Intensive research in the United States then led to the isolation of especially productive Penicillium strains for use in the mass production of the antibiotic. A large number of semisynthetic penicillins have been developed in attempts to overcome the disadvantages of natural penicillins. Scientists develop these penicillins in either of two ways. First, they can stop synthesis of the molecule by Penicillium and obtain only the common penicillin nucleus for use. Second, they can remove side chains from the completed natural molecules and then chemically add other side chains that, among other things, make them more resistant to penicillinase.
Antibiotics are rather easy to discover, but few are of medical or commercial value.
Some are used commercially other than for treating human disease-for example, in veterinary medicine. A few antibiotics are produced by bacteria of the genus Bacillus, and others are produced by molds, mostly of the genera Penicillium and Cephalosporium . The table shows the sources of many antibiotics in use today. Bacteriocins Table of selected bacteriocins Lactic acid bacteria have been used in food production for many years. The bacterial fermentations contribute to flavor and preservation.