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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Beta-lactams

Antibiotics with a four-atom β-lactam ring that stop bacteria from building their cell wall: penicillins, cephalosporins, carbapenems and monobactams.

Updated 2026-11-01

## What they are

The largest and most used family of antibiotics. All share the β-lactam ring and kill bacteria that are growing by blocking the last step of cell-wall assembly.

## Mechanism of action

The β-lactam ring looks like the D-Ala–D-Ala end of the peptidoglycan building block. Penicillin-binding proteins (PBPs, the transpeptidases that cross-link the wall) attack the ring with their active-site serine and stay stuck as an acyl-enzyme, so new cross-links stop forming. The weakened wall, helped by the bacterium's own autolysins, gives way and the cell bursts. Killing depends on the time the drug level stays above the MIC rather than on peak levels.

Target: Penicillin-binding proteins (bacterial DD-transpeptidases)

## Key points

- Three ways bacteria resist: β-lactamases that open the ring (including ESBLs and carbapenemases), altered PBPs with low affinity (PBP2a in MRSA) and, in gram-negatives, fewer porins or more efflux.
- β-lactamase inhibitors such as clavulanic acid, sulbactam, tazobactam and avibactam protect the partner β-lactam (amoxicillin–clavulanate, piperacillin–tazobactam).
- MRSA resists almost every β-lactam because of PBP2a; ceftaroline is the cephalosporin designed to bind it.
- A history of a hypersensitivity reaction to any penicillin contraindicates penicillins; cephalosporins carry a smaller cross-reaction risk and need case-by-case judgement.
- Bactericidal and time-dependent: what counts is how long the level stays above the MIC, and bacteriostatic antibiotics such as tetracyclines can blunt the effect.

## Drugs in this class

- [**Benzylpenicillin (penicillin G)** The original penicillin, given by injection. Its slow-release benzathine form treats syphilis and prevents rheumatic… Penicillin-binding proteins (bacterial DD-transpeptidases)](https://durgical.com/en/pharmacology/benzylpenicillin/)
- [**Amoxicillin** A well-absorbed oral penicillin for ear, sinus and chest infections; combined with other drugs it clears H. pylori… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/amoxicillin/)
- [**Ceftriaxone** A third-generation cephalosporin given by injection that reaches the brain and lasts all day; a mainstay for… Penicillin-binding protein 2 (PBP2) and other PBPs](https://durgical.com/en/pharmacology/ceftriaxone/)
- [**Amoxicillin + clavulanic acid** Amoxicillin shielded by clavulanic acid, which blocks β-lactamases; an oral antibiotic for otitis, sinusitis… Bacterial PBPs (amoxicillin) and β-lactamases (clavulanic acid)](https://durgical.com/en/pharmacology/amoxicillin-clavulanic-acid/)
- [**Ampicillin** An intravenous aminopenicillin, key against Listeria and enterococci; with an aminoglycoside it is used in newborn… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/ampicillin/)
- [**Benzathine benzylpenicillin** An intramuscular depot of penicillin G that releases the drug for weeks: the mainstay for syphilis and for preventing… Penicillin-binding proteins (bacterial DD-transpeptidases)](https://durgical.com/en/pharmacology/benzathine-benzylpenicillin/)
- [**Piperacillin + tazobactam** An antipseudomonal penicillin protected by tazobactam; a broad-spectrum intravenous antibiotic for serious hospital… Bacterial PBPs, mainly PBP3 (piperacillin), and β-lactamases (tazobactam)](https://durgical.com/en/pharmacology/piperacillin-tazobactam/)
- [**Ampicillin + sulbactam** Ampicillin protected by sulbactam for mixed intra-abdominal, gynaecological and skin infections. Bacterial PBPs (ampicillin) and β-lactamases (sulbactam)](https://durgical.com/en/pharmacology/ampicillin-sulbactam/)
- [**Cefalexin** A first-generation oral cephalosporin for skin infections, pharyngitis, otitis and urinary infections due to… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/cefalexin/)
- [**Cefazolin** A first-generation intravenous cephalosporin: the usual choice for antibiotic prophylaxis in surgery and for serious… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/cefazolin/)
- [**Ceftazidime** A third-generation intravenous cephalosporin with activity against Pseudomonas aeruginosa, for serious hospital… Penicillin-binding proteins, mainly PBP3](https://durgical.com/en/pharmacology/ceftazidime/)
- [**Cefepime** A fourth-generation intravenous cephalosporin covering Pseudomonas and gram-positives; used in febrile neutropenia and… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/cefepime/)
- [**Cefixime** A third-generation oral cephalosporin for urinary infections, pharyngitis, otitis and uncomplicated gonorrhoea. Penicillin-binding protein 2 (PBP2) and other PBPs](https://durgical.com/en/pharmacology/cefixime/)
- [**Cefotaxime** A third-generation intravenous cephalosporin that crosses inflamed meninges; used for meningitis and sepsis, including… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/cefotaxime/)
- [**Cefuroxime** A second-generation cephalosporin, oral (cefuroxime axetil) or intravenous, for sinusitis, otitis, pneumonia and early… Penicillin-binding protein 2x (PBP2x) and other PBPs](https://durgical.com/en/pharmacology/cefuroxime/)
- [**Cefoxitin** A second-generation intravenous cephamycin active against anaerobes such as Bacteroides fragilis; used in mixed… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/cefoxitin/)
- [**Ceftaroline** An intravenous cephalosporin active against methicillin-resistant Staphylococcus aureus (MRSA), for skin infections… MRSA PBP2a and other PBPs](https://durgical.com/en/pharmacology/ceftaroline/)
- [**Phenoxymethylpenicillin (penicillin V)** An oral penicillin stable in stomach acid for streptococcal pharyngitis and preventing relapses of rheumatic fever. Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/phenoxymethylpenicillin/)
- [**Cloxacillin** A penicillinase-resistant antistaphylococcal penicillin for infections due to methicillin-susceptible Staphylococcus… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/cloxacillin/)
- [**Dicloxacillin** An oral penicillinase-resistant penicillin for skin and soft-tissue infections due to methicillin-susceptible… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/dicloxacillin/)
- [**Oxacillin** An intravenous penicillinase-resistant penicillin for serious infections by methicillin-susceptible Staphylococcus… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/oxacillin/)
- [**Nafcillin** An intravenous antistaphylococcal penicillin cleared in bile; beware of extravasation and its effect on warfarin. Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/nafcillin/)
- [**Flucloxacillin** A penicillinase-resistant penicillin, oral or injected, for staphylococcal skin, wound and bone infections; it rarely… Penicillin-binding proteins (bacterial transpeptidases)](https://durgical.com/en/pharmacology/flucloxacillin/)
- [**Ceftazidime + avibactam** Ceftazidime protected by avibactam, a non-β-lactam inhibitor active against KPC and OXA-48; reserved for resistant… Bacterial PBPs (ceftazidime) and class A, C and some class D β-lactamases (avibactam)](https://durgical.com/en/pharmacology/ceftazidime-avibactam/)
- [**Cefiderocol** A siderophore cephalosporin that enters bacteria through iron transporters; active against multidrug-resistant… Penicillin-binding proteins, mainly PBP3](https://durgical.com/en/pharmacology/cefiderocol/)
- [**Ceftolozane + tazobactam** A newer cephalosporin combined with tazobactam for urinary, abdominal and pneumonia infections due to resistant… Bacterial PBPs (ceftolozane) and β-lactamases (tazobactam)](https://durgical.com/en/pharmacology/ceftolozane-tazobactam/)
- [**Procaine benzylpenicillin** A procaine salt of penicillin G for intramuscular injection, shorter-acting than the benzathine salt; it can cause a… Penicillin-binding proteins (bacterial DD-transpeptidases)](https://durgical.com/en/pharmacology/procaine-benzylpenicillin/)

## Others in the class

- phenoxymethylpenicillin
- ampicillin
- oxacillin
- dicloxacillin
- piperacillin
- cefazolin
- cefalexin
- cefuroxime
- cefepime
- ceftaroline
- imipenem
- meropenem
- ertapenem
- aztreonam
- clavulanic acid (β-lactamase inhibitor)
- sulbactam (β-lactamase inhibitor)
- tazobactam (β-lactamase inhibitor)
- avibactam (β-lactamase inhibitor)

## Test yourself

Think of the answer, then tap the question to see it.

## Which part of the peptidoglycan building block does the β-lactam ring imitate?

The terminal D-alanyl–D-alanine. The PBP mistakes the ring for its substrate, opens it with its serine and remains acylated, so it can no longer cross-link the wall.

## How does MRSA resist β-lactams, and which β-lactam still works against it?

It carries mecA, which encodes PBP2a, a transpeptidase with very low affinity for β-lactams. Ceftaroline binds PBP2a and keeps activity.

## What does clavulanic acid add to amoxicillin?

It irreversibly blocks many β-lactamases, so amoxicillin is not destroyed by the bacteria that make them.

## Why are β-lactams called time-dependent antibiotics?

Their killing depends on the fraction of time the free drug level stays above the MIC, not on how high the peak is.

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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 - amoxicillin- Amoxicillin capsule amoxicillin- Amoxicillin tablet, film coated](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=649d285c-8fcb-48dd-aa5e-2f34128102f5) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - CEFTRIAXONE injection, powder, for solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=8351aa37-552d-471d-b293-c564dcb6ec29) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - PENICILLIN G POTASSIUM injection, powder, for solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=1442486a-05d6-4dce-8b95-857a30f478c2) (DailyMed (U.S. National Library of Medicine), 2026).

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