210 THE BACTERIOPHAGE AND ITS BEHAVIOR 



assumed the form of B. aerogenes colonies. I also have observed 

 colonies of this type. Their occurrence is very irregular and up to the 

 present it has been absolutely impossible for me to determine the condi- 

 tions governing their appearance.* 



As a matter of fact, there is hardly anything which presents a wider 

 variation in appearance than do mixed cultures. With the staphylo- 

 coccus, for example. Gratia and Jaumain-*^^ have seen mixed cultures on 

 agar which were thick and opaque; others were discrete, like cultures of 

 the streptococcus; some were rich in pigment, others poor; some grew 

 with a homogeneous growth in bouillon, others gave a sedimented 

 growth with a clear supernatant fluid. Apparently almost any type 

 of growth may occur. 



We have just seen that in certain mixed cultures a perfect equilibrium 

 is established. It is, however, rare that these mixed cultures in the form 

 of agglutinated masses become indefinitely cultivable in bouillon in 

 this form. Yet, I have preserved throughout a period of three years, 

 with weekly transfers, such mixed cultures of B. dysenteriae and the 

 bacteriophage and after this time the two antagonists were always 

 present. The multiplication of bacteriophage corpuscles here is 

 extremely slow. 



One of these mixed cultures is centrifuged after about 100 passages. 

 The sedimented material is washed several times with sterile bouillon. 

 The bouillon of the sixth washing filtered through a candle contains no 

 corpuscles, for when it is inoculated into a suspension of B. dysenteriae 

 bacteriophagy does not take place. A trace of the sedimented washed 

 bacilli is implanted into bouillon. It is only after the fourth day that 

 the inoculation of 0.05 cc. of fluid, freed of the agglutinated material by 

 filtration, causes bacteriophagy with a suspension of normal dysentery 

 bacilli. 



This is, however, an extreme case. When the resistance of the 

 bacteria of a mixed culture is not as great corpuscles will be found in 

 the hquid after a few hours. The higher the resistance of the bacteria 

 of the mixed culture the slower is the multiplication of the bacteriophage 

 corpuscles. 



* Gory-^2 i^ag obtained them regularly in working with a normal strain of B. coli 

 and tap-water, which always contains a large number of bacteriophage corpus- 

 cles virulent for this bacterium. After a number of passages in the media con- 

 taining the sewage the transformation of B. coli into B. aerogenes or into a bac- 

 terium which could not be distinguished from the latter took place. It would 

 appear that the B. aerogenes resulted from a mutation of the B. coli subjected to 

 the action of the bacteriophage. 



