Expansion
All inhibit cell wall synthesis, differing in spectrum and stability
Expansion
Penicillins, in order of broadening spectrum
- Benzylpenicillin and phenoxymethylpenicillin: streptococci, meningococcus, syphilis
- Flucloxacillin: beta lactamase stable, so it covers Staphylococcus aureus. The skin and soft tissue agent
- Amoxicillin: adds some Gram negative cover
- Co-amoxiclav: clavulanate is a beta lactamase inhibitor, adding Staphylococcus and anaerobes
- Piperacillin-tazobactam: broad, including Pseudomonas
Cephalosporins, broadening with each generation while losing Gram positive cover
- 1st (cefalexin): mainly Gram positive
- 2nd (cefuroxime)
- 3rd (ceftriaxone, cefotaxime, ceftazidime): good Gram negative and CSF penetration, so ceftriaxone is the meningitis agent. Ceftazidime covers Pseudomonas
- 4th and 5th: ceftaroline covers MRSA
Carbapenems (meropenem, ertapenem): the broadest, reserved for ESBL producers and severe sepsis. Ertapenem does not cover Pseudomonas.
None cover organisms without a conventional cell wall: Mycoplasma, Chlamydia, Legionella, or MRSA (except ceftaroline), which is why atypical pneumonia needs a macrolide.
Cross reactivity between penicillins and cephalosporins is around 1 to 3 per cent, much lower than the 10 per cent often quoted.