Introducing the thematic cluster

T he one-day symposium Gut in Focus, which was held at the Karolinska Institutet on February 2nd, 2015, focused on certain aspects related to the gut, by far the largest organ of the body. When I started studying medicine more than 60 years ago, I heard two statements about the gut: (1) The gut has a brain, if not a mind, of its own and (2) Bacteria constitute up to 50 per cent of feces’ mass and may be of importance for its texture. Together, these two statements did not make sense to me. Nine years later, I came to the Karolinska Institutet as a bacteriologist, working in the team around Nobel Laureate Professor Sune Bergström and also Professor Bengt E Gustafsson and his germfree animals. Both of them were eagerly studying biochemical similarities and differences between germfree and conventional animals. It became my job to find microbe(s) that could close the gap. The functional importance of gut microbes was obvious for these two pioneers, and today, it is generally accepted that ‘Man Microbes Superorganism’, and there is increasing consensus that ‘Gut Microbiota Superorgan’. In our first Gut in Focus symposium in early 2012, the focus was on ‘The Gut and the Brain’. The 2015 symposium focused on the gut itself. The first session dealt with the establishment of a gut microbiota, and the second session with selected functional aspects. The third session was devoted to restoration of the gut. Most of the contributions are published in this Special Issue of Microbial Ecology in Health and Disease. As usual, access is free!