Is Lincomycin (Lincosin) a Macrolide?
No, lincomycin (Lincosin) is not a macrolide—it is a lincosamide antibiotic that is structurally distinct from macrolides despite sharing similar mechanisms of action and resistance patterns. 1
Structural Classification
- Lincomycin belongs to the lincosamide class of antibiotics, which is structurally different from macrolides 1
- Macrolides are defined by a macrocyclic lactone ring of 12 or more elements (14-membered rings like erythromycin and clarithromycin, or 15-membered rings like azithromycin) 1
- Lincosamides (lincomycin and clindamycin) have a completely different chemical structure that does not contain this characteristic lactone ring 2, 3
Shared Functional Properties (Why the Confusion Exists)
Despite being structurally distinct, lincosamides and macrolides share important functional similarities:
- Both bind to the 50S ribosomal subunit to inhibit bacterial protein synthesis 1, 4
- Specifically, lincomycin binds to the 23S RNA of the 50S subunit, similar to macrolides 4
- Both are predominantly bacteriostatic but can be bactericidal at higher concentrations 2, 5
- Cross-resistance exists between lincosamides and macrolides due to shared resistance mechanisms, particularly erm gene-mediated methylation of the 23S ribosomal RNA 1
Clinical Implications of the Distinction
- Macrolide-resistant organisms should be tested for inducible resistance to lincosamides using the D-zone test, as resistance may be shared despite the structural differences 4
- The British Thoracic Society notes that "such resistance may also be associated with resistance to lincosamides such as clindamycin" when discussing macrolide resistance mechanisms 1
- This cross-resistance pattern (MLSB phenotype) means that if a pathogen is resistant to macrolides, it may also be resistant to lincosamides, even though they are different drug classes 4, 2
Common Pitfall
The most important clinical pitfall is assuming that because lincosamides and macrolides are structurally different, they won't have cross-resistance—in reality, organisms with erm-mediated macrolide resistance will often be resistant to lincosamides as well, requiring careful susceptibility testing before prescribing either class 1, 4