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©2013 UpToDate ® Print Email Mechanisms of ampicillin and aminoglycoside resistance in enterococci Resistance Typical MIC (µg/mL) Mechanism or enzyme Species Comment Ampicillin 4 - >128 β-lactamase production E. faecalis Rare. Specific β-lactamase test is required (eg, nitrocefin) ≥16 Low affinity PBP, increased production of PBP5, or penicillin insensitive transpeptidation E. faecium For isolates with MIC ≤64, high dose ampicillin might be effective Aminoglycosides* HLR Gentamicin >500 Bifunctional enzyme AAC(6')-Ie-APH(2")-Ia E. faecium Common; streptomycin remains active• E. faecalis >500 APH(2")-Id E. faecium Rare; streptomycin remains active• E. casseliflavus 64 - 256 APH(2")-Ic E. gallinarum It may not be detected by routine screening tests E. faecium Streptomycin remains active• E. faecalis >500 APH(2")-Ib E. faecium Streptomycin remains active• HLR Streptomycin >1000 Adenylyltransferase (ANT), ribosomal mutation E. faecium Gentamicin remains activeΔ E. faecalis HLR: high level resistance; APH: aminoglycoside phosphotransferase; AAC: aminoglycoside acetyltransferase. * Screening for HLR to aminoglycosides should be performed on a routine bases when a synergistic effect is required following the NCCLS recommendations [National Committee for Clinical Laboratory Standards. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically: Approved standard. Document M7-A6. NCCLS, Pennsylvania] (see text). • Testing for HLR to streptomycin should be performed. Δ Testing for HLR to gentamicin should be performed.