Over the last ten years, books and conferences dedicated to fermented foods have been multiplying, based on new knowledge about the intestinal microbiota. Eating fermented foods would not only preserve and strengthen the digestive flora, but also cure certain diseases. One of these popular foods is kefir, whose grains ferment milk or fruit. These kefir grains are mainly cultivated and shared in the home, which means that the knowledge associated with the cultivation of these microbial ecosystems is produced mainly by amateurs, outside the traditional places where microbiological knowledge is developed (laboratory, industry). The objective of this work will be to understand the dynamics at work in the amateur domestication of these microorganisms, through a combination of ethnographic and microbiological approaches. The research will be carried out in partnership with Agroscope, with an online community of kefir practitioners and with the ColLaboratory of the UNIL.