viii PREFACE 



and destroyed, others are not. Some are subject to destruction by spe- 

 cific enzymes. The substances are largely bacteriostatic in action, to a 

 lesser extent bactericidal j some are also fungistatic and fungicidal. 



Some of the substances are highly toxic to animals. Others are either 

 nontoxic or of limited toxicity and are active in vivo. Some hemolyze 

 red blood cells, others do not. Those that are hemolytic and moderately 

 toxic may be useful for application to local infections. Those that are 

 neither hemolytic nor toxic and are active in vivo may have great im- 

 portance in combating certain diseases in animals and man. 



Some substances are formed by only a few specific organisms, others 

 may be formed under proper conditions of nutrition by many different 

 organisms. Some antagonists produce only one type of antibiotic sub- 

 stance, others form two or even more chemically and biologically dif- 

 ferent substances. 



The ability of an antagonist or its products — antibiotic substances — to 

 destroy a parasitic microorganism in vivo is influenced by the nature of 

 the host as well as by the type and degree of the infection. The manner 

 in which antagonists destroy or modify parasites varies greatly, depend- 

 ing frequently upon the nature of the antibiotic substances produced. 



It is thus clear that the subject is extremely complicated, involving 

 numerous interrelationships among different biological systems of both 

 higher and lower forms of life. 



In the following pages an attempt is made to present the broad inter- 

 relationships among microorganisms living in association, either in sim- 

 ple mixed cultures or in complex natural populations, with special at- 

 tention to the antagonistic effects. Emphasis is laid upon the significance 

 of these associations in natural processes and upon their relation to dis- 

 ease production in man and in his domesticated plants and animals. The 

 chemical nature of the active — antibiotic — substances produced by vari- 

 ous antagonists is described and the nature of the antagonistic action as 

 well as its utilization for practical purposes of disease control is dis- 

 cussed. However, because concepts of the significance of these phenom- 

 ena are changing so rapidly, no pretense has been made of examining 

 completely the practical applications of this important subject. 



Due to the fact that more detailed studies have been made on the 

 production, nature, and utilization of penicillin, more information is 



