Aminopenicillins: Ampicillin and Amoxicillin

Aminopenicillins

• Ampicillin (IV) and Amoxicillin (PO) are aminopenicillins that cover some Gram positives (Streptococcus, Enterococcus, Listeria) but not MSSA.
• They have limited Gram negative coverage, with notable holes in Klebsiella, Moraxella, and SPICE A organisms.
• Aminopenicillins are used to treat upper respiratory infections, sinusitis, otitis media, cellulitis, Listeria infections, UTIs, and early Lyme disease.
• Amoxicillin is the best-absorbed beta lactam (75-90% bioavailability) and is used to treat Enterococcal infections if susceptible (E. faecalis generally susceptible, E. faecium usually not).
• Aminopenicillins can be used with aminoglycosides for synergy in Enterococcal endocarditis.

Anti-Staphylococcal Penicillins

• Methicillin / Nafcillin / Oxacillin (IV) and Dicloxacillin (PO) are anti-Staphylococcal penicillins that cover MSSA and Streptococcus.
• They are used to treat MSSA infections (unless PCN sensitive, which is rare), cellulitis, osteomyelitis, endocarditis, and bacteremia from MSSA.
• They do not cover MRSA and Coag negative Staph is usually resistant (>80%).
• Dicloxacillin is a reasonable oral choice for non-severe cellulitis; for serious MSSA infections, the entire course of therapy must be given intravenously.
• Nafcillin tends to be better tolerated than Oxacillin (less hepatitis and rash).

Anti-pseudomonal PCNs

• Piperacillin and Ticarcillin are anti-pseudomonal PCNs that are usually combined with beta lactamase inhibitors, which confers broader activity.
• The beta-lactamase component does not add activity against Pseudomonas, so if Pseudomonas is sensitive, Piperacillin alone could be used.