The known antibiotics produced by actinomycetes range in complexity from very simple compounds, such as nitropropionic acid (bovinocidin), to very complex proteins, such as the lytic enzymes of actinomycetin. Certain chemical types are linked with a specific range of antimicrobial activity. For example, the polyenes are mainly antifungal in nature, and the exceptions reported in the literature may very well be due to antibacterial impurities. Also the streptothricins are active against both gram-positive and gram-negative bacteria and even fungi, whereas the nonpolyenic macrolides are, as a whole, active only against gram-positive bacteria. In spite of these general correlations between types of structure and activity, it is not yet possible to predict in all cases the range of antimicrobial activity of an antibiotic purely on the basis of its chemical structure. Likewise, it is not yet possible to synthesize compounds, the biological properties of which would be known beforehand.
In this chapter, the antibiotics are grouped according to their chemical similarities. More detailed information and references will be found in Part B of this book and in the reviews written on this subject recently by Abraham and Newton ( 1958), Chain ( 1958), Van Tamelen ( 1958), Harman ( 1959), and also in the Merck Index ( 1960). From a chemical point of view, antibiotics can be grouped according to various criteria: (1) the elements contained in their molecules, (2) the most important groupings in these molecules, and (3) the structure of their molecular skeleton.
An examination of the empirical formulas of the antibiotics of actinomycetes listed in Part B of this book reveals that most of these substances contain carbon, hydrogen, oxygen, and nitrogen (153 compounds). Next in frequency are substances containing only carbon, hydrogen, and oxygen (31 compounds), followed by substances containing carbon, hydrogen, oxygen, nitrogen, and sulfur (18 compounds). This survey takes into account only those substances for which an empirical formula was at least proposed; however, their number is probably high enough to be representative of the whole group. A few substances have unusual elementary compositions: chloramphenicol, chlortetracycline, demethylchlortetracycline, and exfoliatin contain nonionic chlorine. Bromine can be substituted for this chlorine in chlortetracycline to form bromtetracycline, and possibly a similar substitution can be made in the other substances. Exfoliatin contains only carbon, hydrogen, oxygen, and chlorine, whereas the other antibiotics of this group contain, in addition, nitrogen. Two antibiotics have unique elementary composition: grisein contains sulfur and iron, and phleomycin contains