When the Human Genome Project1 was launched in autumn 1990 with the aim of identifying and mapping all of the genes of the human genome, no-one would have thought that we would discover a new microcosmos revolving around and mingling with our human cells. Of course, already long before this project, it was well-known that our body is not sterile and there are many bacteria living within and on it. These bacteria were however mainly classified as being malicious, threatening our health and causing problems.2 Until the 70s of the last century, a germ-free personal environment was considered as most desirable, and strong cleaning products became quite popular. Only starting in the early 1980s, these ideas and information were carefully reevaluated. When the first results of the Human Microbiome Project3 that started in 2008 were published and we learnt that not only do we host an amazing amount of microbes in and on our body but that these tiny cohabitants at least equal or possibly even exceed the amount of our own human cells, intense research has been dedicated to understand their function for human health and disease.4 With the rapid development of novel metagenomic methods, microbiome analysis has become much easier, quicker, more comprehensive and reliable than with traditional techniques such as sampling and culturing that only allow the identification of certain isolated species that can be cultured.
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