80 THE BACTERIOPHAGE 



Two different strains of anti-dysentery bacteriophage have 

 been preserved for over two and a half years and during that time 

 they have undergone more than 100 successive passages. Never- 

 theless, during this period, repeated tests have shown in these 

 cultures the constant coexistence of extremely virulent ultrami- 

 crobes and of bacteria completely refractory to several very viru- 

 lent strains of the anti-dysentery bacteriophage. 



In these mixed cultures there is a stable equilibrium between 

 the virulence of the one and the resistance of the other, and in 

 such cultures the changes in appearance previously noted will 

 never be observed. They might, indeed, be spoken of as "sym- 

 biotic cultures," 3 for the bacteriophage can not be cultivated in 

 series unless it multiplies and it can not multiply unless it para- 

 sitizes bacteria. Moreover, it is only necessary to disturb the 

 equilibrium in favor of the bacterium, by such means as have 

 been mentioned, to cause a rapid disappearance of the ultrami- 

 crobes, rendering them henceforth incapable of cultivation. 



All of the bacteria present in an agglutinated culture are to be 

 found in the agglutinated clumps, none are free in the medium. 

 Subcultures into bouillon from the clear fluid always remain 

 sterile, no colonies develop when transferred to agar, and micro- 

 scopic examination fails to reveal any formed elements. All the 

 bacteria there present have assembled in the agglutinate. The 

 clear fluid contains only the extremely virulent ultramicrobes, 

 as may be proved by the inoculation of a bacterial suspension 

 which quickly becomes lysed. On the contrary, as we have seen 

 above, a bouillon subculture made from the agglutinate always 

 results in the growth of a mixed culture. 



When a mixed, agglutinated culture is inoculated into a pure 

 culture of the bacteriophage, that is, into a suspension previously 

 inoculated and which has undergone complete lysis, the growth 

 consists of an agglutinated culture, just as though the inoculation 

 had been made into fresh sterile bouillon. 



3 This is possible if we interpret symbiosis in a broad sense, as Noel 

 Bernard has. The definition of symbiosis given by this author applies 

 admirably to mixed cultures: "An intermediary condition at which two 

 antagonistic organisms arrive, with an equilibrium of their forces, toler- 

 ating each other in a prolonged common existence." 



