THE BACTEEIOPHAGE AND THE BACTERIUM 91 



division) where there is noted an autogamous reproduction, which 

 implies, at least, a rudimentary sexuality. In autogamy there 

 are necessarily differentiated elements which fuse. According 

 to Schaudinn the loss of sexuality in bacteria is an indication of 

 a degenerative change resulting from the adaptation to a para- 

 sitic existence. In accordance with the hypothesis suggested 

 above, there would be a return to sexual reproduction as a means 

 of defense, under the influence of the parasitism to which the 

 organisms have been subjected. Such sexual reproduction would 

 be, then, extremely frequent in vivo, as frequent indeed as a strug- 

 gle between the bacterium and a bacteriophage occurs in the 

 body. In Part II of this monograph it will appear that this 

 struggle is continuous. 



THE ACQUISITION OF RESISTANCE 



How can this acquisition of immunity by a bacterium be 

 explained? Numerous experiments have shown that if a certain 

 quantity of a slightly active culture of a bacteriophage is introduced 

 into a relatively heavy (1000 to 2000 million per cubic centimeter) 

 suspension of bacilli, the ultramicrobes, readily demonstrated at 

 first by the presence of plaques on plantings on agar, disappear 

 from the medium after an interval of time varying from one 

 hour to two or three days, and that they can not later be dem- 

 onstrated. Subcultures give normal cultures of bacteria. On 

 the other hand, we have seen that with a very virulent bacterio- 

 phage the ultramicrobes disappear from the fluid between ten and 

 twenty minutes after introduction into a suspension, but that they 

 reappear in about twenty times as great a number in from one to 

 one and a half hours later they have multiplied within the 

 interior of the bacteria. In the case of a bacteriophage of low 

 virulence it seems, therefore, that penetration of the bacteria 

 takes place but that multiplication can not be effected. The 

 bacterium resists and the ultramicrobe is actually destroyed in 

 vivo. These parasitized bacteria which "recover" acquire by 

 this an immunity. We will see later that they are even capable 

 of producing antilysins. 



Another fact has been sometimes observed which shows that 

 certain bacteria are able to become "carriers." As has been said, 



