Gentamicin should be administered for the initial 2 weeks of therapy with either -lactam or vancomycin- containing regimens (Class I; Level of Evidence C). Enterococci Although there are >15 species within the Enterococcus genus, E faecalis and E faecium are the major species isolated from clinical sources in IE patients. Enterococci are the third leading cause of IE and account for 10% of cases in non- IDUs. E faecalis causes 97% of cases of IE; E faecium, 1% to 2%; and other species, 1%. Regimens recommended for enterococcal IE are shown in Tables 12 through 15. Enterococci should be routinely tested in vitro for susceptibility to penicillin or ampicillin and van- comycin (MIC determination) and for high-level resistance to gentamicin to predict synergistic interactions (see below). Because of the striking increase in resistance of enterococci to vancomycin, aminoglycosides, and penicillin, additional susceptibility tests may be necessary to identify alternative antimicrobial regimens. For strains of enterococci resistant to -lactams, vancomycin, or aminoglycosides, it is reason- able to test for susceptibility in vitro to daptomycin and line- zolid. Linezolid is bacteriostatic in vitro against enterococci, whereas daptomycin is bactericidal in vitro in susceptible strains. Although rarely identified, -lactamase producing enterococci may account for relapse of infection. Routine screening for -lactamase production is not sensitive enough, and specialized testing will be needed for detection. Compared with VGS and -hemolytic streptococci, enterococci are relatively resistant to penicillin, ampicillin, and vancomycin. These streptococci usually are killed by monotherapy with these antimicrobials, whereas enterococci are inhibited but not killed. Killing of susceptible strains of enterococci requires the synergistic action of penicillin, ampi- cillin, or vancomycin in combination with either gentamicin or streptomycin. Enterococci are relatively impermeable to aminogly- cosides. 