patients and was transplanted to rats with depleted gut microflora; depressed behaviors developed in the microbiota transplanted rats