from Peter Mullany and Adam P. Roberts writing in Horizontal Gene Transfer in Microorganisms:
Mobile genetic elements are discrete segments of nucleic acid that can translocate from one part of the genome to another, and in the case of conjugative elements between genomes in different cells. The vast majority of our knowledge of mobile genetic elements is derived from experimentation on cultivable bacteria and is therefore incomplete. In this chapter we will discuss the methods available for identifying and isolating these elements from metagenomic samples.
Further reading: Horizontal Gene Transfer in Microorganisms