370 GROWTH OF BACTERIOPHAGE 



The experimental determination of the efficiency of plating is described in a 

 later section (see p. 379). The coefficient varies from 0.3 to 0.5. This means 

 that three to live out of every ten infected bacteria produce plaques. The fact 

 that the efficiency of plating is relatively insensitive to variations in the tempera- 

 ture of plate incubation, density of plating coli, concentration of agar, etc. indi- 

 cates that a definite fraction of the infected bacteria in the broth cultures do not 

 readily go through to lysis when transferred to agar plates. For most experiments 

 only the relative assay is significant; we have therefore, given the values derived 

 directly from the plaque counts without taking into account the efficiency of 

 plating, unless the contrary is stated. 



Growth Measurements 



The main features of the growth of this phage in broth cultures of 

 the host are shown in Fig. 2. After a small initial increase (discussed 

 below) the number of infective centers (individual phage particles, 

 plus infected bacteria) in the suspension remains constant for a time, 

 then rises sharply to a new value, after which it again remains constant. 

 Later, a second sharp rise, not as clear-cut as the first, and finally a 

 third rise occur. At this time visible lysis of the bacterial suspension 

 takes place. A number of features of the growth process may be 

 deduced from this and similar experiments, and this is the main con- 

 cern of the present paper. 



The Initial Rise 



When a concentration of phage suitable for plating was added to a 

 suspension of bacteria, and plated at once, a reproducible plaque count 

 was obtained. If the suspension with added phage was allowed to 

 stand 5 minutes at 37°C. (or 20 minutes at 25°C.) the number of 

 plaques obtained on plating the suspension was found to be 1.6 times 

 higher. This initial rise is not to be confused with the first "burst" 

 which occurs later and increases the plaque count 70-fold. After the 

 initial rise, the new value is readily duplicated and remains constant 

 until the start of the first burst in the growth curve (30 minutes at 

 37° and 60 minutes at 25°). 



This initial rise we attribute not to an increase in the number of 

 infective centers, but to an increase in the probability of plaque forma- 

 tion {i.e. an increase in the efficiency of plating) by infected bacteria 

 in a progressed state; that is, bacteria in which the phage particle has 

 commenced to multiply. That this rise results from a change in the 



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