Macrolides
One-Sentence Definition
Macrolides (and related ketolides) inhibit the 50S ribosomal subunit, used for atypical respiratory pathogens, some STI alternatives, and select Gram-positive infections — with rising resistance via erm methylases and efflux.
Simple Explanation
They block the bacterial ribosome factory — handy for atypicals like Mycoplasma and Legionella, but resistance is common in streptococci and some STI contexts.
Detailed Scientific Explanation
| Examples | Typical use |
|---|---|
| Azithromycin | CAP atypicals, chlamydia, some GI infections |
| Clarithromycin | H. pylori regimens (Helicobacter pylori) |
| Erythromycin | Legacy; GI motility side uses |
Coverage: atypicals (Mycoplasma pneumoniae, Legionella pneumophila, Chlamydia trachomatis), some Gram-positives; not reliable for serious S. aureus bacteremia alone.
Resistance: erm ribosomal methylation; mef efflux — affects streptococci, staphylococci, GAS treatment failures.
Mechanism
Bind 50S → block translocation → bacteriostatic (often bactericidal at high concentrations for some bugs).
Clinical Importance
- Outpatient CAP when atypical coverage needed
- Not for invasive meningococcal disease monotherapy
- QT prolongation interactions (azithromycin generally lower risk than erythromycin)
AMR Relevance
- M. abscessus macrolide inducible resistance (erm) — phenotypic testing critical (Nontuberculous Mycobacteria)
- Azithromycin resistance in Gonorrhea / chlamydia regions — follow guidelines
Related MOCs
Active Recall Questions
- Two atypical pneumonias covered?
- erm-mediated resistance mechanism?
- Why macrolides fail for serious staph bacteremia alone?