Submitted: 12/31/12 Revised: 03/08/13 Accepted: 03/11/13

Phylogenetic reconstruction of three highly conserved proteins involved in bacterial conjugation (relaxase, cou-pling protein and a type IV secretion sys-tem ATPase) allowed the classification of transmissible elements in relaxase MOB families and mating pair formation MPF groups. These evolutionary studies point to the existence of a limited number of module combinations in transmissible elements, preferentially associated with specific genetic or environmental back-grounds. A practical protocol based on the MOB classification was imple-mented to detect and assort transmis-sible plasmids and integrative elements from γ-Proteobacteria. It was called “Degenerate Primer MOB Typing ” or DPMT. It resulted in a powerful tech-nique that discovers not only backbones related to previously classified elements (typically by PCR-based replicon typing or PBRT), but also distant new members sharing a common evolutionary ances-tor. The DPMT method, conjointly with PBRT, promises to be useful to gain information on plasmid backbones and helpful to investigate the dissemina-tion routes of transmissible elements in microbial ecosystems. Traditional Trends in Plasmid Classification Plasmid biology emerged as a discipline with the study of fertility and resis-tance factors present in Enterobacteria, Pseudomonas and Staphylococci. At that emerging point, plasmids were assigned to different incompatibility groups (Inc), based on their ability to coexist in the same cell.1 Incompatibility was generally Ordering the bestiary of genetic elements transmissible by conjugatio