220 NATURE OF ANTIBIOTIC ACTION 



of acne bacilli from accompanying staphylococci (153) and for the sepa- 

 ration of streptococci from staphylococci (266). 



Actinomycin was used to separate gram-negative from gram-positive 

 bacteria (945). 



Tyrothricin has been utilized (858) for the isolation of A'', gonor- 

 rhoeae from contamination with gram-positive bacteria. Usually a 

 1 : 15,000 dilution of the substance in "chocolate" agar was found quite 

 satisfactory for this purpose. By the use of tyrothricin, gram-negative 

 microorganisms have been isolated from the nasopharynx even in the 

 presence of overgrowth by gram-positive cocci. Hemophilus injiuenzae 

 is resistant to the action of tyrothricin, and its isolation is facilitated by 

 adding to the media on which it is cultured tyrothricin in a dilution 

 which inhibits the cocci (808). 



Streptothricin was found to help in distinguishing B. mycoides from 

 B. subtilis (950). 



SUMMARY 



Comparatively little is yet known of the mode of action of antibiotic 

 substances. This field offers great opportunities for research and utiliza- 

 tion of bacterial activities. The solution to such important problems as 

 the morphology of the bacterial cell, taxonomic relations of bacteria, 

 various physiological reactions of microorganisms, the mechanism of 

 causation of disease, and the very control of disease-producing micro- 

 organisms — all fundamental problems in microbiology — will be fur- 

 thered by knowledge of the action of specific antibiotic substances upon 

 bacteria and other microorganisms. 



