Beta-Lactam Antibiotics and Beta-Lactamase Inhibitors
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Direct answer
Beta-lactam antibiotics are bactericidal drugs whose beta-lactam ring inhibits penicillin-binding proteins, the enzymes that cross-link peptidoglycan in the bacterial cell wall. They include penicillins, cephalosporins, carbapenems and monobactams, and their spectrum depends on the class and generation. Beta-lactamase inhibitors such as clavulanate, sulbactam and tazobactam protect susceptible partner drugs from enzymatic breakdown.
What you must remember
- Mechanism: inhibition of penicillin-binding proteins, which stops peptidoglycan cross-linking and kills actively dividing bacteria.
- Penicillinase-resistant penicillins: cloxacillin and flucloxacillin, used for methicillin-sensitive Staphylococcus aureus.
- Aminopenicillins: ampicillin and amoxicillin, which cover Listeria, enterococci and Haemophilus influenzae.
- Cephalosporins: grouped into generations, with broader gram-negative cover from the second generation and anti-pseudomonal activity from ceftazidime onwards.
- Carbapenems: the broadest class, including meropenem and ertapenem, with no activity against MRSA.
- Monobactam: aztreonam is active against aerobic gram-negative bacteria only.
- Beta-lactamase inhibitors: clavulanate, sulbactam, tazobactam and avibactam; clavulanate has little intrinsic antibacterial activity.
- Adverse effects: hypersensitivity including anaphylaxis, and imipenem seizures when renal function is impaired.
Comparing the major classes
A 60-year-old man with a urinary infection caused by an ESBL-producing Escherichia coli needs an agent whose spectrum matches the organism. The table compares representative drugs by class and the main organisms they cover, which helps narrow the choice.
| Class | Representative drug | Main organisms covered |
|---|---|---|
| Penicillinase-resistant penicillin | Cloxacillin | Methicillin-sensitive Staphylococcus aureus |
| Aminopenicillin | Amoxicillin | Enterococci, Listeria, Haemophilus influenzae |
| Third-generation cephalosporin | Ceftriaxone | Pneumococcus, meningococcus, Enterobacterales |
| Fourth-generation cephalosporin | Cefepime | Enterobacterales and Pseudomonas aeruginosa |
| Carbapenem | Meropenem | ESBL-producing gram-negative bacilli |
| Monobactam | Aztreonam | Aerobic gram-negative bacilli |
Choice depends on local sensitivity patterns, the site of infection, the patient's allergy history and renal function, since many doses need adjustment.
Narrow-spectrum agents are preferred once the organism and its sensitivity are known, to limit resistance. Route matters too: benzylpenicillin is acid-labile and is given parenterally, whereas amoxicillin is well absorbed orally. Serious infections are usually managed with a choice guided by culture results, and the first empirical agent is revised once sensitivities return.
Where the exam tries to catch you
Questions on this topic usually test the spectrum of each class, the MRSA rule and the reason for combining drugs with inhibitors.
- Trap: all cephalosporins cover MRSA. Correct: only the fifth-generation agents ceftaroline and ceftobiprole cover MRSA, and the others do not.
- Trap: a penicillin-allergic patient cannot receive any cephalosporin. Correct: cross-reactivity is low, especially with dissimilar side chains, and aztreonam is an option for gram-negative infection.
- Trap: carbapenems are the first choice for MRSA. Correct: carbapenems have no activity against MRSA, which is covered by vancomycin, linezolid or ceftaroline.
- Trap: imipenem can be given alone. Correct: it is given with cilastatin, which blocks renal dehydropeptidase and prevents the drug being inactivated in the kidney.
Frequently asked questions
What is the mechanism of action of beta-lactam antibiotics?
Beta-lactam antibiotics inhibit penicillin-binding proteins, the transpeptidases that cross-link peptidoglycan in bacterial cell walls. This makes them bactericidal against actively dividing bacteria. The beta-lactam ring is the structural feature that gives the class its name.
Why does ceftriaxone need caution in neonates?
Ceftriaxone needs caution in neonates, particularly those with hyperbilirubinaemia, because it can displace bilirubin from albumin. It should also not be given together with intravenous calcium, which can cause precipitation. Alternatives are chosen when these risks apply.
Which cephalosporin covers MRSA?
Ceftaroline, a fifth-generation cephalosporin, is the cephalosporin with activity against MRSA. Ceftobiprole shares this property. Other cephalosporins, including ceftriaxone and cefepime, lack anti-MRSA activity and cannot be relied on for MRSA infection.
Why is cilastatin given with imipenem?
Imipenem is given with cilastatin because renal dehydropeptidase-1 would otherwise inactivate imipenem in the kidney. Cilastatin inhibits this enzyme, preserving drug levels in the urine. Imipenem with cilastatin carries a risk of seizures when renal function is impaired.
What is aztreonam used for?
Aztreonam is a monobactam active against aerobic gram-negative bacteria, including Pseudomonas aeruginosa. It has no activity against gram-positive organisms or anaerobes. Its side chain differs from that of penicillins, so it is used when gram-negative cover is needed in a patient with penicillin allergy.