142 BACTERIOPHAGES 



added salt, by means of the centrifugation technique. The sedi- 

 ments and supernatants were assayed on both salt-free agar and 

 on agar containing the optimal amount of salt. The fraction of 

 phage adsorbed was calculated from the plaque counts on agar 

 containing salt. It was found that appreciable adsorption of 

 phage occurs in salt-free tryptose broth. The striking observa- 

 tion is that phage adsorbed to its host cell in the presence of salt 

 has a high efficiency of plating on salt-free agar, while phage 

 absorbed in the absence of salt has a very low EOP on salt-free 

 agar, as does unadsorbed phage. It would appear from these 

 experiments that phage adsorbed in salt-free broth does not 

 initiate infection of the host cell and may be reversibly adsorbed. 



The possibility of reversing adsorption was tested by adsorbing 

 the phage in the absence of salt, sedimenting the bacteria, and 

 washing the sediment with broth to remove free phage. The 

 sediment was then diluted in salt-free broth, incubated for 20 

 minutes, and centrifuged again. Assays on both supernatant 

 and sediment indicated that about 50 per cent of the adsorbed 

 phage had been eluted. Phage adsorption in salt-free broth is 

 partly reversible and infection of the host cell by the adsorbed 

 phage does not occur in the absence of salt. 



Hershey, Kalmonson, and Bronfenbrenner (1944) also found 

 that phage T2 treated in the presence of salt with very dilute 

 anti-T2 serum showed an increased rate of adsorption under 

 suboptimal conditions, and a greatly increased EOP on low-salt 

 agar. This effect may be related to the activation of cofactor- 

 requiring strains of T4 by one of the anti-T4 antibodies (Jerne, 

 1956). The observations with T2 might be well worth reinvesti- 

 gating but are now of doubtful meaning owing to possible com- 

 plications caused by inhibitors of adsorption often present in 

 phage stocks (Sagik, 1954). The same could be said, in fact, of 

 much of the work reported in this chapter. 



The experiments on reversible adsorption described above 

 were abandoned by the authors because it appeared that the 

 transition from reversible to irreversible adsorption could not 

 occur on addition of salt, and therefore that reversible adsorption 



