## 4 Discussion  
Recently, gut-microbiota plays an important role in neuroscience research as a component of the microbiota-gut-brain axis [3]. Our current knowledge about the microbiota-depression relationship is mainly based on animal model studies because very few research has been conducted on human patients. Some findings on animal models cannot fit in with the real human condition, so it can be considered as one of the most intriguing and controversial topics [16]. In this study, the relationship between the microbiota of German Shepherds and other dog breeds was investigated for the first time. In addition to that, the microbiota of the German Shepherd group was compared with that of humans suffering from depression disorder [17]. The choice of German Shepherd is due to the results derived from the quantitative molecular evaluation of fatty acids in platelets of people with mood disorders and several animals (sheep, horse, donkey, dog, cat, bovine, rat, pig and guinea pig). This investigation was conducted using a complex mathematical function, as a Self-Organizing Map (SOM) and an index (B2). This allowed classifying the different depression disorders (major depression and bipolar depression) and identifying their molecular pathway. The data obtained from animals were put into the SOM, where human disorders were classified as pathological areas, bipolar depression area and healthy human area. The results showed that sheep, cats, bovines, horses and donkeys were distributed in the opposite area on the SOM than people with mood disorders, while, guinea pigs, rats and pigs occupied the area characterized by depressive subjects. Only two different breeds of dogs (German Shepherd and Alaskan Malamute) were collocated in the part of the map typical of humans affected by Major Depression (MD). This result was not found in any other animals [13]. In the literature, the human research that associates depression and microbiota composition is controversial. In 2015, [PERSON] and colleagues analyzed the gut microbiota composition of humans with a diagnosis of active major depressive disorder and responded-MDD [16]. This study evidenced that both groups have an increase in genera _Phascalcrcobacterium_, _Roseburia_ and _Parabacteroides_ and a decrease in genera _Ruminococcus_, _Prevotella_, _Dialister_ and _Faecalibacterium_[16]. The research held by [PERSON] et al. (2016) [18] and [PERSON] et al. (2019) [19] observed an increased relative abundance of Lactobacillus in depressed patients. Furthermore, [PERSON] et al. (2019) [19] found a higher prevalence of _Bacteroides_ in people suffering from depression. In this study, the gut microbiota structure of 27 dogs (12 German Shepherd and 15 mixed breeds) was analyzed. Moreover, we have compared the gut microbiota of our German Shepherds with the gut microbiota of depressed humans, previously analyzed by [PERSON] et al. [16]. The phylogenetic profiles of the canine gut microbiota observed in the cohort of this research were found to be aligned with those already reported in the literature for healthy dogs [20; 21] but with a slightly higher abundance of Proteobacteria and a lower abundance of Bacteroides. According to our results, the gut microbiota of the GS group is characterized by higher fecal microbial diversity, estimated using the Shannon Index, rather than the MB group. Interestingly, gut microbiota diversity has been usually considered beneficial for human health but some studies have denied this statement [19; 22]. The GS group was characterized by an enrichment of Streptococcus and Lactobacillus genera compared to the MB group. Bacteria belonging to the genus Streptococcus are known as serotonin producers, while those belonging to Lactobacillus are gamma-aminobutyric acid (GABA) producers. These molecules are recognized as neurotransmitters, able to regulate emotional behaviors [23; 24; 25]. Moreover, the GS group was characterized by a decrease in the _Bacteroides_ genus, associated with metabolic diseases quite common in depression [26].