resistant strain revealed three mutated genes: cls, gdpD and liaF. A cardiolipin synthetase mutation was identified but when the mutant gene was placed in daptomycin-sensitive enterococcus strain the MIC did not change. The same cls mutation was observed in other E. faecalis and E. faecium strains resistant to daptomycin. The other two gene mutations, gdpD (glyceropho- sphoryl diester phosphodiesterase) and liaF (lipid II cycle- interfering antibiotic protein), did have an impact on MICs when reconstituted in the daptomycin-susceptible strain. The liaF mutation increased the MIC from 1 to 4 mg/ml. The gdpD mutation did not increase the MIC, but the combination of both proteins increased the MIC to 12 mg/ml. Mutations in both gdpD and liaF were also identified in other resistant strains of E. faecalis and E. faecium, but were not demonstrated in the Palmer study. Thus, cardiolipin synthetase, GdpD and LiaF are cell membrane proteins associated with daptomycin resistance. Given that a number of different membrane-associated proteins have been linked to reduced daptomycin susceptibility in staphylococci, it seems likely that more genes conferring enterococcal resistance to daptomycin will be identified in the future. Tigecycline. Tigecycline, a novel glycylcycline antibiotic, gained FDA approval in 2005 for complicated intra-abdominal infections, skin and soft tissue infections, and community-acquired pneu- monia. It has been used off-label to successfully treat MRSA and GRE infections.115 Typical tigecycline MICs for enterococcus range from 0.125 mg/ml to 0.25 mg/ml, while MICs . 0.5 mg/ml are considered resistant. Early surveillance studies of tigecycline showed no cases of resistant enterococcus,116 although two case reports of E. faecalis strains with MICs of 2 mg/ml and 6 mg/ml, respectively, have been described.117,118 A more recent study from Taiwan reviewed antimicrobial resistance among 219 VRE isolates and found two isolates with a tigecycline MIC of 0.5, and one isolate with an MIC of 1, with a trend toward increasing tigecycline MIC over time.119 The mechanism of tigecycline resistance in enterococcus is unknown. In staphylococcus, tigecycline resistance is mediated by a novel family of efflux pumps,120 but these genes have not been demonstrated in enterococcus. Other antibiotics. Resistance occurs to other antibiotics including macrolides, tetracyclines, chloramphenicol, fosfomycin, rifampin and quinolones. These resistance mechanisms will not be described in this review, as resistance to these antimicrobial agents is so common that they are seldom involved in treatment of enterococcus infections. Management Because of the differences in resistance patterns between E. faecium and E. faecalis, it is imperative to differentiate the pathogen to the species level and perform susceptibility testing on strains isolated from patients with clinical infections. Treatment of enterococcal infections depends upon (1) the species, (2) the resistance patterns present in the clinical isolate and (3) the location and severity of the infection. Uncomplicated enterococcal infections may be adequately treated with monotherapy, whereas severe infections such as endocarditis benefit from a synergistic regimen. In uncomplicated, fully susceptible E. faecalis and E. faecium infections, ampicillin remains the preferred therapy. In the uncommon presence of β-lactamase, combination with a β-lactamase inhibitor such as sulbactam may improve outcomes. When complicated infections such as endocarditis occur in susceptible enterococcal infections, an aminoglycoside should be added to a cell wall active agent for synergistic killing, as has been the standard for almost 60 years.20 Among aminoglycosides, only gentamicin and streptomycin should be considered for synergistic therapy. Historically, twice-daily or three times daily aminoglyco- side dosing regimens have been used. In streptococcus infections, once-daily aminoglycoside dosing was shown to be effective in humans.121 For enterococcal endocarditis, though, once-daily aminoglycoside dosing has only been studied in animal models with evidence for122–124 and against125,126 its use. Differences between these studies likely reflect the pharmacodynamic and pharmacokinetic differences between the animal models. In humans, the efficacy of once-daily aminoglycoside dosing has not been established for enterococcus infections. As such, guidelines continue to recommend three times daily dosing for gentamicin and twice-daily dosing for streptomycin.127 In the presence of high-level penicillin resistance, synergy has been observed in animal models with the combination of aminoglyco- sides and other cell wall active antibiotics including vancomycin or daptomycin.128 In the instance of complicated enterococcal infections resistant to high-levels of gentamicin and streptomycin, an alternative agent must be used for synergistic activity. Despite relative resistance to both agents, the combination of ceftriaxone and ampicillin has been shown to be efficacious in animals.129 In case reports and prospective case-series, 56 patients have been treated with this combination with success rate of 71.4% (40/56);130-132 although these numbers may reflect publication bias. The presumed benefit of ceftriaxone-ampicillin combination therapy is attributed to full saturation of PBPs 2–4, which cannot be achieved with either agent alone. By inhibiting all PBPs, the bacteria have no alternative enzyme with which to build a cell wall. In in vitro and animal model studies, similar synergistic bactericidal activity has been shown with other cell wall active combination therapies, including ceftriaxone-fosfomycin133 and ampicillin-imipenem,134 but not with ampicillin-ertapenem.135 Treatment of glycopeptide-resistant enterococcus. Vancomycin- resistant enterococci pose particular problems for treatment because the strains which harbor VanA and VanB resistance are also typically resistant to other classes of antibiotics. While only linezolid and quinupristin-dalfopristin have FDA approval for treatment of GRE infections, other antimicrobial agents including daptomycin, tigecycline, fosfomycin, quinolones, tetracyclines and new fifth generation cephalosporins exhibit in vitro activity and have been used with success in individual cases. In uncomplicated cases, monotherapy based upon the antibiotic susceptibility profile is appropriate. In complicated cases such as endocarditis, the ideal therapy for GRE has not been determined. Both linezolid and quinupristin-dalfopristin have been shown to be efficacious in treatment of complicated GRE infections and are FDA approved for this indication.56,136 Linezolid has been 428 Virulence Volume 3 Issue 5