298 PHYSIOLOGY OF BACTERIA 



a larger number of '^reacting molecules" per cell though 

 such cases appear quite possible (see p. 367). Rahn 

 (1930) has pointed out that the staphylococcus gets 

 its name from the tendency to form clusters, and that 

 spore-formers usually grow in threads. Each thread or 

 cluster will contain several cells, yet on the agar plate, 

 it will produce only one colony. The plate count does 

 not give us the number of individuals in these cases, but 

 the number of clusters, and the clusters contain several 

 ^'reacting molecules,'^ namely one for each cell. The 

 fault lies in our inadequate method of counting living 

 cells. 



A very simple and convincing proof for this assumption is given 

 by Wyckoff and Rivers (1930). They spread a young culture of 

 Bad. coll on an agar surface, exposed it to cathode rays, and con- 

 sidered that the colonies developing on the agar represented the 

 survivors. A microscopic investigation proved that the cells spread 

 on the agar had not been clustered. The survivor curve was nearly a 

 straight line (Fig. 35). A similar plate was held in the incubator 

 for a short time before being exposed. The originally single cells 

 of the bacterium had had a chance to form small groups. The 

 survivor curve of this plate was bulging like those known for larger 

 organisms. 



(h) ADAPTATION AND SELECTION OF RESISTANT STRAINS 



According to the strict chemical interpretation of 

 the logarithmic order of death, the last survivor is no 

 more resistant than the first cell that died. Surviving is 

 merely a question of chance. According to the theory of 

 graded resistance, the last survivor is much more 

 resistant than the first, in fact, it is the fittest survivor. 

 Although with higher animals and plants, this method 

 of selecting resistant strains is almost certain to give 

 good results (provided due consideration is given to the 

 laws of inheritance), the experiments with bacteria 

 have not been very successful. 



