206 BACTERIOPHAGES 



plasm through a hole in the wall. Phage multiplication in- 

 variably involves marked changes in the appearance of the bac- 

 terial nucleus and, usually, a marked decrease in the stainability 

 of the cytoplasm; however, the sequence of changes varies 

 greatly from one phage to another. The cytological changes ob- 

 served are characteristic of the phage rather than of the host cell, 

 are under genetic control, and are of taxonomic significance be- 

 cause phylogenetically related phages produce similar cytological 

 patterns. 



A careful comparison of the appearance of living cells by dark 

 field or phase contrast microscopy with the appearance of fixed 

 and dried cells by electron microscopy or in stained preparations 

 suggests that the procedures of fixing and drying are likely to re- 

 sult in the production of artefacts. This places limitations on 

 the interpretation of some of the cytological evidence that have 

 not always been evident to workers in this field. Any interpr-e- 

 tation should take into consideration the large amount of infor- 

 mation about the mechanisms of phage multiplication which has 

 been obtained by other means. 



Up to the present the nuclear staining techniques have been 

 the most productive of new information about the response of the 

 host cell to invasion by bacteriophage. However, it is probable 

 that recent improvements in electron microscope technology will 

 permit studies of the phage-infected bacterium at an order of 

 resolution approaching that of biochemical techniques. 



As is true of most recent phage research, a tremendous amount 

 of effort has been devoted to the cytological examination of a 

 very few phage strains. It would be desirable to apply the 

 techniques now available to a broader range of phages and host 

 strains and conditions. 



