276 Associative and Antagonistic EflFects 



the organisms affected by them, such as mycocidin, or on the basis of 

 their chemical composition, such as chloramphenicol. They differ 

 greatly in their chemical properties, toxicity to animals, and in vitro 

 vs. in vivo activities. 



Cup method against fungi. 



Many microorganisms isolated from the soil have been found ca- 

 pable of producing antibiotics. Among the actinomycetes, for 

 example, 10-50 per cent of all organisms tested were found to have 

 such properties. Both spore-forming bacteria and non-spore-formers 

 are able to produce antibiotics. Those produced by the spore-formers 

 include tyrothricin, bacitracin, subtilin, and polymyxin; those pro- 

 duced by the non-spore-formers include pyocyanase, pyocyanin, 



