INTRODUCTORY 



Detailed studies of the microbiological 

 population of the soil have revealed not only 

 the presence of numerous actinomycetes but 

 also the fact that the growth of certain of 

 these organisms exerts a depressive effect 

 upon the growth of other microorganisms, 

 notably l)acteria and fungi. Casual o))ser\'a- 

 tions of cultures of actinomycetes isolated by 

 Gasperini from 1892 to 1895, Aliiller in 1908, 

 Greig-Smith from 1911 to 1917, Lieske in 

 1921, Gratia and Dath from 1924 to 1927, 

 and Rosenthal in 1925 further demonstrated 

 that these organisms have the capacity to 

 produce chemical substances, now known 

 as antibiotics, which inhibit the growth of 

 other organisms. That these observations 

 were not isolated instances but were char- 

 acteristic of a large number of the actino- 

 mycetes was also established in the ^'arious 

 sur^'eys carried out by a group of Russian 

 investigators from 1935 to 19o9. 



Systematic investigations of the effect of 

 actinomycetes upon other soil organisms, 

 carried out in our own laboratories since 

 1935 (Waksman, 1937, 1941, 1947), resuUed 

 in 1940 in the isolation, in crystalline form, 

 of a pigmented antibiotic which was named 

 actinomycin. This was followed by the iso- 

 lation in our laboratories of streptothricin 

 in 1942, micromonosporin and streptomycin 

 in 1943, grisein in 1946, neom_ycin in 1948, 

 and later of a number of other antibiotics, 

 notably streptocin, ehrlichin, fradicin, can- 

 dicidin, and candidin. Some of these antibi- 

 otics, especially streptomycin and neomycin, 

 have found extensive pi'actical application 

 in the control of numerous human, animal, 

 and plant diseases; more recently actinomy- 

 cin was shown to possess activity against 

 certain forms of cancer; candicidin and can- 

 didin give promise as antifungal agents. 



Nearly all of them are of scientific interest. 

 Numerous other antibiotics soon were iso- 

 lated in various other laboratories through- 

 out the world. 



In recent years, the field of antibiotics has 

 undergone spectacular developments. The 

 ever growing imi)ortance of these comjxuuids 

 in the control of human and animal diseases 

 as well as of certain plant diseases, in animal 

 nutrition, in food pi'eser\^ation, in the preser- 

 vation of biological materials, and in other 

 fields of human endeavor has revolutionized 

 medical practice and many (jf the habits of 

 modern life. The antibiotics have added un- 

 told wealth to our economy and have re- 

 sulted directly in the prolongation of mil- 

 lions of human lives. They have also intro- 

 duced a new concept in our understanding 

 of microbial life in natural environments and 

 have added greatly to our understanding of 

 certain chemical reactions in biological sys- 

 tems. 



These de\'elopments in the field of medical 

 science and the important practical applica- 

 tions in agriculture are due primarily to 

 specific biological and chemical properties of 

 the antibiotics, particularly their antimi- 

 crobial activities. Among these, their selec- 

 tWe destructive action against various 

 microbial pathogens and their relative 

 harmlessness to the hosts attacked by the 

 pathogens are of particular significance. 

 Antibiotics affect various microbes at dif- 

 ferent rates. They are not generalized anti- 

 septics and disinfectants. Each antibiotic is 

 characterized by a selective antimicrobial 

 spectrum, or the ability to inhibit the growth 

 of or destroy certain microbes but not others. 

 The various microbes differ, moreover, in 

 the degree of their sensitivity to each anti- 

 biotic. On prolonged contact with a given 



