CHAPTER 10 



CHEMICAL NATURE OF ANTIBIOTIC SUBSTANCES 



CLASSIFICATION OF ANTIBIOTIC SUBSTANCES 



Antimicrobial agents are of either chemical or biological origin. The 

 first comprise inorganic (heavy metals, halogens) and organic (phenols, 

 arsenicals, dyes, aromatic oils) compounds. The second include a variety 

 of products of higher plants (quinine, chaulmoogra oil, wheat flour pro- 

 tein), higher animals (lactenin, lysozyme), and microorganisms, to 

 which the term "antibiotic" is specifically applied. 



The property possessed by culture filtrates of many bacteria of inhib- 

 iting the growth of bacterial cells has long been recognized (506). The 

 suggestion has even been made that all bacteria, when tested at the right 

 age and under proper conditions of culture, are able to produce anti- 

 bacterial substances (70). It is now definitely established, however, 

 that this property is characteristic of only certain strains of specific bac- 

 teria, fungi, and actinomycetes. 



Antibiotic substances of microbial origin are primarily bacteriostatic 

 in nature. They are selective in their action. Some substances affect 

 largely gram-positive bacteria j their action upon gram-negative bacteria 

 is more limited as regards both the kinds affected and the concentration 

 required to bring about growth inhibition. Other substances may inhibit 

 alike the growth of certain members of both groups of bacteria. One is 

 fully justified, therefore, in speaking of a characteristic bacteriostatic 

 spectrum for each antibiotic substance. The production of antibiotic sub- 

 stances by specific microorganisms is influenced by the strain of the or- 

 ganism, the composition of the medium, the temperature of incubation, 

 the age of the culture, aeration, and certain other factors. Antibiotic sub- 

 stances also vary greatly in their mode of action upon the bacterial cells, 

 in their toxicity to animals, and in their practical utilization for the treat- 

 ment of human and animal diseases. 



The more important antibiotic substances are described briefly in 

 Table 32. They may be classified on the basis of their origin from spe- 

 cific microorganisms, their chemical properties, or their biological ac- 



