334 THE BACTERIOPHAGE 



a similar relationship in a lytic mixture of Bad. coli and a coli phage. In both cases, 

 massive lysis was initiated when the number of phage particles per bacterial cell reached 

 a certain value. In the Bad. coli system it was 1,600. The steady rates of phage pro- 

 duction, as Burnet (1934) points out, are not incompatible with the conception of hbera- 

 tion of phage in bursts from infected bacteria, but may well represent the statistical average 

 of large numbers of sudden bursts occurring at different times. Krueger and Fong (1937) 

 later showed that the relationship they had observed between bacterial count and phage 

 count was peculiar to their conditions of measurement, which happened to have been 

 near the optimum both for phage production and bacterial growth ; and, as we have 

 already noted, they were apparently able by altering these conditions to demonstrate 

 increase of phage in the absence of bacterial multipUcation. This important observation, 

 that phage is liberated in large amounts from infected bacteria in the absence of bacterial 

 multipUcation, has been repeated by Spizizen (1943a), but in view of the large number 

 of previous negative reports, it may be wise to await its further confirmation. 



We may distinguish three stages in the production of phage, namely, adsorption of 

 phage on the bacteria (Fig. 51), a period of constant phage count in which phage grows 

 in or on the bacteria, and release of phage when the phage count rises (Fig. 52). 



Adsorption. — The rate of adsorption depends on the concentration of both phage and 

 bacteria. The adsorption capacity of a bacterial species for a phage to which it is sensitive 

 varies with the strain (see for instance, Hershey and Bronfenbrenner 1943), and the physio- 

 logical state of the bacterial cell. Delbriick (1940) found that an actively growing culture 

 of Bad. coli adsorbed 200 particles per cell, while the ceUs of starved culture of the same 

 strain adsorbed only 20 particles per cell. When a phage filtrate is adsorbed by excess 

 of bacteria a certain amount of residual phage is left, which was regarded by Krueger 

 (1931) as evidence that adsorption was reversible. An alternative view (Schlesinger 

 1932-33a, b, and Delbriick 1942) regards the residual phage as having lessened affinity 

 for the bacterium. 



Constant Period. — The constant period varies in the same way that division of bacteria 

 varies with temperature (Ellis and Delbriick 1939). 



Release of Phage. — At the end of the constant period, the release of phage occurs 

 steadily for a certain jieriod, during which all the infected bacteria one after the other 

 are lysed, and then ceases, a step being followed by a halt in the curve of phage pro- 

 duction. The released phage is adsorbed to other bacteria, and in due time a second 

 " step release " occurs, and so on, each succeeding step being less clearly marked as the 

 cycles of adsorption, growth and release in each infected bacterium overlap one another 

 in time. Within certain limits, the constant period and the size of the step release are 

 independent of the number of phage particles adsorbed, and it appears that if more 

 than one particle is adsorbed, only one of them actively induces the production of more 

 phage. Lysis of the growing culture as a whole occurs when the phage concentration 

 reaches a threshold Umit for the number of bacteria present. In his studies of Bad. coli 

 phage systems Delbriick (1940), demonstrated in bacteria, at a given stage of growth, 

 a close similarity of the adsorption capacity of the bacterium, the threshold Umit for 

 lysis, and the yield of phage at the end of the constant period, and suggested that each 

 bacterium might produce as many phage particles as it had receptors to which adsorption 

 could take place. Using a similar system, Hershey and Bronfenbrenner (1943) did not 

 observe any simple relation between phage yield and adsorptive capacity. 



It may be noted that phage particles, when present in adequate concentration, are able 

 to kUl bacteria without producing lysis. Andre wes and Elford (1932) have demonstrated 

 this direct kiUing action by studies with a citrate-sensitive coU-phage. Mixtures of this 

 phage with a sensitive Bad. coli, when plated on citrate agar gave normal growth, without 

 lysis, so long as the phage concentration was below a certain Umit. But when this Umit 

 was exceeded 95-99 per cent, of the Bad. coli were kiUed, in the sense that they failed to 

 develop on the citrate agar, although there was, of course, stiU no lysis. 



