Tetracyclines and Glycylcyclines

Tetracyclines (such as tetracycline and doxycycline) and the glycylcyclines (such as tigecycline) are broad-spectrum bacteriostatic antibiotics that bind the 30S ribosomal subunit and block the attachment of incoming aminoacyl-tRNA to the ribosome. Glycylcyclines are tetracycline derivatives engineered to overcome the two most common tetracycline-resistance mechanisms.

Definition

Tetracyclines are polycyclic antibiotics that bind the 30S subunit and prevent aminoacyl-tRNA from binding the ribosomal A site, arresting elongation; glycylcyclines are semisynthetic tetracycline derivatives with a substituent that restores activity against organisms resistant by efflux or ribosomal protection.

Scope

This topic covers the ribosomal target, the mechanism of elongation blockade, the broad spectrum of these agents, the principal resistance mechanisms (efflux and ribosomal protection), and how the glycylcyclines were designed to evade them. It is a pharmacological reference entry, not prescribing guidance.

Core questions

• How do tetracyclines block elongation at the 30S subunit?
• Why are tetracyclines considered broad-spectrum and bacteriostatic?
• What are the two dominant tetracycline-resistance mechanisms?
• How do glycylcyclines such as tigecycline overcome classical tetracycline resistance?

Key concepts

• 30S A-site binding
• Blockade of aminoacyl-tRNA accommodation
• Bacteriostatic elongation arrest
• Broad-spectrum activity (including atypical organisms)
• Tet efflux pumps
• Ribosomal protection proteins
• Glycylcycline evasion of efflux and protection

Mechanisms

Tetracyclines bind the 30S ribosomal subunit at a site overlapping the aminoacyl (A) site, where structural studies of the 30S subunit localized the primary binding position. By occupying this site they sterically prevent the incoming aminoacyl-tRNA from being accommodated into the ribosome, so each round of elongation is interrupted; because this arrest is reversible and does not destroy the ribosome, the effect is bacteriostatic. Bacteria resist tetracyclines chiefly by two routes: energy-dependent efflux pumps (encoded by tet genes) that expel the drug, and ribosomal protection proteins that dislodge the drug from its binding site. Glycylcyclines such as tigecycline carry a bulky substituent on the tetracycline core that strengthens ribosomal binding and renders them poor substrates for common efflux pumps and resistant to ribosomal protection, restoring activity against many tetracycline-resistant strains.

Clinical relevance

Tetracyclines and glycylcyclines are valued for broad-spectrum coverage that includes several atypical and intracellular pathogens, and their mechanism explains both their bacteriostatic behaviour and the design rationale of the glycylcyclines as agents for resistant organisms. This entry describes the pharmacology of the classes for reference and is not a guide to drug selection or dosing.

Evidence & guidelines

Mode of action, spectrum, and the efflux and ribosomal-protection resistance mechanisms are compiled in comprehensive reviews of the tetracycline class, while the 30S binding position is established by crystal structures of the small subunit; class pharmacology is summarized in standard references.

History

Chlortetracycline, the first tetracycline, was discovered in the late 1940s, and the class quickly became a mainstay of broad-spectrum therapy. As efflux- and ribosomal-protection-mediated resistance spread, the glycylcyclines were developed as later-generation derivatives, with tigecycline introduced in the mid-2000s to address those resistance mechanisms. The molecular details of tetracycline binding to the 30S subunit were clarified by ribosomal crystallography around 2000.

Key figures

• Benjamin M. Duggar
• Ian Chopra
• Marilyn Roberts

Seminal works

• chopra-roberts-2001
• carter-2000

Frequently asked questions

How do glycylcyclines like tigecycline get around tetracycline resistance?

They keep the tetracycline core but add a bulky side chain that binds the ribosome more tightly and makes the drug a poor target for the common efflux pumps and ribosomal protection proteins that defeat older tetracyclines, so they remain active against many resistant strains.

Why are tetracyclines described as broad-spectrum?

Because they block a step of elongation common to many bacteria, they inhibit a wide range of Gram-positive and Gram-negative organisms and also several atypical and intracellular pathogens, giving the class an unusually broad reach.