Why does resistance to a macrolide often also mean resistance to clindamycin?

Because macrolides and lincosamides bind overlapping sites on the 50S subunit, a single change such as methylation of a key 23S rRNA adenine by an erm enzyme can reduce binding of both classes at once, producing the linked MLS resistance phenotype.

How do macrolides stop a bacterium from making proteins if they do not block the very first peptide bond?

They sit in the tunnel through which the growing protein leaves the ribosome, so once the chain has been extended a few residues it runs into the bound drug and elongation stalls, halting production of the full-length protein.

Macrolides and Lincosamides

Macrolides (such as erythromycin, clarithromycin, and azithromycin) and lincosamides (such as clindamycin) are antibiotic classes that bind the 50S ribosomal subunit near the entrance of the nascent-peptide exit tunnel and block elongation of the growing protein chain. They are generally bacteriostatic and share overlapping binding sites and resistance mechanisms despite their distinct chemistry.

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Definition

Macrolides are antibiotics built around a large macrocyclic lactone ring, and lincosamides are structurally distinct agents; both bind the 50S subunit at the exit tunnel adjacent to the peptidyl transferase centre and block the elongation of nascent polypeptides, generally producing a bacteriostatic effect.

Scope

This topic covers the ribosomal binding site, the mechanism by which these agents arrest translation, the typically bacteriostatic action, and the cross-resistance that links the two classes. It is a pharmacological reference entry and does not provide prescribing or dosing guidance.

Core questions

Where on the 50S subunit do macrolides and lincosamides bind, and how does that block translation?
Why are these agents usually bacteriostatic?
What molecular change accounts for cross-resistance between macrolides and lincosamides?
How does target-site methylation confer the MLS resistance phenotype?

Key concepts

50S subunit and 23S rRNA binding
Nascent-peptide exit tunnel obstruction
Bacteriostatic elongation arrest
Overlapping macrolide and lincosamide binding sites
MLS_B resistance phenotype
erm-encoded 23S rRNA methylation
Efflux and enzymatic resistance

Mechanisms

Macrolides and lincosamides bind within the large (50S) ribosomal subunit at a site formed largely by the 23S ribosomal RNA, located at the entrance to the tunnel through which the nascent polypeptide leaves the ribosome, close to the peptidyl transferase centre. Crystal structures of the 50S subunit bound to these antibiotics showed that the drug occludes this tunnel, so that as the peptide chain elongates it collides with the bound antibiotic and translation stalls; because the ribosome is blocked rather than destroyed, the effect is usually bacteriostatic. The binding sites of macrolides and lincosamides overlap, which is why a single resistance change can affect both classes: methylation of a key adenine residue in the 23S rRNA by erm-encoded methyltransferases reduces drug binding and produces the combined macrolide-lincosamide-streptogramin B (MLS_B) resistance phenotype. Additional resistance arises through active efflux and through enzymatic drug inactivation.

Clinical relevance

Macrolides and lincosamides are widely used against respiratory, skin, and certain atypical and anaerobic infections, and their shared binding site explains both their overlapping spectrum and the cross-resistance that can compromise the whole group at once. This entry describes the pharmacological mechanism of the classes for reference and is not a basis for individual treatment decisions.

Evidence & guidelines

The exit-tunnel binding site is established by crystal structures of 50S-antibiotic complexes, and the resistance mechanisms, particularly erm-mediated rRNA methylation and the MLS phenotype, are compiled in reviews of macrolide resistance and in standard pharmacology references.

History

Erythromycin, the prototype macrolide, was introduced in the early 1950s, and the lincosamides lincomycin and later clindamycin followed. The genetic and biochemical basis of MLS resistance through rRNA methylation was worked out over subsequent decades, and the precise location of the drugs at the exit tunnel of the 50S subunit was visualized directly when high-resolution structures of antibiotic-bound large subunits became available around 2001.

Key figures

Ada E. Yonath
Alexander Mankin
Bernard Weisblum

Seminal works

schlunzen-2001
fyfe-2016

Frequently asked questions

Why does resistance to a macrolide often also mean resistance to clindamycin?: Because macrolides and lincosamides bind overlapping sites on the 50S subunit, a single change such as methylation of a key 23S rRNA adenine by an erm enzyme can reduce binding of both classes at once, producing the linked MLS resistance phenotype.
How do macrolides stop a bacterium from making proteins if they do not block the very first peptide bond?: They sit in the tunnel through which the growing protein leaves the ribosome, so once the chain has been extended a few residues it runs into the bound drug and elongation stalls, halting production of the full-length protein.