482 BACTERIOPHAGES 



the end of the latent period a loopful of the adsorption mixture 

 is placed on the surface of the hardened agar. After a wait of 

 a few minutes for the broth to be soaked up by the agar, a 

 cover glass is placed in contact with the agar and sealed with 

 petrolatum. This preparation is placed on a warm stage 

 microscope so that incubation of the culture can continue at 

 37 °C. The bacteria are now observed under the high power 

 or oil immersion lens and a suitable field containing 50-100 

 bacteria is selected. A map is drawn of this field. This should 

 be completed before the end of the latent period. Then the 

 bacteria are examined at regular intervals. When a bacterium 

 lyses, its image fades out during a minute or so. The time of 

 lysis is then recorded on the map. After lysis of the bacteria 

 has ceased a curve can be drawn of the number of surviving 

 bacteria as a function of time since the initiation of infection. 

 This curve will be the reciprocal of the one-step growth curve 

 for the same phage at the same temperature, indicating that 

 the lysis of bacteria coincides with the liberation of phage. 

 If a warm stage microscope is not available, the entire experiment 

 can be carried out at the temperature of the room and a one- 

 step growth experiment run at the same temperature. Tem- 

 perature control is important since the temperature coefiicient 

 of the latent period is of the same order of magnitude as the 

 temperature coefficient of the bacterial generation time. In the 

 case of a coli phage, both the generation time of the host 

 cell and latent period of the phage doubled when the tem- 

 perature was decreased from 37 to 25 °C. (Ellis and Delbriick, 

 1939). 



2. Observation by turbidimetric measurements. The meas- 

 urement of bacterial concentrations by the decrease in light 

 transmission is quite precise but not very sensitive since the 

 minimal concentration which can be measured with any ac- 

 curacy with a photoelectric colorimeter is about 10^ bacteria/ 

 ml. Photoelectric measurement of the intensity of scattered 

 light in a nephelometer is far more sensitive, enabling accurate 

 estimation of bacterial concentration over the range from 5 X 



