Bacteria : Clones and Mutation 



331 



presence or absence of flagella, so that the 

 detection of mutants by examining individual 

 bacteria is seriously limited to those which 

 affect particular morphological traits to a 

 measurable degree. 



After a suspected mutant is found under 

 the microscope, it is essential to isolate it 

 and determine, from the members of the clone 

 it produces, whether or not the new trait 

 appears in the offspring, that is, was caused 

 by a mutant. There are several methods by 

 which an individual bacterium may be iso- 

 lated. This can be done directly by the 

 tedious, but exact, procedure of removing a 

 single bacterium from a culture with the aid 

 of a micromanipulator and placing it in fresh 

 culture medium. Single bacteria can also be 

 obtained by two, less exact, but quicker, in- 

 direct methods. If the bacteria are growing 

 in a liquid medium, it is possible to dilute the 



culture sufficiently so that a sample contains 

 relatively few individuals. When this sample 

 is poured onto the surface of a petri dish con- 

 taining nutrient agar, the individual cells will 

 be located on the agar at random, usually so 

 spaced that the visible clone each produces 

 will be discrete (Figure 36-3). It is also 

 possible to take a small amount of a broth 

 culture and spread the bacteria in the sample 

 in the following way. A small amount of 

 the culture is picked up by a sterile innocu- 

 lating loop and streaked across the surface of 

 fresh agar (Figure 36-4). At some places 

 on the agar single bacteria will have been 

 deposited far enough apart so that they give 

 rise to separate colonies. Which of these 

 methods is actually used depends upon the 

 precision required to obtain the isolation of 

 a particular bacterium or bacterial type. 

 The study of the individual bacterium is 



FIGURE 36-2. The geometric in- 

 crease in tlie number of bacteria 

 due to vegetative reproduction. 



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