• broad spectrum
• eg demeclocycline, doxycycline, tetracycline, oxytetracycline
• indicated for use against some gram positive and gram negative organisms
• Interfere with protein synthesis by binding to 30S subunit of bacterial ribosome  inhibit binding of transfer RNA to the messenger RNA-ribosome complex
• have difficulty penetrating mammalian cell wall (lack the active transport system that is needed)  selective toxicity
• food impairs absorption
• Development of resistance is common
• adverse effects – GI irritation, yellow or brown discolouration of teeth, liver and renal toxicity, photosensitivity

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