Pheromone-responsive plasmids are found predominantly in E. faecalis. Chromosomally encoded lipoprotein fragments (“pheromones”) released by recipient cells are sensed by nearby donor cells and stimulate production of aggregation substance (Asa1, PrgB and others), encoded by the plasmid.32 Aggregation substance interacts with enterococcal binding substance (EBS) on the surface of the recipient cell and stimulates recipient-donor contact that promotes conjugation.30 These plasmids transmit genetic information in a highly efficient manner between E. faecalis strains (1023/donor cell during 4 h of mating), but are largely restricted to this species. pRUM plasmids in E. faecium are similar to pheromone-responsive plasmids in E. faecalis in that they transfer at a high frequency but exhibit a narrow host range.31 In contrast, broad host range plasmids are capable of transferring genetic information to other gram-positive and even gram-negative species,30 but at a lower frequency (1027/donor cell during 4 h of mating) than pheromone-responsive plasmids. Transfer of these plasmids requires close contact between cells. Inc.18-type plasmids are well-known broad host range plasmids that have been implicated in the transfer of vancomycin resistance determinants to S. aureus in recent years.33 Three types of transposons are responsible for most gene mobility in enterococci, Tn3 family transposons, composite transposons, and conjugative transposons.31 The prototypical Tn3 family transposons are Tn917 [conferring macrolide, lincosamide and streptograminB resistance (MLSB)] and Tn1546 (conferring glycopeptide resistance), whereas the prototypical conjugative transposon is Tn916, which confers resistance to minocycline and tetracycline.34 Composite transposons can readily be formed by the interaction of related IS elements that are liberally sprinkled throughout the genome of most clinical enterococcal strains. The movement of these IS elements not only confers mobility to resistance genes, but it promotes cointegration of plasmids with other plasmids and with the bacterial chromosome. β-lactams. Enterococci may develop increased resistance to penicillins through acquisition of β-lactamases or PBP4/5 mutations. Plasmid-mediated bla genes (encoding β-lactamases) were first described in E. faecalis in 1983.35 Since that time, enterococcal β-lactamase production has been rare and described predominantly in E. faecalis. The bla genes in enterococcus are identical to those in S. aureus8 and are often encoded by remnants of staphylococcal β-lactamase transposon Tn552. High-level penicillin resistance in E. faecium is most commonly associated with accumulation of point mutations in the penicillin binding region of PBP5.5 A variety of point mutations have been described in both E. faecium36,37 and E. faecalis.38 Although these point mutations likely originated de novo in individual bacteria under selective pressure from antibiotics, chromosome-to-chro- mosome transfer of low affinity pbp5 genes has been documented in vitro and likely contributes to the dissemination of high-level penicillin resistance in E. faecium.39 Ceftaroline and ceftobiprole, fifth generation cephalosporins, have activity against enterococcus, but may be prone to emergence of resistance with increased clinical use. Clark et al.40 subjected E. faecalis to serial passages of ceftaroline and identified two resistant isolates (one with an MIC of 8 mg/ml and the other with an MIC of 32 mg/ml). Ceftobiprole shows good in vitro activity against E. faecalis with no reports of resistance to date,41,42 but is ineffective against penicillin-resistant clinical strains of E. faecium.43,44 Aminoglycosides. While intrinsic mechanisms result in low- level aminoglycoside resistance, acquisition of mobile genetic elements typically underlies high-level aminoglycoside resistance in both E. faecium and E. faecalis. Ensuing MICs range from 2,000 mg/ml to as high as 128,000 mg/ml.1 Among the genes that encode high-level resistance, the most concerning are those that result in gentamicin and streptomycin resistance because these antibiotics are used for synergistic therapy of serious enterococcal infections. High-level gentamicin resistance most frequently occurs through acquisition of a bifunctional gene encoding APH(2'')-Ia-AAC(6')- Ie.45 These enzymes inactivate gentamicin (and structurally related aminoglycosides) by phosphorylation at the 2'hydroxy position of gentamicin and simultaneous acetylation of the 6'hydroxy position of the other aminoglycosides.46,47 The modified antibiotic is no longer capable of binding to its target on the 30S ribosomal subunit and thereby loses antibacterial activity. Strains that contain aph(2'')- Ia-aac(6')-Ie are clinically resistant to all aminoglycosides except for streptomycin.48 The aph(2'')-Ia-aac(6')-Ie gene is most commonly flanked by IS256 in a composite transposon designated Tn4001 in S. aureus and Tn5281 in E. faecalis. Several other genes have been identified that confer gentamicin resistance, including aph(2'')-Ic, aph(2'')-Id and aph(2'')-Ib.49–51 In comparison with aph(2'')-Ia-aac(6')-1e, these genes are minor contributors to gentamicin resistance in enterococci. Their prevalence varies by geographical region. Importantly, MICs for enterococci harboring aph(2'')-Ic may be as low as 256 mg/ml, an MIC which would be interpreted as gentamicin-susceptible by labs that use an MIC of 500 mg/ml as a cut-off to determine high- level gentamicin resistance. Despite the lower MIC, bacteria expressing these enzymes are resistant to the synergistic activity of cell wall active agents and gentamicin.48 Thus, in geographical area where aph(2'')-Ic is present, laboratories should be alerted to lower the threshold MIC for gentamicin to enhance detection of enterococci that would be resistant to synergy. High-level resistance to streptomycin occurs most commonly through enzymatic modification of the antibiotic or by single point mutations to the ribosome. Two well-described adenylyl- transferases, Ant(6')-Ia and Ant(3'')-Ia, are capable of inactivating streptomycin (and structurally related aminoglycosides).48,52 Enterococci can also develop ribosomal mutations that result in streptomycin resistance. Whereas resistance caused by aminogly- coside-modifying enzymes (AME) will typically have MICs in the 4,000 to 16,000 mg/ml range, ribosomal mutations result in MICs of 128,000 mg/ml.53 Other acquired AMEs have been identified in enterococci, including Aph(3')-IIIa, an aminoglycoside phosphotransferase that confers resistance to kanamycin54 and Ant(4'')-Ia, a nucleotidyltransferase that confers resistance to tobramycin, amikacin, neomycin and kanamycin.55 As these enzymes do not confer gentamicin or streptomycin resistance, they are of less clinical significance. 424 Virulence Volume 3 Issue 5