MUTATIONS OF BACTERIA 493 



point of attack of the virus on the bacterium. After survival of such random 

 individuals, however, we must assume that their offspring are hereditarily 

 immune, since they do not even adsorb the virus. 



These alternative hypotheses may be grouped by first considering the origin 

 of the hereditary difference. Do the original variants trace back to mutations 

 which occur independently of the virus, such that these bacteria belong to a 

 few clones, or do they represent a random sample of the entire bacterial popu- 

 lation? The first alternative may then be subdivided further, according to 

 whether the original variants do or do not interact with the virus. Disregarding 

 for the moment this subdivision, we may formulate two hypotheses: 



1. First hypothesis {mutation): There is a finite probability for any bac- 

 terium to mutate during its life time from "sensitive" to "resistant." Every 

 offspring of such a mutant will be resistant, unless reverse mutation occurs. 

 The term "resistant" means here that the bacterium will not be killed if ex- 

 posed to virus, and the possibility of its interaction with virus is left open. 



2. Second hypothesis {acquired hereditary immunity)'. There is a small finite 

 probability for any bacterium to survive an attack by the virus. Survival of 

 an infection confers immunity not only to the individual but also to its off- 

 spring. The probability of survival in the first instance does not run in clones. 

 If we find that a bacterium survives an attack, we cannot from this information 

 infer that close relatives of it, other than descendants, are likely to survive the 

 attack. 



The last statement contains the essential difference between the two hy- 

 potheses. On the mutation hypothesis, the mutation to resistance may occur 

 any time prior to the addition of virus. The culture therefore will contain 

 "clones of resistant bacteria" of various sizes, whereas on the hypothesis of 

 acquired immunity the bacteria which survive an attack by the virus will be 

 a random sample of the culture. 



For the discussion of the experimental possibility of distinction between 

 these two hypotheses, it is important to keep in mind that the offspring of a 

 tested bacterium which survives is resistant on either hypothesis. Repeated 

 tests on a bacterium at different times, or on a bacterium and on its offspring, 

 could therefore give no information of help in deciding the present issue. Thus, 

 one has to resort to less direct methods. Two main differences may be derived 

 from the hypotheses: 



First, if the individual cells of a very large number of microcolonies, each 

 containing only a few bacteria, were examined for resistance, a pronounced 

 correlation between the types found in a single colony would be expected on 

 the mutation hypothesis, while a random distribution of resistants would be 

 expected on the hypothesis of acquired hereditary immunity. This experiment, 

 however, is not practicable, both on account of the difficulty of manipulation 

 and on account of the small proportion of resistant bacteria. 



Second, on the hypothesis of resistance due to mutation, the proportion of 

 resistant bacteria should increase with time, in a growing culture, as new 

 mutants constantly add to their ranks. 



