Antistaphylococcal Penicillins

Agents: nafcillin, oxacillin, dicloxacillin, methicillin, cloxacillin

It did not take long for  Staphylococcus species to become resistant to penicillin. Within a few years of penicillin becoming widely available, staphylococcal strains began to produce beta-lactamases, rendering penicillin useless in these infections. The basic structure of penicillin was modified to resist these destructive enzymes, leading to the anti-staphylococcal penicillins. This modification gave these drugs activity against staphylococci that pro-duce penicillinases (beta-lactamases active against penicillins), but did not add to the poor Gram-negative activity of the natural penicillins.

Spectrum

Good: MSSA, streptococci

Poor: GNRs, enterococci, anaerobes, MRSA

Adverse Effects

Similar to those of other beta-lactams, with a possibly higher incidence of AIN.

Important Facts

• Antistaphylococcal penicillins have a short half-life and must be dosed frequently. This presents a problem, because they cause phlebitis. Does your patient have phlebitis? Try a first-generation cephalosporin instead.

• Most antistaphylococcal penicillins are eliminated from the body in large part by the liver and do not need to be adjusted in cases of renal dysfunction.

• These drugs are interchangeable therapeutically. Therefore, Staphylococcus aureus that is susceptible to methicillin (which is no longer used) is susceptible to oxacillin, nafcillin, and the rest. That is, MSSA = OSSA = NSSA, etc.

What They’re Good For

Infections caused by MSSA, such as endocarditis and skin and soft-tissue infections.

Don’t Forget!

Beta-lactams kill staphylococci more quickly than vancomycin, so patients with MSSA infections who lack serious beta-lactam allergies should be switched to beta-lactams, such as antistaphylococcal penicillins.

References

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