INTRODUCTION i> 



According to the precursor theory bacteriophages are en- 

 dogenous, existing in bacteria as precursors which either sponta- 

 neously or after stimulation are transformed into characteristic 

 lytic substances, much as trypsinogen can be converted into 

 trypsin. This theory was supported by Gildemeister (1921), 

 Bordet (1925), Northrop (1939a), Kreuger and Scribner (1939), 

 and Felix (1953). Much of the experimental support for it was 

 derived from studies on lysogenic bacteria, to be discussed pres- 

 ently. 



According to the virus theory the bacteriophages are autono- 

 mous microbes analogous to plant and animal viruses but obli- 

 gately parasitic on bacteria. This hypothesis was adopted very 

 early in his work by d'Herelle and seemed obvious to most 

 virologists once something had been learned about the biology 

 of viruses in general. In the light of current knowledge this 

 theory amounts to very little, however, since both autonomy and 

 parasitism as applied to viruses are terms difficult to define. 

 Some current views are presented below in connection with 

 theories concerning the origins of viruses. 



As is often the case in the historical development of a scientific 

 field each of the rival hypotheses can now be construed as part 

 of the truth; each was incomplete by itself. The current con- 

 cept of the nature of bacteriophage is a synthesis of the old ideas 

 plus a few novel ideas developed recently. Somewhat as a 

 typical bacillus may exist in the spore state or the vegetative 

 state, a typical bacteriophage may exist in three states, called 

 prophage, vegetative phage, and mature phage. Phages exist 

 outside of the host cell in mature form, metabolically inert and 

 resembling very crudely the spore state of bacteria. Following 

 adsorption to the host cell, part of the phage particle penetrates 

 within and may begin to multiply. When it does, the multiply- 

 ing, intracellular state of the phage particle diff'ers in many re- 

 spects from mature phage and has been termed vegetative phage 

 in recognition of its almost unlimited reproductive capacity. 

 Some phages, called temperate, are distinguished by the fact 

 t^at they can exist in a third state, prophage, as well. When a 



