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Chapter 5.5 : Screen Tests To Detect High-Level Aminoglycoside Resistance in Enterococcus spp.
High-level aminoglycoside resistance (HLAR) in enterococci is most commonly detected by assessing growth at high concentations of gentamicin (500 µg/ml) and sometimes streptomycin (1,000 µg/ml in broth and 2,000 µg/ml in agar) ( 1 ). Strains that show HLAR to gentamicin will not be synergistically killed with combinations of cell wall-active drugs (generally ampicillin, penicillin, or vancomycin) plus gentamicin. HLAR to gentamicin also means HLAR to tobramycin, netilmicin, amikacin, and kanamycin. HLAR to streptomycin equals resistance to combinations of cell wall-active drugs and streptomycin. See additional information in Appendix 5.5-1 . The agar screen test is described in detail here, and reference is made to the disk diffusion and the broth microdilution MIC test procedures for detecting HLAR.