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 produce penicillinase. Also, remember that these drugs do not have useful activity against Pseudomonas aeruginosa.
Spectrum
Good: streptococci, enterococci Moderate: enteric Gram-negative rods, Haemophilus POOH staphylococci, anaerobes
Adverse Effects
Similar to other beta-lactams. They 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 intravenous therapy, and amoxicillin for oral therapy. Europeans disagree; they use amoxicillin intravenously also.
- Ampicillin is a drug of choice for susceptible enterococci.
What They're Good For
Infections caused by susceptible Gram-negative rods, enterococci, and streptococci. Because resistance among Gram-negative rods is prevalent, aminopenicillins are used only infrequently in complicated nosocomial infections. Amoxicillin frequently is 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.