16 BACTERIOPHAGES 



Studying any new bacterium-phage system is the effect of vary- 

 ing th^salt concentration in the growth medium. An unfavor- 

 able sah concentration can prevent phage adsorption. It is 

 probable that antiphage antibody acts in part by interfering 

 with adsorption, and antibacterial antibodies can also prevent 

 adsorption by coating the bacterial receptor sites. 



The specificity of the phage-bacterium relationship is in most 

 cases determined by the adsorption process. When a coliphage 

 for instance fails to attack a particular strain of E. coli, the failure 

 is usually due to lack of adsorption. Certain exceptions to this 

 generalization will be noted in later chapters. The adsorption 

 process is remarkably specific. A single mutation on the part 

 of a host cell can result in loss of ability to adsorb a particular 

 bacteriophage, without loss of susceptibility to other closely re- 

 lated phages. The bacteriophage by a single mutation can 

 gain the ability to adsorb to a particular host without other de- 

 tectable change in viral properties. 



Another interesting aspect of adsorption specificity is the re- 

 quirement for adsorption cofactors by certain phages. A particu- 

 larly well studied example is a tryptophan requiring variant of 

 coliphage T4 which fails to adsorb to its host cell unless trypto- 

 phan or certain other substances are present in the medium. 

 The tryptophan reacts reversibly with the phage, changing it 

 from a particle incapable of adsorption to a particle which ad- 

 sorbs with high efficiency (T. F. Anderson 1948a). This "'sen- 

 sitization" of the phage particle to adsorption can be counter- 

 acted by indole (Delbriick, 1 948) . Since E. coli can convert tryp- 

 tophan into indole, we have a bacterium converting a sub- 

 stance essential for the attack by a hostile virus into a substance 

 which renders the virus innocuous. This is antibiosis on a 

 primitive level. 



An important observation by T. F. Anderson (1951) gives 

 further evidence for a high order of specificity in the adsorption 

 process. Stereoscopic electronmicrographs show that the tailed 

 bacteriophages T2, T4, and T6 attach to the host cell by the tips 

 of their tails only. 



