This work provides evidence on the existence of specific differences in intestinal microbiota composition according to whether the patient owns a pet or not, and its potential association with MetS. Moreover, we identified specific gut microbiota features associated particularly to owning a dog and that pet ownership was linked to a lower risk of MetS and obesity.
Currently, the relationship between microbiota and disease is an emerging field in research. Studies in humans have identified a direct interaction between microbiota dysbiosis and the incidence of diseases such as non-alcoholic fatty liver disease and inflammatory bowel disease [23, 24]. In addition, microbiota dysbiosis has also been related to a higher incidence of metabolic diseases and CVD, including MetS and obesity [25–27].
Previous data have indicated that factors such owning a pet seems to affect gut microbiota composition . Additionally, it has also been described that pet ownership is associated to lower risk of suffering CVD, mainly by providing social support and motivation for physical activity [3, 4]. Despite this increasing knowledge, the potential improvement in gut microbiota profile associated to pet ownership in cardiovascular disease patients has not yet been studied. In addition, to the best of our knowledge, none of the published studies have investigated the relationship between pet ownership and MetS.
It is interesting to note that differences in gut microbiota, in owner of pets, have been linked to be a protective factor against the development of diseases such as allergies and obesity in infants [5, 6]. In line with this, our results provide evidence that, in cardiovascular disease patients, pet owners and dog owners may have less risk of MetS than non-pet owners (OR = 0.1-0.42-0.94). Furthermore, there was a small age difference between groups (5 years), which we do not consider relevant to influence our findings.
Moreover, our study also showed that this difference in MetS prevalence between pet owners and the group with no pets was accompanied by differences in the gut microbiota. In fact, the group with no pets was characterized by a gut microbiota with a preponderance of Ruminococcus, Anaerotruncus and an unknown genus of Enterobacteriaceae compared to the group of pet owners. The abundance of Ruminococcus and Anaerotruncus in human microbiota has been previously related to a higher prevalence of MetS [17, 29] and the abundance of Enterobacteriaceae has been positively linked to the development of obesity in children and pregnant women [30–32].
In our study, the gut microbiota of pet owners was characterized by higher levels of Coprococcus and Serratia. The genus Coprococcus has been shown to be a protective factor against MetS and T2DM  and is a short-chain fatty acid producer that modulates insulin resistance . Lower levels of Coprococcus are strongly associated with fasting serum levels of glycerol, monounsaturated fatty acids and saturated fatty acids, and inversely associated with polyunsaturated fatty acids . Serratia, on the other hand, is a potentially pathogenic genus; however, its presence in healthy people has been related to be a protective factor against obesity . Thus, our findings are in line with previous studies describing an altered abundance of these bacterial taxa in metabolic disease, as the group of pet owners had less risk of MetS and obesity.
Considering the potential of different animals to change the human gut microbiota [5, 6, 28], and particularly the fact that has been shown to be a protective factor against various diseases such as allergies , we explored the specific bacterial differences between a subgroup of patients who only owned dogs versus non-pet owners in general. Here, our data showed that the genera Oscillospira, Coprococcus and Methanobrevibacter, together with the Archaea domain, were found in higher proportions in the gut microbiota of dog owners. The higher prevalence of Archaea is probably due to the higher proportion of Methanobrevibacter, an interesting methanogen genus and SCFA producer  which has been linked to lower body mass index (BMI) and lower levels of triglycerides, while a lower fecal concentration of this genus has been found in prediabetic subjects .
In addition, Oscillospira genus has been correlated with leanness, lower BMI and lower prevalence of obesity, and is able to degrade host glycans (such as fucose, sialic acids and glucuronic acid) [39, 40].
In agreement with these findings, the increased presence of Oscillospira, Methanobrevibacter and Coprococcus in the samples of dog owners could explain the lower prevalence of MetS and obesity in this population [33–35]. Very few studies have explored the gut microbiota of people who come into contact with pets, and in all of them, infants were studied. However, these studies have shown that two of the four genera that our data showed as more prevalent in pet owners and dog owners, Coprococcus and Oscillospira, have been previously linked to pet ownership, which reinforces the hypothesis of the transfer of microbiota from pets to owners. In fact, higher levels of Coprococcus and Oscillospira have been previously linked to infants who live in a household where there is a dog [5, 6].
By contrast, the other two genera, Serratia and Methanobrevibacter, have not yet been reported as being higher in humans who have contact with pets. Even though Serratia has not been identified in humans in contact with pets, this genus has been found in healthy dogs, which constitute the majority of the pets owned by our patients, which suggests that it could be transferred from pets to their owners [41, 42]. Like Serratia, higher abundance of Methanobrevibacter has not been found previously in humans living with dogs; however, this genus can also be present in the dog microbiota .
Our study has limitations and could be considered a preliminary study. One lies on the fact that the population in which we performed the study has CVD and may already present several potential alterations in their gut microbiota associated with this disease. Further studies are needed to clarify the relationship between cardiovascular disease and pet ownership, with particular reference to the changes in the gut microbiota that may be produced as a consequence.