148 BACTERIOPHAGES 



ever, in the earlier experiments the adsorption mixture had been 

 diluted 100-fold before centrifugation, thus permitting elution of 

 any reversibly adsorbed phage. That this accounts for the dis- 

 crepancy was demonstrated by Garen and Puck (1951) who 

 showed that phage Tl adsorbed at 3 ° C. could be largely eluted 

 on dilution into cold broth, but that phage adsorbed at 37 ° C. 

 could not. Thus the temperature coefficient is low for reversible 

 adsorption and high for irreversible adsorption. 



Other methods devised for separating the two kinds of adsorp- 

 tion are the following : 



7. Zinc ions specifically prevent irreversible adsorption of 

 Tl (but not of T2). Phage Tl adsorbed to B at 37° C. in the 

 presence of zinc ions is largely eluted on dilution, whereas Tl 

 adsorbed in the presence of calcium ions is not. Zinc competes 

 with calcium and magnesium ions for sites on the surface of the 

 bacterial cell, thereby protecting the bacterium against in- 

 vasion by phage Tl . 



2. Suspensions of bacteria heavily irradiated with ultra- 

 violet light still adsorb phage Tl quite rapidly at 37 ° C, but on 

 dilution into cold broth all the phage is eluted. With unir- 

 radiated bacteria, the adsorption is irreversible under the same 

 conditions. Irradiation must destrov some bacterial substance 

 essential for irreversible adsorption. 



3. Strain B of E. coli has at least two different spontaneous 

 mutants that are resistant to phage Tl, B/1,5 resistant to both 

 Tl and T5, and B/1 resistant to Tl only. Phage Tl fails to ad- 

 sorb to B/1,5 under any condition tested, but adsorbs reversibly 

 to B/1. Thus, mutation to B/1 results in loss of some factor 

 essential for irreversible adsorption without seriously affecting 

 reversible adsorption. In B/1,5 both kinds of adsorption fail. 

 Similarly, B/2 does not adsorb T2. There is also no reversible 

 adsorption of T4 to B/4 (Stent and WoUman, 1952). 



Similar experiments on the adsorption of phage T4 to its host 

 cells in +he presence and absence of tryptophan show that no 

 attachment of the phage to the host cell takes place unless 

 tryptophan is present. This indicates that trytophan is essen- 



