Natural Penicillins

Agents: penicillin G, penicillin V

All of us have heard of the discovery of penicillin by    Sir Alexander Fleming in 1929. Once penicillin was isolated years later, it had a major impact on society,  particularly in the treatment of wound infections.  The importance of this discovery became apparent during World War II, when the Allies had access  to penicillin and the Axis did not. Unfortunately,  staphylococci quickly became resistant to penicillin,  initiating the search for new beta-lactams and leading to the confusing array of these drugs available today. The development of resistance has narrowed the spectrum of effectiveness of natural penicillins considerably over the past 60 years, such that staphylococci are almost universally resistant to them. Occasionally we still see an isolate of Staphylococcus aureus that is susceptible to penicillin.

Spectrum

Good: Treponema pallidum, most streptococci

Moderate: Streptococcus pneumoniae, enterococci

Poor : almost everything else

Adverse Effects

Similar to those of other beta-lactams.

Important Facts

• Natural penicillins have a very short half-life and must be dosed frequently or given by continuous infusion. Long-acting depot formulations (procaine, benzathine) are available for intramuscular administration. It is important to know the differences among these formulations because the doses vary considerably. It is even more important not to give procaine or benzathine products intravenously, which can be fatal.

• Penicillin V is the oral form of penicillin G.  Other drugs have supplanted it in most, but not all, uses.

• Penicillin G remains the drug of choice for syphilis.

• Because of resistance, penicillin is a poor empiric choice for most infections. Not all text-books and references have been updated to reflect the changes in penicillin use that have resulted from widespread resistance.

What They’re Good For

Syphilis, particularly neurosyphilis. During penicillin shortages, hospitals often reserve its use  for this indication. Penicillin is also used in susceptible streptococcal infections such as pharyngitis or endocarditis.

Don’t Forget!

Other, more conveniently dosed narrow-spectrum beta-lactams are available for bugs treatable with penicillin.

References

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