SYLLABUS 117 



elude, therefore, that adsorption is a pseudo first order reaction, i.e., 

 that there occurs a collision of two bodies, one of which (the bacterial 

 receptors) is in large excess over the other (the phage). 



In case neither bacteria nor phage have motility, one may calcu- 

 late the approximate frequency of collision of the two bodies due to 

 Brownian movement in suspensions of known concentration (Schles- 

 inger, 1932). This calculated frequency is of the same order of mag- 

 nitude as the rate of adsorption observed under the most favorable 

 conditions. Thus, almost every collision between phage and host must 

 lead to specific fixation. 



It is possible to show, however, that under other conditions active 

 phage may collide with sensitive bacteria without being adsorbed. Ex- 

 periments wdth starved bacteria (Delbriick, 1940a) and at low tem- 

 peratures (Wollman and Stent, unpublished) show that the reduction 

 in the adsorption rate observed is much more severe than that predicted 

 from the decrease of the collision frequency. Under unfavorable con- 

 ditions therefore, the fraction of the collisions leading to adsorption can 

 be greatly reduced. 



It is found that the adsorption rate of phages no longer increases 

 with the bacterial concentration after the cell density has exceeded a 

 certain limit. (Anderson, 1949; and Wollman and Stent, unpublished). 

 Since the collision rate should continue to increase under these condi- 

 tions, one is led to postulate a second, rate limiting, step of the adsorp- 

 tion reaction. 



The finding that no adsorption at all occurs in mixtures which 

 are agitated violently (Anderson, 1949) can be interpreted to mean 

 that stirring prevents phage and host from staying together long enough 

 to undergo steric fitting. 



16. The bacterial change from sensitivity to resistance. — Bacteria 

 can change by mutation from a form in which they are sensitive to a 

 given phage to a form in which they are resistant to it. By resistance 

 we mean here that the viability of the bacterium is not affected by the 

 phage. The word is sometimes used also in a less stringent sense. For 

 instance, in the sense that the bacterium has become unsuitable for 

 multiplication of the phage although it is still attacked and even killed 

 by it. 



In strain B ana the phages of the T series, resistance in the strict 

 sense is always associated with failure of the resistant bacteria to ad- 

 sorb the phage to which they are resistant. These resistant mutants are 

 obtained by plating out 100 or 1,000 million bacteria of the sensitive 



