48 INTERRELATIONSHIPS AMONG MICROORGANISMS 



appeared to be thermostable j Nadson and Adamovic (625) showed 

 that certain metabolic products of microorganisms, even when heated to 

 120° C, may have a strong influence upon the subsequent growth of 

 the organisms. 



Fungi are capable of producing not only growth-inhibiting but also 

 growth-promoting substances (509, 547). By means of certain proce- 

 dures, it was found possible to separate the two (654). The tendency of 

 fungus hyphae to turn away from the region in which other hyphae of 

 the same fungus were growing was explained as a negative reaction to 

 chemical substances produced by the growing fungus (304). This nega- 

 tive chemotropism was shown to be due to thermolabile staling sub- 

 stances (352). The phenomenon of staling was often spoken of as vacci- 

 nation of medium (45), and was ascribed to the action of protein degra- 

 dation products. 



These and other experiments led to the conclusion that many micro- 

 organisms are capable of producing substances that are injurious to their 

 own development (iso-antagonistic) or, and sometimes much more so, 

 to other organisms growing close to them (hetero-antagonistic). The 

 growth of certain fungi and bacteria in practically pure culture, even in 

 a nonsterile environment, was believed to be due to this phenomenon. 

 It is sufficient to mention the lactic and butyric acid bacteria, the citric 

 acid-producing species of Asfergillus, the lactic and fumaric acid- 

 producing species of Rhizofus, and the alcohol-producing yeasts. The 

 chemical substances produced by these organisms in natural substrates 

 may be looked upon as protective metabolic products of microorgan- 

 isms in their struggle for existence. Such products play a highly sig- 

 nificant part in the metabolism of various organisms, especially those 

 that grow parasitically upon living plant and animal bodies. 



Among the various types of antagonism, the one resulting in the pro- 

 duction of active substances that can be isolated and purified has re- 

 ceived the greatest consideration recently. These substances have been 

 designated as toxins, poisons, antagonistic agents, bacteriostatics, and 

 antibiotics. The chemical nature of some has been elucidated, but that 

 of many others is still unknown. Some of these substances are destroyed 

 by boiling, by exposure to light, or by filtration, whereas others are re- 

 sistant to heat and to ultraviolet raysj some are readily adsorbed by 



