Bacteria are single-celled organisms that do not have a nucleus and organelles and hence are classified as the prokaryotes. The human intestinal tract is dominated by anaerobic bacteria since there is not much oxygen. The first of the reviewed studies investigated the oral microbiomes. The analysis of 1179 species that are identified by their 16S rRNA sequences in order to be entered into a database called Human Oral Microbiome Database (HOMD). The second study presented here investigates fecal samples microbiomes of a female infant. Depending on a treatment of the child, different groups of bacteria appear sequentially on daily basis. The microbial composition of the intestinal tract is also highly dependent on dietary habits. In the third rewieved study fecal samples from children in rural villages in Africa and urban centers of Europe were comparated regarding the microbe diversity. Different dietary habits in two populations of children affected their microbe diversity, but both populations contain the four most common groups of bacteria, namely: Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria. The last reviewed paper brings research on how intestinal microbiome affects the developing brain, its function and behavior in germfree (GF) and pathogen-free (SPF) mice. Significant differences in motor abilities, noradrenaline, dopamine, 5-HT, synaptophysin and PSD-95 levels were found between the two groups of mice.