Mechanisms of ampicillin and aminoglycoside resistance in enterococci

Resistance	Typical MIC (mcg/mL)	Mechanism or enzyme	Species	Comment
Ampicillin	4 to >128	Beta-lactamase production	E. faecalis	Rare. Specific beta-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 to 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 active^Δ
			E. faecalis

MIC: minimum inhibitory concentration; PBP: penicillin-binding protein; 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 National Committee for Clinical Laboratory Standards (NCCLS) recommendations^[1] (Refer to UpToDate topic on mechanisms of antibiotic resistance in enterococci).
¶ Testing for HLR to streptomycin should be performed.
Δ Testing for HLR to gentamicin should be performed.

Reference:

National Committee for Clinical Laboratory Standards. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically: Approved standard. Document M7-A6. NCCLS, Pennsylvania.

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Mechanisms of ampicillin and aminoglycoside resistance in enterococci