How grapevine got pimples: the interkingdom horizontal transfer of an unusual symbiont

Humans have established a long-lasting co-habitation with a variety of animals, plants and microbes. The human-gut microbe interaction has been recently explored, and cases have been reported of microbial host transfer from domesticated animals to humans. Much less is known regarding human-plant microbial transfers. Here we report a recently established symbiosis between the human-associated opportunistic pathogen Propionibacterium acnes and the highly domesticated grape Vitis vinifera. We detected P. acnes in many grape plants using pyrosequencing of 16S rDNA and fluorescent in-situ hybridization (FISH) which localized P. acnes in specific endosphere habitats (pith and bark). We also show that recA genes in uncultivable endophytic P. acnes of grape are pseudogenes, suggesting a recent obligate symbiosis which we tentative date as being not older than 7000 years ago, an age compatible with the domestication of grape by humans. Our results represent the first documented inter-kingdom horizontal host transfer of a human symbiont