An Enterococcus faecalis blood culture isolate is reported as penicillin susceptible and vancomycin susceptible. Before recommending ampicillin plus gentamicin synergy for endocarditis therapy, which additional susceptibility test must be performed?
- A Screening for high-level aminoglycoside resistance using a 120 microgram gentamicin disk ✓
- B Standard 10 microgram gentamicin disk diffusion
- C Serum bactericidal titres against the isolate
- D Time-kill assay at one quarter the MIC of gentamicin
Correct answer: A. Screening for high-level aminoglycoside resistance using a 120 microgram gentamicin disk
Explanation
High-level aminoglycoside resistance (HLAR) predicts failure of the aminoglycoside-beta-lactam synergistic combination. It is screened with high-content disks: gentamicin 120 micrograms or streptomycin 300 micrograms, with zones below 10 mm indicating resistance. The conventional low-concentration disk result does not predict synergy because HLAR is usually due to aminoglycoside-modifying enzymes such as aph(2'')-aac(6'), which raise the MIC far above achievable serum levels even when the routine disk appears susceptible.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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