Tetracyclines (TCN) -- Tetracycline, Minocycline, Demeclocycline, Doxycycline, Oxytetracycline, and Chlortetracycline (the latter two are only used topically and in ophthalmic preparations).

Mechanism of Action -- The TCNs bind to the 30S subunit of bacterial ribosomes in a manner similar to the AG antibiotics.  Specifically, they prevent the binding of transfer RNA to the 30S subunit, thereby inhibiting bacterial protein synthesis.  Refer to Diagram from the AG Section.

Spectrum -- The TCNs are broad spectrum agents effective against Gram positive (less activity) and Gram negative (greater activity) bacteria, plus activity against Rickettsia, Chlamydia, Legionella, Plasmodium, and Corynebacterium spp and Vibrio choleræ.  It is the activity against Corynebacterium that makes the TCNs effective in the treatment of acne.  Resistance has developed extensively among Gram positive organisms.  Consequently, the TCNs are used primarily in the treatment of infections caused by Gram negative and other organisms.

Therapeutics -- The TCNs are used in the treatment of brucellosis, cholera, legionnaire's disease (Legionella pneumophila), tularemia, rocky mountain spotted fever (RMSF -- rickettsial), lyme disease (Borellia spp.), and psittacosis (chlamydial, from parrots), and acne.

Adverse Effects --

GI upset, can be severe including burning and ulceration -- They may be taken with food but NOT with dairy products, which will limit their absorption through calcium chelation.

Photosensitivity -- especially with demeclocycline and doxycycline

Hepatotoxicity -- less with oxytetracycline and tetracycline

Nephrotoxicity -- not as bad as with the AG

Effect on teeth and bones -- TCNs are strong chelators of calcium.  If administered to children with developing teeth, the TCN may become incorporated with the calcium causing discolouration.  Exposure in utero may inhibit fœtal bone growth (this may be reversible if duration of therapy is relatively short).

Other Adverse Effects -- Pseudotumour cerebri (increased intracranial pressure), WBC dyscrasias, and minocycline may cause a dose-dependent ototoxicity (more common in females).

Mechanism of Action -- Similar to the TCNs

Spectrum -- Primarily effective against Gram negative organisms.

Theratpeutics -- Spectinomycin is used in human medicine primarily as a second or third choice (following the beta-lactams and FQ) for the treatment of gonorrhœa.  It is given parenterally only.

Adverse Effects -- Relatively few, may see urticaria, chills, fever, dizziness, nausea, insomnia.