**6. Antimicrobial resistance plasmids**

Mobile element acquisition is a major contributor to acquired resistance in enterococci. Multi-drug resistant strains possess larger genomes.

The gene that encodes resistance to Vancomycin and Teicoplanin in enterococci is known as the *van* gene with many variant subtypes ranging from *van*A to *van*E, *van*G and then *van*L, M and N. Of these *van*A and B are predominant. Both are easily transmissible via the plasmid carrying transposons. While *van*A carries resistance to both, *van*B carries resistance to Vancomycin only. The predominant transposons are numbered Tn1545 and 1547. These genes encode a change in the peptidoglycan of the cell wall.

*E. faecalis*: The resistance rates to Vancomycin is steadily increasing in *E. faecalis* strains too, reaching about 5% now. *E. faecalis* generally is associated with *van*A and *van*B gene clusters and less frequently with *van* L, M, N.

*E. faecium*: Resistance to Vancomycin and Teicoplanin is widespread among *E. faecium* strains (VREfm) and in USA, Australia and some regions of Europe, prevalence *Enterococcal Infections: Recent Nomenclature and Emerging Trends DOI: http://dx.doi.org/10.5772/intechopen.104792*

is more than 50%. Resistance to these antibiotics is encoded on transposons which ride on plasmids and are easily transferable to other species like staphylococci. In a hospital environment, such resistant strains have the survival advantage and they replace the normal flora. In such a situation, outbreaks due to VREfm may occur especially in critical care units. WGS could identify two distinct clades within VREfm, Clade 1 being more hospital associated and Clade 2 being community (animal) associated. The crux of the problem is that VREfm has evolved to fit the niche in a hospital by easily acquiring plasmids, prophages, genomic islands, transposons and even has its own enterococcus cassette chromosome. It has lost the clustered regularly interspaced short palindromic repeats (CRISPR) self-defence systems that protect from genomic modification [11–13].
