18.01.4.1 Tetracyclines

Probably the best known example of a tetracycline that is used clinically is doxycycline. The tetracyclines, such as doxycycline inhibit the binding of the transfer (t)RNA, which has the amino acid attached, to the bacterial ribosomes, and hence inhibits the synthesis of proteins in the bacteria. The tetracyclines are considered bacteriostatic not bactericidal. Thus, they halt the multiplication of the bacteria, rather than killing the bacteria. The tetracyclines are broad spectrum i.e. stop the replication of many bacteria. The tetracyclines are used orally in the treatment of respiratory tract infections. They are also used in the treatment of acne. However, doxycycline can cause gastrointestinal upsets (nausea, vomiting and diarrhoea). Furthermore, the tetracycyclines combine with calcium and can cause discolouration of teeth and decrease bone growth. For this reason, they are not given to young children or pregnant women. Another problem with the tetracyclines is the development of widespread resistance.

The resistance to the tetracyclines is due to the induction of transporters in the bacteria membrane that remove the tetracyclines from the bacterium. When the tetracyclines are being transported out of the bacteria, there will be reduced concentrations of the tetracyclines in bacterium cell. As the effectiveness of the tetracycline depends on the concentration at the target site, when they are transported out of the cell, the concentration in the cell will drop, and there will be reduced effectiveness.