CHAPTER X 



ADSORPTION OF PHAGE TO HOST CELL 



The first step in the infectious cycle is the adsorption of the 

 phage particle to the host cell. Adsorption may be defined as 

 attachment of phage particles to bacterial surfaces so that phage 

 and bacteria will sediment together. As described in Chapter 

 IV, phages adsorb to their host cells by the tips of their tails. 

 The attachment is made to specific receptor substances on the 

 cell surface, often parts of the cell wall (Chapter IX). 



Adsorption is usually measured by centrifuging a mixture of 

 bacteria and phage, and counting the phage in sediment or 

 supernatant fractions, or both, by means of plaque counts. The 

 reduction in plaque count in the supernatant fluid can be used 

 to measure adsorption in excess of 30 per cent ; for smaller frac- 

 tions the measurement by difference is too inaccurate, and the 

 sedimented phage must be counted directly. If the fraction 

 adsorbed is very small, antiserum must be used as well (Hershey 

 and Davidson, 1951). Other methods of measurement do not 

 require centrifugation. One such method depends on the fact 

 that infected bacteria can be destroyed by agents such as chloro- 

 form that do not inactivate free phage particles (Fredericq, 

 1952a). Another makes use of antiserum to inactivate un- 

 adsorbed phage (Delbriick, 1945b). Adsorption can also be 

 measured in terms of the fraction of bacteria killed by the phage. 

 Many of these methods, however, depend on steps in the infective 

 cycle taking place subsequent to adsorption as defined above. 

 It cannot be assumed, without test, that any other measurement 

 will yield information equivalent to the reduction in plaque 

 count of the supernatant fluid on centrifugation of the adsorption 

 mixture. Moreover, the results of this measurement may vary, 



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