Macrolides, Fluoroquinolones and Tetracyclines
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Direct answer
Macrolides, fluoroquinolones and tetracyclines are three antibiotic classes that act on bacterial protein synthesis or DNA replication. Macrolides and tetracyclines inhibit ribosomes at the 50S and 30S subunits, while fluoroquinolones block DNA gyrase and topoisomerase IV. The exam-critical toxicities are QT prolongation with macrolides and fluoroquinolones, tendon rupture with fluoroquinolones, and tooth discolouration and bone growth inhibition with tetracyclines in young children.
What you must remember
- Macrolide mechanism: erythromycin, clarithromycin and azithromycin bind the 50S subunit and block translocation, with reliable activity against atypicals such as Mycoplasma, Chlamydia and Legionella.
- Fluoroquinolone spectrum: ciprofloxacin is the anti-pseudomonal member, while levofloxacin and moxifloxacin cover Streptococcus pneumoniae better, and moxifloxacin adds anaerobic cover.
- Tendon rupture: the Achilles tendon is the classic site, with risk rising in patients over 60, those on corticosteroids and transplant recipients.
- QT prolongation: erythromycin and clarithromycin are the macrolides to remember, and moxifloxacin and sparfloxacin are the fluoroquinolones with the strongest QT signal.
- Tetracycline restriction: avoid in pregnancy and under 8 years because of enamel staining and bone growth inhibition; doxycycline needs no renal dose adjustment.
- Chelation rule: calcium, iron, magnesium, aluminium and antacids reduce absorption of tetracyclines and fluoroquinolones, so doses are separated.
- Indian clinical anchors: doxycycline for scrub typhus and leptospirosis, azithromycin for enteric fever, and azithromycin in trachoma control programmes.
Choosing a drug by target, spectrum and toxicity
Three classes overlap in atypical coverage, which is why exam stems often offer them as alternatives. When two classes seem to fit an organism, the tiebreaker is usually the toxicity or the patient's age. The table separates them by target, key spectrum and the toxicity most likely to be tested.
| Feature | Macrolides | Fluoroquinolones | Tetracyclines |
|---|---|---|---|
| Target | 50S ribosome | DNA gyrase and topoisomerase IV | 30S ribosome |
| Key spectrum | Atypicals, Campylobacter, H. pylori | Gram-negatives, pneumococci | Rickettsiae, Chlamydia, Brucella |
| Signature toxicity | QT prolongation, CYP3A4 inhibition | Tendinopathy, QT, dysglycaemia | Tooth staining, paediatric bone effects |
| Typical Indian use | Pertussis, atypical pneumonia | Complicated UTI, gram-negative infections | Scrub typhus, leptospirosis |
Clarithromycin is the macrolide to remember for H. pylori eradication and CYP3A4 interactions. Erythromycin estolate is the formulation linked with cholestatic hepatitis, which is why it appears in toxicity questions. Doxycycline is the tetracycline used most often in practice because it needs no renal adjustment, and it remains the default choice for rickettsial disease in adults.
Where the exam tries to catch you
NEET-PG stems usually hide the drug class behind a patient's age, a co-medication or an organism.
- Trap: a 70-year-old on prednisolone who develops heel pain is treated as plantar fasciitis. Correct: age over 60 with corticosteroids is the classic setup for Achilles rupture, so the fluoroquinolone is stopped at the first tendon symptom.
- Trap: a child with atypical pneumonia is given doxycycline by default. Correct: azithromycin is the safer choice in children, and tetracyclines are reserved for life-threatening infections where benefit clearly outweighs staining risk.
- Trap: a macrolide is added to an azole or a statin without thought. Correct: clarithromycin inhibits CYP3A4 and prolongs QT, so the interaction and ECG are checked before the combination is used.
Frequently asked questions
Which antibiotic class causes tendon rupture most often in NEET-PG?
Fluoroquinolones cause tendon rupture most often, and the Achilles tendon is the classic site. Risk rises with age over 60, corticosteroid use and transplant status, so the drug is stopped at the first tendon pain.
Why are tetracyclines avoided in children under 8 years?
Tetracyclines bind calcium in developing teeth and bone, causing permanent yellow-grey staining of enamel and inhibiting bone growth. Short courses are accepted for life-threatening infections such as rickettsial disease, where the benefit outweighs the staining risk.
Which macrolide inhibits CYP3A4 most strongly?
Clarithromycin inhibits CYP3A4 far more strongly than azithromycin, which has minimal enzyme interaction. This is why clarithromycin interacts with statins and carbamazepine, and why azithromycin is preferred when interactions matter.
Which fluoroquinolone is preferred for pneumococcal pneumonia?
Levofloxacin and moxifloxacin are preferred over ciprofloxacin for pneumococcal pneumonia because they have stronger activity against Streptococcus pneumoniae. Ciprofloxacin remains the anti-pseudomonal choice among fluoroquinolones.
Why are calcium antacids separated from doxycycline or ciprofloxacin doses?
Calcium, iron, magnesium and aluminium form chelates with doxycycline and ciprofloxacin in the gut, which reduces absorption. The doses are separated, with the exact gap set by the product label.