656 GROWTH OF PHAGE AND LYSIS 



ture occurs. If the initial ratio of phage/bacteria is smaller than the satu- 

 ration value, clearing occurs only slightly earlier than in a one to one mixture, 

 and it is accompanied by a noticeable increase in the plaque count. 



On the other hand if the initial ratio of phage/bacteria is greater than the 

 saturation value, clearing occurs much earlier and is not accompanied by 

 an increase in plaque count. 



In both cases the final plaque count is considerably smaller than the in- 

 itial one; we have, in effect, a phage destruction by the adsorption that 

 causes lysis. 



One can see the difference between the two types of lysis with the naked 

 eye. A culture of rod shaped bacteria, like B. coli, shows flow lines on 

 shaking due to the orientation of the rods under the influence of the shearing 

 forces of unequal flow. In lyses under the influence of great excess of phage 

 these flow lines disappear before the culture clears up, because the rods are 

 transformed into spherical bodies before they disappear, as described in the 

 section on microscopic observations. 



Growth of Phage and Lysis of Bacteria When Equivalent Numbers Are Mixed 



It can be predicted that a disturbance must arise when equivalent 

 amounts of phage and bacteria are mixed, due to the fact that the phage 

 that are liberated from the first lysing bacteria will cause an excess of phage 

 over bacteria to be present. These phage will be adsorbed on bacteria that 

 are already infected and will therefore not show up in a plaque count assay. 

 They will moreover interfere with the phage growth in these bacteria and 

 in some of them cause a lysis from without. 



Qualitatively the following can be predicted. We have seen that the 

 phage will be liberated at a constant rate (after the lapse of the minimum 

 latent period of 17 minutes). They will be adsorbed at a rate that is pro- 

 portional to the phage concentration and to the bacterial concentration. 

 The phage concentration is constantly increasing and the bacterial con- 

 centration is constantly decreasing (due to lysis). The adsorption rate 

 will therefore pass through a maximum and the net free phage production 

 rate will pass through a minimum. The net result is the appearance of a 

 point of inflection, i.e. a secondary step in the phage growth curve, in some 

 cases even a temporary decrease in the free phage if the rate of adsorption 

 at any time exceeds the rate of phage liberation. Because of the loss of 

 phage by adsorption and partial lysis from without the total step size must 

 be smaller than in a one step growth curve where the bacteria are in excess 

 and where multiple adsorption is prevented by extreme dilution, after ad- 

 sorption of the parent phage. 



70 



