Understanding how microbes disperse in ecosystems is critical to understand the dynamics and evolution of microbial communities. However, microbial dispersal is difficult to study because of uncertainty about the vectors that may contribute to their migration. This applies to both microbial communities in natural and human-associated environments. Here, we studied microbial dispersal among French sourdoughs and flours used to make bread. Sourdough is a naturally fermented mixture of flour and water. It hosts a community of bacteria and yeasts whose origins are only partially known. We analyzed whether flour is a carrier of sourdough yeast and bacteria and studied whether microbial migration occurs between sourdoughs. The microbial community of a collection of 46 sourdough samples, as well as that of the flour from which each was made, was studied by 16S rDNA and ITS1 metabarcoding. No sourdough yeast species were detected in the flours. Sourdough lactic acid bacteria (LAB) were found in only five flour samples, and they did not have the same amplicon sequence variant (ASV) as found in the corresponding sourdough. The species shared between the sourdough and flour samples are commonly found on plants and are not known to be alive in sourdough. Thus, the flour microorganisms did not appear to grow in the sourdough microbial community. Dispersal between sourdoughs was also studied. Sourdoughs shared no yeast ASV, except in few cases where groups of three to five bakers shared some. These results suggest that there is little migration between sourdoughs, except in a few situations where bakers may exchange sourdough or be vectors of yeast dispersal themselves.

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