270 BACTERIOPHAGES 



compete for cations. In addition to its ability to interfere with 

 adsorption, pectin may also afTect phage multiplication as 

 evidenced by the low burst sizes obtained when pectin was 

 added after adsorption. The inhibition of T2 multiplication 

 by citrus pectin was previously reported by Maurer and Woolley 

 (1948). Their conclusion that the pectin might act by inducing 

 a lysogenic state was not supported by experimental data and 

 can now be explained by the findings of Reiter. 



Our final consideration in the prevention of adsorption deals 

 with those agents which can antagonize the organic cofactors 

 for adsorption. Delbriick (1948) found that indole can prevent 

 the adsorption of certain of the T4 strains that require tryptophan 

 as a cofactor. The relation between indole and tryptophan in 

 activation of phages is not clear (Chapter XVI). 



2. Prevention of Penetration 



Prevention of penetration refers to the ability of a chemical 

 agent to pre\'ent the injection of the phage DNA into the bac- 

 terial host. In the case of T5, as discussed in Chapter XI, this 

 process requires calcium ions and can be stopped by the addition 

 of citrate. Other polyvalent cations are also involved, as evi- 

 denced by the inhibitory action of the chelating agent Versene 

 on T2 production (Kozloff and Henderson, 1955). When T2 

 is adsorbed to bacteria in the presence of the chelator, the bac- 

 teria are killed but no production of phage ensues. That this is 

 indeed interference with injection is shown by the fact that 90 

 per cent of the adsorbed phage DNA can be removed from the 

 bacteria by the Blendor technique as compared to 30 per cent 

 in the absence of versene. Kozloff and Henderson interpreted 

 this result to mean that metal complexes participate in the 

 injection of DNA in a manner analogous to the action of cad- 

 mium cyanide on T2 particles. Versene presumably acts by 

 combining with metals on the bacterial surface. Other chelat- 

 ing agents may act in the same way. The action of chymo- 

 trypsin as an inhibitor of rhizobium phage is suggestive of an 

 interference with penetration. Chymotrypsin docs not prevent 



