Human and small animal microbiomes

The host’s first major exposure to a complex microbiota occurs during birth through contact with the maternal microbiome, which represent a primary mecha­nism for the intergenerational microbiota transfer in mammals and, afterwards, bacterial colonization progresses from childhood to adulthood [24].

The most important group of organisms in microbiome studies is called the dynamic symbionts, whose symbiotic nature may vary along a spectrum from mutualism and commensalism to parasitism and amensalism [25]. Usually, microbes perform synthetic or catabolic metabolic activity through direct microbe­host interactions. Catabolism and bioconversion of compounds from the diet make nutrients more available to the host through the processes of fermentation, hydroly­sis, metabolism of drugs and toxins, among others. Some microbiota members can synthesize important cofactors or bioactive signaling molecules such as vitamins and active amines. In addition, this can trigger changes in the host’s gastrointestinal epithelial and immune responses [26].

The combination of factors such as age, genetics, physiological status (includ­ing innate and adaptive immune system), lifestyle, diet, host environment and disease status can result in variation in microbiomes between hosts [27]. Human gut microbiota is extremely diverse, with an estimated 1,000 bacterial species in the gut with 2,000 genes per species yields an estimate of 2,000,000 genes, which is 100 times the commonly estimated 20,000 human genes [27]. In dogs, gut microbiome contains around 1,200,0 00 genes [20] and recent studies suggest that canine and feline gut fecal microbial phylogeny (e.g. predominance of Firmicutes, Bacteroidetes, Proteobacteria and Actinobacteria) and functional capacity (e.G. major functional groups related to carbohydrate, protein, DNA and vitamin metabolism, virulence factors and cell wall and capsule) are similar to those of the human gut [28].

3.