Il6 BENZER ET AL. 



III. Adsorption 



13. The term "adsorption" refers to that part of the Ufe cycle in 

 which phage and bacterium form an irreversible union. Experiments 

 with X-rayed viruses (see 14) demonstrate that this step is empirically 

 recognizable, and that it involves certain side effects (see 38, 39). 



Whether or not such a union can occur between a particular phage 

 and bacterium depends on highly specific factors. A host strain nor- 

 mally sensitive to one particular type of phage may mutate to resistance 

 (i. e. to viability in the presence of the virus) while retaining its sen- 

 sitivity to other phage types. Such a bacterial strain no longer forms 

 an irreversible bond with the phage to which it has become resistant. 

 Sensitive bacteria, on the other hand, can unite with the phage even 

 under conditions where no reproduction of the phage is possible. Ad- 

 sorption proceeds, for example, when phage and bacteria are suspended 

 in buffer containing no metabolic substrates. If either virus or cell 

 has been mistreated in some way, such as X-ray or ultraviolet irradia- 

 tion of the phage, or heat- or alcohol-killing of the bacteria, adsorption 

 still occurs. It is, in fact, possible to make extracts from bacteria which 

 can unite irreversibly and specificially in vitro with active phages (see 

 17). 



The adsorption process involves, therefore, some specific structures 

 on the surface of host and virus, perhaps analogous to those responsible 

 for the steric fitting encountered in antibody antigen reaction. It does 

 not involve metabolic processes on the part of the bacterium. 



14. Adsorption without killing of the bacterium. — It has been dis- 

 covered by Watson (1950) that a large fraction of the phage particles 

 inactivated with X-rays is adsorbed by the bacteria wdthout killing 

 them. Proof that they are actually adsorbed is obtained by showing 

 that these particles can saturate the adsorptive capacity of the bac- 

 teria. Bacteria killed by heat (one hour at 6o°C) are able to adsorb 

 particles, up to a maximum of about 500 for each bacterium; Wat- 

 son could show that heat-killed bacteria can have their adsorptive 

 capacity saturated by exposure to phage inactivated by X-rays. He 

 demonstrated this saturation by the subsequent reduction of their ad- 

 sorptive capacity for active phage. 



15. Mechanism of the adsorption process. — A study of the velocity 

 with which active phages are adsorbed to sensitive bacterial cells shows 

 that the fraction of the phages remaining free decreases exponentially 

 with time. In a certain range of bacterial concentrations, moreover, 

 the adsorption rate is proportional to the cell concentration. We con- 



