There are 4 classes of penicillins, based upon their ability to kill various types of bacteria. From narrow to broad range of effectiveness they include:
- Natural Penicillins (Penicillin G, Procaine, Penicillin G, Penicillin V, Benzathine). The natural penicillins were the first agents in the penicillin family to be introduced for clinical use. The natural penicillins are based on the original penicillin-G structure. They are effective against gram-positive strains of streptococci, staphylococci, and some gram-negative bacteria such as meningococcus. Penicillin V is the drug of choice for the treatment of streptococcal pharyngitis. It is also useful for anaerobic coverage in patients with oral cavity infections.
- Penicillinase-Resistant Penicillins (Cloxacillin, Dicloxacillin, Methicillin, Nafcillin, Oxacillin). Methicillin was the first member of this group, followed by oxacillin, nafcillin, cloxacillin and dicloxacillin. The penicillinase-resistant penicillins have a more narrow spectrum of activity than the natural penicillins. Their antimicrobial efficacy is aimed directly against penicillinase-producing strains of gram-positive cocci, particularly Staphylococcal species and these drugs are sometimes called anti-staphylococcal penicillins.
- Aminopenicillins (Ampicillin, Amoxicillin, Bacampicillin). The aminopenicillins were the first penicillins discovered to be active against gram-negative bacteria (such as E. coli and H. influenzae). Aminopenicillins are acid-resistant so administered orally. Orally administered amoxicillin and ampicillin are used primarily to treat mild infections such as otitis media, sinusitis, bronchitis, urinary tract infections and bacterial diarrhea. Amoxicillin is the agent of choice for the treatment of otitis media.
- Extended Spectrum Penicillins (sometimes called anti-pseudomonal penicillins). Extended Spectrum Penicillins include both alpha-carboxypenicillins (carbenicillin and ticarcillin) and acylaminopenicillins (piperacillin, azlocillin, and mezlocillin). These agents have similar spectrums of activity as the aminopenicillins but with additional activity against several gram negative organisms of the family Enterobacteriaceae, including many strains of Pseudomonas aeruginosa. Like the aminopenicillins, these agents are susceptible to inactivation by beta-lactamases. These agents may be used alone or in combination with Aminoglycosides.
When in doubt, ask your pharmacist.