Antibiotics are antimicrobial compounds used in the treatment and prevention of bacterial infection. They can either kill or they can inhibit the growth of bacteria. Several antibiotics are also effective against fungi and protozoans. Some are toxic to humans and animals, even when given in therapeutic dosages. Antibacterial antibiotics are commonly classified based on their mechanism of action, chemical structure, or spectrum of activity. Most antibiotics target bacterial metabolic functions or growth processes. Those antibiotics that interfere with the function of essential bacterial enzymes, or that target the bacterial cell wall or the cell membrane, are usually bactericidal. Those that target protein synthesis are usually bacteriostatic. Further categorizations can be made based on target specificity. Use of antibiotics is essential in retaining the purity of particular selection factors in populations of cells. In recent years antibiotics have been found to be valuable in a wide range of life science disciplines. Fermentek has had particular success in manufacturing antibiotics using fermentation processes.