14:2/ Structure of Viruses 249 



into the dish and spread in a thin uniform film over the surface of the 

 gel. (This is called a plate.) The plate is then covered, and the 

 bacteria are allowed to grow for one or more days. If their initial 

 concentration was of the order of 100 per ml, each will land on a separate 

 spot on the plate and give rise to a small colony (also called a clone) 

 which spreads out around the original bacterium. The clone has a 

 size, shape, and color characteristic of the given type of bacteria. The 

 clones can be counted visually after they have developed. Thus, the 

 original number of bacteria in one ml can be determined. 



Figure 2. Bacterial plates for three different dilutions. The 

 left hand plate represents a 10 7 :1 dilution which shows too 

 few clones for meaningful counting, whereas the right hand 

 one, a 10 5 : 1 dilution, has far too many. However, the middle 

 one, diluted 10 6 :1, shows about 50 clones. By counting 

 duplicate plates at this dilution, one can find the original con- 

 centration of bacteria at the time of plating. 



It is unlikely that the initial bacterial concentration will be in a range 

 suitable for plating. Accordingly, a series of dilutions are made, each 

 differing by a factor of 10. A few members of such a series are shown in 

 Figure 2. On plates where the dilution is too great, too few clones 

 develop to make counting statistically meaningful. On plates made up 

 with too high a concentration of bacteria, many spots originate from two 

 or more of the bacteria placed on the plate, and many clones overlap. In 

 the extreme case, the entire plate will be covered with bacteria. Figure 2 

 illustrates typical plates after they have been incubated for two days. 



When phages are studied by a plating method, bacteria are used at a 

 concentration which would completely cover the plate in the absence of 

 phage particles. If phages are present, clear areas develop on the sur- 

 face of the plate. These result because each phage particle multiplies 

 inside a bacterium until the cell wall is eventually ruptured. For every 

 bacterium infected, as many as 300 new phage particles are sometimes 

 produced. The new phage particles then enter other bacteria surrounding 



