PRODUCTION OF ANTIBIOTICS 167 



Of the five genera of actinomyeetes, the genus Streptomyces has been 

 most productive of antibiotics, 80 or more preparations being credited so 

 far to species of this genus. Species of Nocardia come next, with seven or 

 eight antibiotic preparations. These are followed by species of Micromono* 

 spora, with four or five preparations, and Thermoactino?nyces, with one 01 

 two. None of the species of Actinomyces (anaerobes) have so far yielded any 

 antibiotics. All those that have found practical application as chemothera- 

 peutic agents are produced by species of Streptomyces. 



The antibiotics produced by actinomyeetes represent a great variety of 

 chemical compounds. These compounds differ greatly in their physical 

 properties, including solubility in water and in organic solvents; in their 

 chemical composition; in their antimicrobial activities both in vivo and 

 in vitro; and in their toxicity to animals. These differences are frequently 

 not only qualitative but also quantitative in nature. Hence, the differences 

 emphasized in these keys are often not sufficiently sharp to make such 

 separation distinct. Frequently, a compound placed in one position in the 

 keys could just as easily have been placed in another on the basis of certain 

 minor differences. The difficulties involved in the classification of these 

 antibiotics become particularly sharp when one realizes that many of them 

 have not been crystallized. Some antibiotics have not been sufficiently 

 described, either because of lack of information or because of certain 

 priority questions, whereas others still represent fairly crude preparations. 



The logical basis for the classification of these antibiotics is their anti- 

 biotic spectrum or range of activity against different organisms, supple- 

 mented by their physical and chemical properties. 



