Table 1. Mechanisms of resistance to E. faecium and E. faecalis (continued) Antibiotic resistance Linezolid Daptomycin Mechanism of resistance rRNA point mutations Methylated rRNA Altered membrane- bound protein Altered membrane- bound protein Altered membrane- bound protein Associated enzyme G2576T, G2505A, L4(F101L) Cfr Cardiolipin sythetase GdpD LiaF Phenotype Linezolid resistance Linezolid, lincosamides, streptogramin A resistance Contributes to Daptomycin resistance through an unknown mechanism Daptomycin resistance, effect is amplified in combination liaF mutation Daptomycin resistance when combined with gdpD mutation Intrinsic, sporadic or associated MGE Sporadic pEF-01 Sporadic Sporadic Sporadic Host range References E. faecalis E. faecium E. faecalis E. faecium E. faecalis E. faecium E. faecalis E. faecium E. faecalis E. faecium 89–91 94 104, 105 105 105 Gram-positive species, downregulation or absence of a two- component signal transduction VncR/S autolytic system also contributes to penicillin tolerance,15 but this mechanism has not been demonstrated in enterococcus. Tolerance may be induced when penicillin is administered by pulsed-dosing. As such, penicillin-naive enterococcal strains may appear susceptible in vitro but develop tolerance after exposure to the drug.16,17 Aminoglycosides. Both E. faecium and E. faecalis are intrinsically resistant to clinically achievable concentrations of aminoglycosides. In E. faecalis, MICs vary for the aminoglyco- sides, with the greatest degree of resistance seen to streptomycin (MIC up to 500 mg/ml). Intrinsic resistance in E. faecalis is attributed to an inability of the aminoglycoside to enter the cell (where they act by inhibiting ribosomal protein synthesis), as demonstrated in experiments by Moellering and colleagues in the early 1970s.18,19 When enterococci were exposed to radiolabeled aminoglycoside with or without penicillin, higher intracellular aminoglycoside concentrations were achieved in the presence of the cell wall synthesis inhibitor. The combination of cell wall active agents and aminoglycosides also resulted in bactericidal activity (bactericidal synergism). These studies provide physiologic context to the long-standing observations of improved clinical outcomes with aminoglycoside-penicillin combination therapy.20 Some enterococci also express chromosomally-encoded enzymes that increase the MIC of aminoglycosides and prevent synergism. Ubiquitous among E. faecium, the aminoglycoside 6' acetyltransferase [AAC(6')-Ii] confers resistance to tobramycin with MICs as high as 1000 mg/ml and to kanamycin.21 Additionally, an efmM-encoded m5C methyltransferase in E. faecium confers low-level resistance to dibekacin, tobramycin and kanamycin.22 EfmM methylates the 16S rRNA resulting in a sterically-hindered ribosome target site.22 Intrinsic enzyme-mediated high-level resistance to neither gentamicin nor streptomycin has been described in enterococci. As such, these drugs retain synergistic activity in enterococci and have consequently emerged as the drugs of choice to achieve synergism in severe infections caused by either E. faecium or E. faecalis.23 Lincosamides and streptogramins. E. faecalis are intrinsically resistant to clindamycin (a lincosamide), quinupristin (streptogra- min B class) and dalfopristin (streptogramin A class) through activity conferred by expression of the lsa gene. lsa is related structurally to ATP-binding cassette (ABC)-efflux pumps, suggest- ing drug efflux as a possible mechanism,24 and was found in 180/ 180 strains of E. faecalis and 0/189 other enterococcus species, suggesting the gene is intrinsic to E. faecalis.24 In general, for clinical resistance to quinupristin-dalfopristin to occur, the bacteria must be resistant to both streptogramin A and streptogramin B. E. faecium harbors a different putative ABC-efflux pump encoded by the intrinsic msrC gene.25 This gene, a close relative of msrA and msrB in staphylococci, confers low-level resistance (MIC 1–2 mg/ml) to streptogramin B compounds, explaining the elevated quinupristin- dalfopristin MICs seen when E. faecium acquires a separate determinant that confers streptogramin A resistance alone. Trimethoprim-sulfamethoxazole. Most bacteria lack the ability to absorb folate from the environment and as such require de novo folate synthesis in order to produce nucleic acids. The antibiotic combination trimethoprim-sulfamethoxazole inhibits two sequential steps in the tetrahydrofolate synthesis pathway, thereby inhibiting folate synthesis and synergistically killing a broad spectrum of bacterial species. Enterococci are unusual in that they can absorb folic acid from the environment, bypassing the effects of trimethoprim-sulfamethoxazole.26 Therefore, in vitro testing of enterococcal susceptibility to trimethoprim-sulfa- methoxazole in a media devoid of folate will yield a susceptible result.27 Despite this apparent in vitro susceptibility, trimetho- prim-sulfamethoxazole is ineffective in treating serious entero- coccus infections.28,29 Acquired Resistance Acquired resistance in enterococci (that which is not intrinsic to the species) can occur through sporadic mutations or through acquisition of foreign genetic material. Horizontal gene exchange among enterococci occurs through the transfer of pheromone- sensitive or broad host range plasmids, or through the movement of transposons. With few exceptions, multiple plasmids and transposons can be identified in clinical strains. These elements may interact with each other and with the bacterial chromosome to form composite mobile elements. For recent reviews of the plasmids and transposons in enterococci, refer to Palmer30 and Hagstead,31 respectively. www.landesbioscience.com Virulence 423