A clinical isolate of Enterococcus faecalis from a patient with urinary tract infection shows high-level resistance to gentamicin (MIC greater than 500 microgram/mL) on sensitivity testing. What is the therapeutic implication of this finding?
- A High-level gentamicin resistance eliminates the synergistic bactericidal effect of combining a cell-wall active agent with an aminoglycoside ✓
- B The organism is resistant to all aminoglycosides and no beta-lactam can be combined with any aminoglycoside for synergy
- C The isolate must be treated with vancomycin regardless of ampicillin sensitivity
- D High-level gentamicin resistance indicates the organism is also intrinsically resistant to ampicillin
Correct answer: A. High-level gentamicin resistance eliminates the synergistic bactericidal effect of combining a cell-wall active agent with an aminoglycoside
Explanation
High-level aminoglycoside resistance (HLAR) in enterococci, defined as gentamicin MIC greater than 500 microgram/mL or streptomycin MIC greater than 1000-2000 microgram/mL, means the aminoglycoside cannot enter the bacterial cell in sufficient concentration to achieve synergy with a cell-wall active agent (ampicillin or vancomycin). This eliminates the standard synergistic combination therapy. It does not automatically mandate vancomycin if the isolate remains ampicillin-sensitive, nor does it imply ampicillin resistance.
Reference: Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 7th ed.
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