**2. Materials and Methods**

The key research question of this literature review is: What factors may influence the microbiome and could be associated with the development and/or progression of ADHD? The databases Pubmed and Scopus were searched until the 1 July 2019 with the following MeSH and search terms: "Microbiome", "microbiota", "gut-brain axis", "microbiota-gut-brain-axis", "ADHD". Most of the research papers included in this review were published between 2010 and 2019.

As our primary focus was on bacteria, we excluded studies that concentrated on archaea and eukaryota. The incorporated studies had to fulfill all of the following inclusion criteria:


A total of 208 citations were included in this article.
