Aminopenicillins

Agents: amoxicillin, ampicillin

Though the antistaphylococcal penicillins improve on the Gram-positive coverage of natural penicillins, they do not add to their Gram-negative coverage. Aminopenicillins are more water-soluble and pass through porin channels in the cell wall of some Gram-negative organisms. However, they are susceptible to beta-lactamases, and resistance to them has become fairly common in many institutions. Aminopenicillins are rarely active against staphylococci, because these almost always produce penicillinases. Also, remember that these drugs do not have useful activity against  Pseudomonas aeruginosa.

Spectrum

Good: streptococci, enterococci

Moderate: enteric GNRs, Haemophilus

Poor : staphylococci, anaerobes

Adverse Effects

Similar to those of other beta-lactams. Aminopenicillins have a high incidence of diarrhea when given orally.

Important Facts

• Though ampicillin can be given orally, amoxicillin is a better choice. It is more bioavailable,  better tolerated, and administered less frequently. Use ampicillin for IV therapy, and amoxicillin for oral therapy. Europeans dis-agree; they use amoxicillin intravenously also.

• Ampicillin is a drug of choice for susceptible enterococci.

What they’re good For

Infections caused by susceptible GNRs, enterococci, and streptococci. Because resistance among GNRs is prevalent, aminopenicillins are used only infrequently in complicated nosocomial infections.  Amoxicillin is frequently prescribed for infections of the upper respiratory tract, including streptococcal pharyngitis (strep throat) and otitis media (ear infection).

Don’t Forget!

To achieve bactericidal activity against enterococci,  ampicillin (or any other beta-lactam) has to be combined with an aminoglycoside. This should be done in serious infections such as endocarditis.

References

Gallagher ,J.C. and MacDougall ,c. (2012). Antibiotics Simplified. Second Edition. Jones & Bartlett Learning, LLC.

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