A patient with Enterococcus faecalis endocarditis is planned for ampicillin plus gentamicin combination therapy. Screening for high-level aminoglycoside resistance shows a gentamicin MIC of 1024 microgram/mL. What does this result predict?
- A The isolate will also be resistant to ampicillin
- B Gentamicin should be replaced with a higher dose to achieve synergy
- C Loss of the expected synergistic bactericidal effect of ampicillin plus gentamicin ✓
- D The result reflects only intrinsic low-level resistance and does not affect synergy
Correct answer: C. Loss of the expected synergistic bactericidal effect of ampicillin plus gentamicin
Explanation
High-level aminoglycoside resistance (MIC above 500 microgram/mL for gentamicin, above 2000 for streptomycin) indicates acquisition of aminoglycoside-modifying enzymes such as aph(2")-aac(6'). Cell wall active agents like ampicillin increase aminoglycoside uptake, producing bactericidal synergy only if the organism remains susceptible at high drug levels. Once high-level resistance is present, adding gentamicin adds toxicity without killing benefit. Ampicillin susceptibility is independent of this screen.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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