Large-scale analysis of plasmid relationships through gene-sharing networks.

Plasmids are vessels of genetic exchange in microbial communities. They are known to transfer between different host organisms and acquire diverse genetic elements from chromosomes and/or other plasmids. Therefore, they constitute an important element in microbial evolution by rapidly disseminating various genetic properties among different communities. A paradigmatic example of this is the dissemination of antibiotic resistance (AR) genes that has resulted in the emergence of multiresistant pathogenic bacterial strains. To globally analyze the evolutionary dynamics of plasmids, we built a large graph in which 2,343 plasmids (nodes) are connected according to the proteins shared by each other. The analysis of this gene-sharing network revealed an overall coherence between network clustering and the phylogenetic classes of the corresponding microorganisms, likely resulting from genetic barriers to horizontal gene transfer between distant phylogenetic groups. Habitat was not a crucial factor in clustering as plasmids from organisms inhabiting different environments were often found embedded in the same cluster. Analyses of network metrics revealed a statistically significant correlation between plasmid mobility and their centrality within the network, providing support to the observation that mobile plasmids are particularly important in spreading genes in microbial communities. Finally, our study reveals an extensive (and previously undescribed) sharing of AR genes between Actinobacteria and Gammaproteobacteria, suggesting that the former might represent an important reservoir of AR genes for the latter.