202 BACTERIOPHAGES 



As an interesting comparison with the previous studies, Dela- 

 porte (1952) described the cytological changes in an induced 

 colicinogenic strain of £■. coli. Strain ML after ukraviolet induc- 

 tion was incubated at 37 ° C. and sampled at intervals. Intra- 

 cellular colicin is detectable within 15 minutes, and appears in 

 the medium after 75 minutes, increasing in amount until about 

 2 hours after irradiation. During this tim.e the bacteria swell 

 and gradually fill up with Feulgen positive chromatin which 

 seems to have no definite arrangement in the cells. The lysis 

 which liberates the colicin is accompanied by a gradual de- 

 crease in cell size and amount of chromatin per cell. However, 

 it should be mentioned that Kellenberger and Kellenberger 

 (1956) published an electron microscope study of lysates of 

 cohcinogenic strains (including ML). They were unable to 

 identify a colicin particle, possibly because the size of these 

 would be of the same order as the many granules liberated from 

 normal bacteria. In any case, the colicins m.ay not correspond 

 in structure to the phages and there are not enough studies to al- 

 low a clear statement of cytological events. 



3. Electron Microscope Observations 



There is a very extensive literature dealing with electron micro- 

 scope studies of phage- infected bacteria. The pertinent refer- 

 ences have been listed by Beutner, Hartman, Mudd, and Hillier 

 (1953). There are a number of defects which severely limit the 

 value of much of this work. The ability to distinguish phage par- 

 ticles and other minute structures within the relatively large, 

 whole bacteria is limited by electron scattering and by lack of 

 contrast, and the situation is made worse by shadowing. It is un- 

 likely that studies of whole bacteria by electron microscopy can 

 be very rewarding; however, various means of circumventing 

 the difficulty have been devised and these will be discussed. The 

 techniques of studying biological materials with the electron 

 microscope are still in embryonic state. 



One approach is breaking open the bacteria at intervals dur- 

 ing the latent period and studying the liberated material. The 



