Penicillins & Cephalosporins
Pharmacology · Antimicrobials · lean revision notes
Penicillins & Cephalosporins
The beta-lactams are the most clinically important class of antibacterials and a perennial NEET PG favourite. This note builds the topic from the beta-lactam ring outward: mechanism, classification, spectrum, resistance, the beta-lactamase inhibitor combinations, allergy and cross-reactivity, and the high-yield "drug of choice" tables that examiners love.
Definition & the beta-lactam family
A beta-lactam is any antibiotic whose core structure contains a four-membered beta-lactam ring. Four sub-classes exist and all share the same mechanism but differ in spectrum and stability:
| Sub-class | Examples | Ring fusion | Note |
|---|---|---|---|
| Penicillins | Benzylpenicillin, amoxicillin, piperacillin | Beta-lactam + thiazolidine | Oldest, narrow→extended spectrum |
| Cephalosporins | Cefazolin, ceftriaxone, cefepime, ceftaroline | Beta-lactam + dihydrothiazine | 5 generations |
| Carbapenems | Imipenem, meropenem, ertapenem | Beta-lactam + 5-membered C ring | Broadest spectrum |
| Monobactams | Aztreonam | Beta-lactam ring alone (monocyclic) | Gram-negative only; safe in penicillin allergy |
High-yield: Aztreonam is a monobactam with a monocyclic beta-lactam ring. It has essentially no cross-reactivity with penicillins (except a shared side chain with ceftazidime) and is the go-to beta-lactam for a Gram-negative infection in a patient with severe penicillin allergy.
Mechanism of action
Beta-lactams are bactericidal and act on the bacterial cell wall:
- The cell wall is a peptidoglycan mesh; its strength comes from cross-linking of peptide side chains.
- The final cross-linking (transpeptidation) is catalysed by penicillin-binding proteins (PBPs), which are transpeptidase enzymes.
- The beta-lactam ring is a structural analogue of the terminal D-alanyl-D-alanine of the peptidoglycan precursor.
- Beta-lactam covalently acylates the active-site serine of the PBP, irreversibly inactivating the transpeptidase.
- Cross-linking fails → weak cell wall + activation of bacterial autolysins (murein hydrolases) → osmotic lysis and death.
Beta-lactam → binds PBP → blocks transpeptidation → no cross-linking → autolysin activation → bacterial lysis.
High-yield: Beta-lactams are time-dependent killers — efficacy correlates with the time the free drug concentration stays above MIC (T > MIC), not the peak. This is why penicillins are given as frequent doses or continuous/extended infusions, unlike aminoglycosides (concentration-dependent).
Because they act on cell-wall synthesis, beta-lactams are most active against actively dividing organisms and are ineffective against organisms lacking peptidoglycan (e.g., Mycoplasma, Chlamydia, Legionella intracellularly, and true fungi).
Penicillins — classification & spectrum
| Group | Drugs | Spectrum highlights | Beta-lactamase stable? |
|---|---|---|---|
| Natural | Benzylpenicillin (Pen G, IV), Penicillin V (oral), benzathine/procaine penicillin (depot) | Streptococci, Treponema pallidum, Clostridium, Actinomyces, meningococcus | No |
| Antistaphylococcal (penicillinase-resistant) | Methicillin, cloxacillin, dicloxacillin, nafcillin, oxacillin | MSSA | Yes (resist staph penicillinase) |
| Aminopenicillins | Ampicillin, amoxicillin | Extended to some Gram-negatives (E. coli, H. influenzae, Listeria, enterococci) | No |
| Carboxy/Ureidopenicillins (antipseudomonal) | Ticarcillin, piperacillin | Adds Pseudomonas and many Gram-negatives | No |
High-yield: Benzathine penicillin G is the depot form giving sustained low levels for ~3–4 weeks — used for syphilis and rheumatic fever prophylaxis. Procaine penicillin gives a shorter depot (~24 h).
High-yield: Amoxicillin > ampicillin for oral use because of far better and food-independent oral bioavailability. Ampicillin (oral) classically causes a non-allergic maculopapular rash when given in infectious mononucleosis (EBV) — a favourite distractor that does not mean true penicillin allergy.
Memory aid for the natural-penicillin spectrum: organisms remain largely Gram-positive plus a few spirochaetes/anaerobes — resistance has eroded much of the historical Gram-negative cover.
Cephalosporins — generational classification
The classic teaching: as generations rise, Gram-negative cover increases and (until the advanced agents) Gram-positive cover decreases, with better CNS penetration in the 3rd generation onward.
| Generation | Representative drugs | Spectrum | Exam pearls |
|---|---|---|---|
| 1st | Cefazolin (IV), cephalexin (oral) | Gram-positive cocci (MSSA, streptococci), some E. coli/Klebsiella/Proteus (PEcK) | Cefazolin = surgical prophylaxis drug of choice |
| 2nd | Cefuroxime; cephamycins: cefoxitin, cefotetan | Adds H. influenzae, some anaerobes (Bacteroides for cephamycins) | Cefoxitin/cefotetan cover anaerobes (intra-abdominal/PID) |
| 3rd | Ceftriaxone, cefotaxime, ceftazidime, cefixime (oral) | Strong Gram-negative + good CSF penetration | Ceftazidime = antipseudomonal; ceftriaxone/cefotaxime cross BBB |
| 4th | Cefepime | Broad Gram-positive and Gram-negative incl. Pseudomonas | Stable to many beta-lactamases; febrile neutropenia |
| 5th (advanced) | Ceftaroline, ceftobiprole | Covers MRSA (binds PBP2a) | Only beta-lactams with reliable MRSA cover |
High-yield: Ceftaroline is the only cephalosporin with reliable activity against MRSA because it binds the altered PBP2a. Standard cephalosporins do NOT cover MRSA or enterococci ("LAME" bugs that cephalosporins miss: Listeria, Atypicals, MRSA, Enterococci).
High-yield: Ceftriaxone is biliary-excreted (no renal dose adjustment) but is contraindicated in neonates with hyperbilirubinaemia (displaces bilirubin → kernicterus) and must not be co-administered with calcium-containing IV fluids in neonates (fatal precipitates). In neonatal meningitis use cefotaxime instead.
Mnemonic for CSF-penetrating 3rd-gen agents: think ceftriaxone + cefotaxime + ceftazidime are the cephalosporins reliable for meningitis (ceftazidime when Pseudomonas is the concern).
Resistance mechanisms
Bacteria evade beta-lactams by four routes:
- Beta-lactamase (penicillinase/cephalosporinase) production — hydrolyse the beta-lactam ring. This is the commonest mechanism and the target of inhibitor combinations.
- Altered PBP target — e.g., MRSA produces PBP2a (encoded by mecA) with low beta-lactam affinity; penicillin-resistant pneumococci have mutated PBPs.
- Reduced permeability — loss of outer-membrane porins in Gram-negatives (e.g., OprD loss → carbapenem resistance in Pseudomonas).
- Efflux pumps — actively export the drug.
Important beta-lactamase types: ESBLs (extended-spectrum, inactivate 3rd-gen cephalosporins; treat with carbapenems), AmpC (chromosomal, inducible in Enterobacter/Serratia/Citrobacter), and carbapenemases (e.g., KPC, NDM-1, OXA-48) — NDM-1 ("New Delhi metallo-beta-lactamase") is a high-yield Indian-origin term.
High-yield: MRSA resistance = PBP2a (mecA gene), not beta-lactamase — so adding a beta-lactamase inhibitor does not restore activity. Treat MRSA with vancomycin, linezolid, daptomycin, or ceftaroline.
Beta-lactamase inhibitor combinations
These inhibitors have weak intrinsic antibacterial action but bind and inactivate beta-lactamases ("suicide inhibitors"), protecting the partner beta-lactam.
| Inhibitor | Combined with | Brand-style pairing | Covers |
|---|---|---|---|
| Clavulanic acid | Amoxicillin / ticarcillin | Co-amoxiclav | Adds MSSA, H. influenzae, Moraxella, anaerobes |
| Sulbactam | Ampicillin / cefoperazone | Ampicillin-sulbactam | Sulbactam itself has anti-Acinetobacter activity |
| Tazobactam | Piperacillin / ceftolozane | Pip-tazo | Broad incl. Pseudomonas, anaerobes |
| Avibactam (novel, non-beta-lactam) | Ceftazidime | Ceftazidime-avibactam | KPC and OXA-48 carbapenemase producers |
| Vaborbactam / Relebactam | Meropenem / imipenem | — | KPC carbapenemase producers |
High-yield: Classic inhibitors (clavulanate, sulbactam, tazobactam) do NOT inhibit AmpC or metallo-beta-lactamases (NDM-1). The newer agents avibactam/vaborbactam restore activity against KPC/serine-carbapenemases but not against metallo-enzymes like NDM-1.
High-yield: Clavulanic acid is the inhibitor most associated with adverse effects — GI upset and cholestatic hepatitis (co-amoxiclav is a leading cause of drug-induced cholestatic liver injury).
Penicillin allergy & cross-reactivity
Penicillins are the most common cause of drug allergy. Reactions are classified by the Gell-Coombs system:
| Type | Mechanism | Timing | Example |
|---|---|---|---|
| Type I (IgE) | Immediate hypersensitivity | Mins–1 h | Urticaria, anaphylaxis |
| Type II | IgG/cytotoxic | Days | Haemolytic anaemia (high-dose Pen G) |
| Type III | Immune complex | 1–3 weeks | Serum sickness |
| Type IV | T-cell mediated | Days | Maculopapular rash, SJS/TEN |
Cross-reactivity with cephalosporins is the most tested concept:
- Historical quoted rate of ~10% was inflated by early contamination; true cross-reactivity is far lower (~1–2%).
- Cross-reactivity is driven by similarity of the R1 side chain, not the shared beta-lactam ring.
- 1st-generation cephalosporins (and cefamandole) share R1 side chains with penicillins → higher cross-reactivity.
- 3rd/4th-generation cephalosporins have dissimilar side chains → very low cross-reactivity.
High-yield: In a patient with a history of non-severe penicillin allergy, later-generation cephalosporins (different side chain) are generally safe. In severe/anaphylactic penicillin allergy, avoid all beta-lactams except aztreonam (no meaningful cross-reactivity, barring the ceftazidime side-chain overlap).
Management of anaphylaxis (rapid recall): IM adrenaline (0.5 mg, 1:1000) → oxygen + IV fluids → antihistamine + hydrocortisone as adjuncts. Adrenaline is first-line and life-saving; never delay it for steroids.
Pharmacokinetics & adverse effects
- Excretion: mostly renal (tubular secretion blocked by probenecid, which prolongs penicillin levels — a classic MCQ). Exceptions excreted in bile: ceftriaxone, cefoperazone, nafcillin, oxacillin (no renal adjustment).
- CSF penetration: poor when meninges are normal; improves with inflammation. 3rd-gen cephalosporins (ceftriaxone/cefotaxime) achieve therapeutic CSF levels.
- Adverse effects (beyond allergy):
- Jarisch-Herxheimer reaction when treating syphilis with penicillin (fever, myalgia from spirochaete lysis).
- High-dose penicillin → seizures (especially in renal failure), interstitial nephritis (methicillin).
- Disulfiram-like reaction and hypoprothrombinaemia/bleeding with cephalosporins bearing the N-methylthiotetrazole (NMTT) side chain — cefoperazone, cefotetan, cefamandole.
- Broad-spectrum agents → C. difficile pseudomembranous colitis.
High-yield: Cephalosporins with the NMTT side chain (cefoperazone, cefotetan, cefamandole) cause a disulfiram-like reaction with alcohol and vitamin-K-dependent bleeding. Mnemonic for the culprits: think "MTT = MeTronidazole-like reaction" cephs.
Drug-of-choice (clinical scenarios)
This is the single most exam-relevant table.
| Condition | Drug of choice |
|---|---|
| Bacterial meningitis (empirical, adult) | Ceftriaxone/cefotaxime + vancomycin (± ampicillin if Listeria risk: >50 y, neonate, immunocompromised) |
| Neonatal meningitis | Ampicillin + cefotaxime (avoid ceftriaxone) |
| Infective endocarditis — viridans streptococci | Benzylpenicillin (± gentamicin) |
| Endocarditis — MSSA | Cloxacillin/nafcillin (cefazolin if mild allergy) |
| Endocarditis — enterococcal | Ampicillin + gentamicin (or ampicillin + ceftriaxone) |
| Community-acquired pneumonia (CAP), typical | Amoxicillin (outpatient); ceftriaxone + macrolide (inpatient) |
| Surgical prophylaxis | Cefazolin |
| Syphilis | Benzathine penicillin G |
| Listeriosis | Ampicillin (± gentamicin) |
| Gonorrhoea | Ceftriaxone (+ azithromycin/doxycycline) |
| Pseudomonas infection | Piperacillin-tazobactam / ceftazidime / cefepime |
| Actinomycosis | High-dose penicillin G |
High-yield: Empirical adult meningitis = ceftriaxone + vancomycin; add ampicillin to cover Listeria at the extremes of age and in the immunocompromised — a near-guaranteed question.
High-yield: Cefazolin is the universal surgical prophylaxis agent (given within 60 min of incision) because of its MSSA/strep cover, good tissue levels, and favourable safety profile.
Complications & special situations
- Penicillin + aminoglycoside synergy in enterococcal/streptococcal endocarditis: penicillin breaches the cell wall, allowing aminoglycoside entry. Do not mix in the same syringe — aminoglycoside is inactivated.
- Drug interactions: probenecid raises penicillin levels; combining with bacteriostatic agents (e.g., tetracyclines) may antagonise bactericidal action in meningitis.
- Resistant pneumococcus: rising penicillin MIC is why vancomycin is added empirically in meningitis pending sensitivities.
Key differentials / "looks similar" comparisons
- Penicillins vs cephalosporins vs carbapenems: all beta-lactams, all PBP inhibitors; carbapenems are broadest and most beta-lactamase stable; imipenem is combined with cilastatin to block renal dehydropeptidase-1 (prevents nephrotoxic metabolite) — a classic distractor against beta-lactamase inhibitors.
- Vancomycin vs beta-lactams for cell wall: vancomycin binds D-Ala-D-Ala terminus directly (not PBP), so it works against MRSA and beta-lactam-allergic patients; it is not a beta-lactam.
- Aztreonam vs cephalosporins: aztreonam covers only aerobic Gram-negatives, no Gram-positive/anaerobe cover; safe in penicillin allergy.
Recently asked / exam angle
- Mechanism MCQs: "Beta-lactams act by inhibiting transpeptidase (PBP) / cross-linking of peptidoglycan."
- "Cefazolin — surgical prophylaxis"; "Ceftriaxone contraindicated in neonates / with calcium."
- "Only cephalosporin active against MRSA = ceftaroline (binds PBP2a)."
- "Cephalosporins do not cover enterococci, Listeria, MRSA, atypicals (LAME)."
- "NDM-1 / metallo-beta-lactamase not inhibited by clavulanate/avibactam."
- "Ampicillin rash in EBV infectious mononucleosis = not true allergy."
- "Disulfiram-like reaction with cefoperazone/cefotetan (NMTT side chain)."
- "Empirical adult meningitis = ceftriaxone + vancomycin (+ ampicillin for Listeria)."
- "Beta-lactams are time-dependent (T > MIC) bactericidals."
- "Imipenem given with cilastatin (dehydropeptidase-1 inhibitor)."
Rapid revision
- All beta-lactams inhibit PBP transpeptidase → block peptidoglycan cross-linking → autolysin-mediated lysis (bactericidal, time-dependent).
- Aztreonam = monobactam, Gram-negative only, safe in penicillin allergy.
- Benzathine penicillin G = syphilis and rheumatic fever prophylaxis (depot, ~3–4 weeks).
- Amoxicillin has better oral bioavailability than ampicillin; ampicillin rash in EBV is not allergy.
- Cephalosporins: rising generation → more Gram-negative, better CSF penetration; ceftaroline (5th) = only one covering MRSA.
- Cephalosporins miss LAME = Listeria, Atypicals, MRSA, Enterococci.
- Ceftriaxone is biliary-excreted; avoid in neonates (kernicterus, calcium precipitation) — use cefotaxime.
- Cefazolin = surgical prophylaxis drug of choice.
- MRSA = PBP2a (mecA); beta-lactamase inhibitors do not help — use vancomycin/linezolid/ceftaroline.
- Classic inhibitors (clavulanate, sulbactam, tazobactam) don't touch AmpC or NDM-1 metallo-enzymes; avibactam/vaborbactam cover KPC but not metallo-enzymes.
- NMTT side chain (cefoperazone, cefotetan, cefamandole) → disulfiram-like reaction + bleeding.
- Empirical adult meningitis = ceftriaxone + vancomycin (± ampicillin for Listeria); anaphylaxis treated with IM adrenaline first.