ntibiotics are classified based on their mechanism of action in a host. Some of them are bactericides (an agent that destroys bacteria) whereas others are bacteriostatic, meaning they may inhibit the growth of organisms and multiplication of bacteria but do not kill them. They act by interfering with the synthesis of folic acid inside the cell anabolism. Another method of classifying antibiotics is based on mode of action as antibiotics interact with an antigen. This chapter covers the fermentation process, which leads to the synthesis of antibiotics. Penicillin G is made by a fungus, Penicillium chrysogenum. The number of antibiotics from fungal sources is few, although they do include penicillin G and V, and cephalosporin C. These three antibiotics are the major starting materials for the semisynthetic β-lactam antibiotics. The systemic antifungal antibiotic griseofulvin is also of fungal origin. Most antibiotics are produced by fermentation using fungi, including streptomycin and the tetracycline family, among numerous others.