Penicillin groups
Penicillins grouped by spectrum: natural, antistaphylococcal, aminopenicillins and antipseudomonal, and why β-lactamase inhibitors are added.
All penicillins share the β-lactam ring fused to a thiazolidine ring and kill growing bacteria by blocking the penicillin-binding proteins (PBPs) that cross-link the cell wall. Chemical changes to the side chain created groups with different strengths: resistance to staphylococcal penicillinase, better reach into gram-negative bacteria, or activity against Pseudomonas. Knowing the group tells you what a penicillin covers and what it misses.
Shared features
- Bactericidal: they acylate PBPs and stop peptidoglycan cross-linking, so dividing bacteria lyse
- Time-dependent killing: what counts is the time the level stays above the MIC, which is why most are given several times a day or by extended infusion
- Mostly cleared by the kidneys (nafcillin is the exception, cleared in bile)
- Most reach the cerebrospinal fluid only when the meninges are inflamed
Natural penicillins
Benzylpenicillin (penicillin G) is given intravenously or intramuscularly; phenoxymethylpenicillin (penicillin V) resists stomach acid and is oral. Benzathine and procaine benzylpenicillin are intramuscular depot forms that release the drug slowly: benzathine penicillin treats syphilis, including in pregnancy, and prevents relapses of rheumatic fever. Their spectrum is narrow: streptococci, Treponema pallidum, Neisseria meningitidis and many anaerobes of the mouth. Penicillinase-producing staphylococci destroy them.
Antistaphylococcal (penicillinase-resistant) penicillins
A bulky side chain shields the β-lactam ring from staphylococcal penicillinase. Oxacillin and nafcillin are intravenous; cloxacillin, dicloxacillin and flucloxacillin are oral options in many countries. They are drugs of choice for serious methicillin-susceptible Staphylococcus aureus (MSSA) infections and for skin and soft-tissue infections. They are not a substitute for penicillin G, do not work against MRSA, and miss enterococci, Listeria and gram-negative bacteria.
Aminopenicillins
An amino group helps ampicillin and amoxicillin cross the outer membrane of some gram-negative bacteria (Haemophilus influenzae, Escherichia coli, Proteus) when these do not make β-lactamase. They are key drugs against Enterococcus faecalis and Listeria monocytogenes, and amoxicillin is a first choice for otitis media, sinusitis and community pneumonia in many guidelines. Rash is more frequent with them than with other penicillins.
Antipseudomonal penicillin and β-lactamase inhibitors
Piperacillin reaches Pseudomonas aeruginosa and more Enterobacterales; combined with tazobactam it is a broad intravenous antibiotic for hospital pneumonia, intra-abdominal and complicated urinary infections. The inhibitors clavulanic acid, sulbactam and tazobactam have little antibacterial action of their own; they inactivate many β-lactamases so that the partner is not destroyed. Amoxicillin–clavulanic acid and ampicillin–sulbactam gain activity against β-lactamase-producing H. influenzae, MSSA and B. fragilis. None of these inhibitors stops carbapenemases.
Penicillin allergy
About one in ten patients in the United States reports a penicillin allergy, but when evaluated fewer than one in a hundred are truly allergic. The label often comes from a childhood viral rash, a family history or a side effect such as diarrhoea, and it pushes clinicians to broader, less suitable antibiotics. True reactions are immediate (IgE-mediated, up to anaphylaxis) or delayed (severe rashes, organ or blood injury). A careful history, risk stratification, skin testing and supervised oral challenge can remove a wrong label; people with severe delayed reactions such as Stevens–Johnson syndrome are not challenged.
Test yourself
Think of the answer, then tap the question to see it.
Which penicillin group is the drug of choice for serious MSSA infections?
The antistaphylococcal (penicillinase-resistant) penicillins, such as oxacillin, nafcillin or cloxacillin.
Which penicillins cover Enterococcus faecalis and Listeria?
The aminopenicillins, ampicillin and amoxicillin.
Why is clavulanic acid added to amoxicillin?
It inactivates many β-lactamases, so amoxicillin is not destroyed and covers producers such as some H. influenzae, MSSA and B. fragilis.
Which penicillin is combined with tazobactam to cover Pseudomonas aeruginosa?
Piperacillin.
Of patients who report a penicillin allergy, how many are truly allergic when evaluated?
Fewer than one in a hundred; most labels can be removed after careful evaluation.
Example drugs
- Benzylpenicillin (penicillin G)
- Phenoxymethylpenicillin (penicillin V)
- Benzathine benzylpenicillin
- Procaine benzylpenicillin
- Oxacillin
- Nafcillin
- Cloxacillin
- Dicloxacillin
- Flucloxacillin
- Ampicillin
- Amoxicillin
- Amoxicillin + clavulanic acid
- Ampicillin + sulbactam
- Piperacillin + tazobactam
Connections
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - PENICILLIN G POTASSIUM injection, powder, for solution (DailyMed (U.S. National Library of Medicine), 2026); Nafcillin Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); Piperacillin and Tazobactam Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); Clinical Features of Penicillin Allergy (Centers for Disease Control and Prevention (CDC), 2026).
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Representative model, for educational purposes only.