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Chapter 40: Aminoglycosides

A 36-year-old woman is being treated with gentamicin in hospital for a gram-negative infection. Her trough blood concentrations are below the known minimum inhibitory concentration (MIC) of the bacteria. An infectious disease consultant is not concerned about this because

a. the MIC for most bacteria are notoriously too high.

b. gentamicin plasma concentrations are not an effective way to monitor therapy.

c. gentamicin has residual bacteriocidal activity which persists after the plasma concentration has fallen below the MIC.

d. gentamicin is bacteriostatic.

e. the mechanism of action of gentamicin is inhibition of cell wall synthesis.

Answer is c. A postantibiotic effect, that is, residual bactericidal activity persisting after the serum concentration has fallen below the MIC, is characteristic of the aminoglycoside antibiotics; the duration of this effect is concentration-dependent.

A 64-year-old man is suffering from a gas gangrene infection of his right foot due to a mid-calf arterial clot. The bacteria involved is Clostridium perfringens. An aminoglycoside is not a good choice for antibiotic therapy because aminoglycosides are

a. bacteriostatic.

b. not active against anaerobic bacteria.

c. more toxic under anaerobic conditions.

d. only effective if they are injected directly into anaerobic tissue.

e. metabolized more rapidly under anaerobic conditions.

Answer is b. Aminoglycosides must penetrate intracellularly to be effective. Transport of aminoglycosides across the cytoplasmic membrane is an oxygen-dependent active process. Thus, anaerobic bacteria are resistant to the aminoglycosides because they lack the necessary transport system.

A 26-year-old woman has the diagnosis of bacterial meningitis. The organism cultured is a gram-negative Pseudomonas susceptible to gentamicin. There is concern about treatment of this patient with gentamicin because

a. aminoglycosides are not generally active against gram-negative bacteria.

b. aminoglycosides can cause seizures at therapeutic concentrations.

c. patients with meningitis have a tendency to be more susceptible to the nephrotoxic effects of the aminoglycosides.

d. aminoglycosides are polar and do not penetrate the central nervous system.

e. aminoglycosides are rapidly metabolized in the brain and do not reach therapeutic concentrations.

Answer is d. Because of their polar nature, the aminoglycosides do not penetrate into most cells, the CNS, or the eye. Concentrations of ...

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