92 BACTERIA AS ANTAGONISTS 



between inhibition and hemolysis. Of the 1 8 active strains, 1 1 lost the 

 property after two transfers and 2 were exceedingly active. 



More recently, beginning with the work of Dubos, considerable at- 

 tention has been devoted to spore-forming bacteria, resulting in the iso- 

 lation of a number of substances or preparations that have been desig- 

 nated as tyrothricin, gramicidin S, subtilin, bacitracin, bacillin, sim- 

 plexin, subtilysin, and endo-subtilysin (815a). These substances are 

 largely active against gram-positive bacteria j however, some also aifect 

 gram-negative bacteria and fungi. 



Dubos (201) obtained from a soil enriched with various living bac- 

 teria a gram-negative, spore-bearing bacillus (B. brevis) that had a 

 marked lytic effect against gram-positive bacteria, including staphy- 

 lococci and pneumococci. The antagonist was grown for 3 to 4 days in 

 shallow layers of peptone media at 37° C. The bacterial cells were re- 

 moved by centrifuging, and the filtrate was acidified, giving a precipi- 

 tate from which a highly active substance (tyrothricin) was isolated. 

 On crystallization, two preparations were obtained, namely gramicidin 

 and tyrocidine, these making up only a fraction of the tyrothricin com- 

 plex. 



Natural substrates, such as soil, sewage, manure, and cheese, were 

 found (209) to contain various spore-forming bacteria that have 

 marked antagonistic properties against various gram-positive and gram- 

 negative bacteria. Hoogerheide (442) obtained from the soil an aero- 

 bic, spore-forming bacterium that produced a highly active bactericidal 

 substance J it also prevented the formation of capsules by Friedlander's 

 bacterium. This substance appeared to be similar to gramicidin. Grami- 

 cidin S is, however, more like tyrocidine. 



Further studies definitely established that strains of spore-forming 

 bacteria possessing antagonistic properties are widely distributed in the 

 soil and possess certain physiological characteristics that differentiate 

 them from the inactive strains. This is brought out in Table 12. The 

 production of the antibiotic is a function of the growth of the bacterial 

 cell. The yield of the antibiotic is influenced by the composition of the 

 medium j the substance is bound to a protein, the bond between the two 

 being destroyed by trypsin (523). 



B. sub tills has been reported by many investigators to exert an an- 



