A Short Definition of Terms

Abstract

We humans are colonized by myriads of microorganisms in various parts of the body, such as the skin, the mouth, the vagina and the gastrointestinal tract. Even the lung and other hitherto thought to be sterile parts, as the placenta, are now considered to be colonized. Furthermore, our microbiota is not only comprised of bacteria, but also archaea and eukaryotes such as protozoa, fungi and nematodes. Even viruses of all three cellular domains, collectively termed the virome, can be found in the microbiota (Virgin 2014). It has been estimated that the human-associated microbiota, consists of at least 40,000 bacterial strains in 1800 genera (Luckey 1972; Frank and Pace 2008; Forsythe and Kunze 2013), which collectively harbor at least 9.9 million non-human genes (Li et al. 2014). They encode for approximately 500 times the human protein-coding genes which are currently annotated (http://www.ensembl.org). The estimated mass of the microbiota (1–2 kg in an adult body (Forsythe and Kunze 2013)) is comparable to the weight of the adult human brain (ca. 1.5 kg, Parent and Carpenter 1996).