Extrachromosomal elements are ubiquitous in the prokaryotic world and play important roles in the adaptation and survival of cell populations, especially in changing environments. Plasmids are readily found in enterococci, and it is not unusual for clinical and commensal strains (e.g. Enterococcus faecalis and Enterococcus faecium) to harbor a number of such elements. Indeed, plasmid-free isolates are only infrequently identified. Enterococcal plasmids commonly encode: i) resistance to one or more antibiotics; ii) elevated resistance to ultraviolet light; iii) virulence factors, such as cytolysin and aggregation substance; and iv) bacteriocins. In addition, intercellular transmissibility is frequently a plasmid-determined trait. As in many bacterial species, plasmids generally range in size from 3–4 kb to well over 100 kb and may be present at relatively low copy number (1–2 copies) or up to 20 or more per cell. Table 1 presents a list of enterococcal plasmids recently compiled by one of the authors (Teresa M. Coque).
Conjugation is a primary means for intercellular DNA mobility in enterococci—natural transformation has never been reported, and information is only beginning to be reported with regard to transduction involving a bacteriophage (see Enterococcal bacteriophages and genome defense). Some conjugative plasmids transfer efficiently from donor to recipient in broth, whereas others transfer well only on solid surfaces. In the case of E. faecalis, peptide sex pheromones secreted by recipient cells induce conjugation-related mating functions, determined by certain plasmids (e.g. pAD1, pCF10, and a host of others). Another group of plasmids, such as pMG1 and related elements identified mainly in E. faecium, are also able to transfer efficiently in broth, but do not appear to make use of sex pheromones. A group of plasmids exemplified by pAMβ1 do not transfer well in broth, but are able to move if the cells are on a solid surface. Nonconjugative plasmids are also commonly present in enterococci, and some are readily mobilized by conjugative elements in trans or move via co-integration in some cases. Representatives of some of the above-noted elements have been sequenced, and studies relating to their transfer mechanisms have been published. In addition, reports relating to replication and partitioning provide significant information on the ways in which certain transmissible elements are maintained in their host.
Other types of transmissible elements common in enterococci are the so-called conjugative transposons, which are exemplified by the Tn916 family. Usually found integrated in the chromosome, their movement involves an excision event that results in a non-replicative circular intermediate that is able to transfer conjugatively, followed by insertion into the genome of a recipient cell. Originally identified in E. faecalis, these elements, which commonly encode antibiotic resistance traits, have a broad host range and are widespread among numerous bacterial genera. In a similar vein and as found to be the case for many species of bacteria in recent years, enterococci have been shown to carry a plethora of “genomic islands,” some of which are mobile and called “integrative conjugative elements” (ICEs). Some of these represent “pathogenicity islands” that confer significant virulence traits and even antibiotic resistance.
Rapidly accumulating genomic sequencing data are facilitating identification of the enterococcal “mobilome,” which includes not only transmissible elements, but also insertion sequences, transposons, and integrons that move intracellularly. Studies based on functionality, including replication and maintenance, complement this rapidly expanding picture, and the significant extent to which enterococci have participated in horizontal transfer within the bacterial world is becoming readily apparent. Below we attempt to summarize recent developments in various aspects of mobile genetic elements (MGEs) in enterococci and try to provide a perspective that is relevant to bacterial-human interaction.