4.1 Beta-Lactam Antibiotics

How Beta-Lactams Work

All beta-lactam antibiotics share a four-membered beta-lactam ring that binds covalently to penicillin-binding proteins (PBPs) — bacterial enzymes (transpeptidases) that cross-link peptidoglycan strands in the cell wall. Inhibiting PBPs halts cell-wall synthesis, weakens the wall, and triggers autolysin-mediated lysis, making the class bactericidal against actively dividing organisms. Resistance arises through three main routes: beta-lactamase enzymes that hydrolyze the ring, altered PBPs (the mecA-encoded PBP2a of methicillin-resistant Staphylococcus aureus (MRSA)), and porin/efflux changes in gram-negatives.

Beta-lactams exhibit time-dependent killing: efficacy correlates with the percentage of the dosing interval that free drug concentration stays above the minimum inhibitory concentration (MIC), not with peak concentration. This is why dosing frequency matters more than dose size, why prolonged or continuous infusions of piperacillin-tazobactam and meropenem are used for critically ill patients, and why missed doses of penicillins lose efficacy faster than a lower peak would.

Because nearly all beta-lactams are eliminated unchanged by the kidney, renal dose adjustment is a core exam point: creatinine clearance guides reductions for most penicillins, cephalosporins, and carbapenems. Failure to adjust causes accumulation, and high beta-lactam levels in renal impairment are a recognized cause of neurotoxicity and seizures.

The Penicillin Family

• Natural penicillins — benzylpenicillin (penicillin G) and phenoxymethylpenicillin (penicillin V): streptococci, syphilis, and dental/oropharyngeal organisms. Largely destroyed by staphylococcal beta-lactamase, so they do not cover Staphylococcus aureus.
• Aminopenicillins — amoxicillin and ampicillin: add gram-negative coverage (Haemophilus influenzae, Escherichia coli, Proteus mirabilis) plus Listeria and enterococci. Amoxicillin is the workhorse for community respiratory and ear infections. Classic trap: an amoxicillin/ampicillin rash is common in infectious mononucleosis (Epstein-Barr virus) and with concurrent allopurinol — this is a non-IgE-mediated drug eruption, not a true penicillin allergy.
• Anti-staphylococcal penicillins — flucloxacillin and cloxacillin (oral; nafcillin/oxacillin parenterally): resistant to staphylococcal beta-lactamase and therefore first-line for methicillin-susceptible S. aureus (MSSA) skin, bone, and bloodstream infections. Their gram-negative activity is negligible. Flucloxacillin is also associated with cholestatic hepatitis, typically delayed by weeks.
• Antipseudomonal penicillin — piperacillin, almost always combined with the beta-lactamase inhibitor tazobactam: broad gram-positive, gram-negative (including Pseudomonas aeruginosa), and anaerobic cover, making it a standard empirical choice for hospital-acquired pneumonia, febrile neutropenia, and complicated intra-abdominal infection. It causes hypokalemia and can prolong the bleeding time at high doses.

Beta-lactamase inhibitors — clavulanate (with amoxicillin as co-amoxiclav), tazobactam, and sulbactam — have little antibacterial activity alone; they irreversibly inhibit many beta-lactamases, restoring activity against beta-lactamase-producing H. influenzae, Moraxella, E. coli, Klebsiella, and MSSA. They add nothing against MRSA or Pseudomonas species that resist via other mechanisms.

Cephalosporin Generations

Cephalosporins are grouped into generations, and the exam repeatedly tests the spectrum shift:

Key cross-cutting facts: no cephalosporin covers enterococci or atypical organisms (Legionella, Mycoplasma, Chlamydophila). Ceftriaxone is renally and biliarily excreted — it is the major beta-lactam that usually needs no renal dose adjustment — but it concentrates in bile and causes reversible biliary sludging (pseudolithiasis). It is contraindicated in neonates, especially jaundiced or premature infants, because it displaces bilirubin from albumin (raising kernicterus risk) and forms insoluble precipitates with intravenous calcium; it must never be co-infused through the same line as calcium-containing fluids.

Carbapenems and Aztreonam

Carbapenems — meropenem, imipenem/cilastatin, ertapenem — have the broadest beta-lactam spectrum, including extended-spectrum beta-lactamase (ESBL) producers, Pseudomonas (except ertapenem, which notably lacks anti-pseudomonal activity), and anaerobes. They are the go-to agents for ESBL bacteremia. Their signature toxicity is seizure risk — highest with imipenem, lower with meropenem, and amplified by renal impairment, CNS disease, and failure to adjust the dose. Meropenem can also lower valproic acid levels dramatically, an interaction worth remembering. Carbapenems do not cover MRSA, enterococci (except imipenem partially), or atypicals.

Aztreonam is a monobactam active only against aerobic gram-negatives including Pseudomonas. Its clinical niche is its lack of cross-reactivity with penicillins: it is the safe beta-lactam option for a gram-negative infection in a patient with a history of severe immediate penicillin allergy.

Hypersensitivity and Allergy Assessment

Penicillin allergy is reported by roughly 10% of patients, but over 90% of them tolerate penicillin on formal assessment — labels are frequently wrong, and retaining a false label drives broader-spectrum, more toxic, more expensive therapy. Assessment hinges on the reaction phenotype:

• Immediate, IgE-mediated (Type I) — urticaria, angioedema, bronchospasm, anaphylaxis within an hour: avoid penicillins; consider aztreonam or a non-beta-lactam; graded cephalosporin use only after specialist review.
• Severe delayed cutaneous reactions — Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS: never re-challenge with any beta-lactam.
• Mild delayed maculopapular rash (e.g., childhood rash, the amoxicillin-EBV or allopurinol rash): often not a true allergy; many such patients can safely receive penicillins again.

Penicillin-cephalosporin cross-reactivity is driven mainly by similar R1 side chains (e.g., amoxicillin with cefalexin) and is about 1–2% overall — far below the historically quoted 10%. Carbapenem cross-reactivity is under 1%.