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CHAPTER 42 



phage particles are added on top of such a 

 bacterial lawn, each particle will enter a 

 different bacterium, lyse it, and release up to 

 several hundred daughter particles. These 

 will proceed to attack bacteria near the 

 original burst, and subsequently cause them 

 to lyse. When repeated, this cycle will result 

 in a progressively increasing zone of lysis 

 which appears as a clearing or plaque in the 

 bacterial lawn. Each plaque will represent a 

 phage colony derived from one ancestral 

 particle. 



The type of plaque formed depends upon 

 the medium, host, and phage. When, how- 

 ever, all other factors are controlled, it is 

 found that genetically different virulent 

 phages may produce characteristically differ- 

 ent plaques. Differences in plaques may in- 

 volve size, the presence or absence of a halo, 

 the nature of the edges, and color differences 

 on colored agar. One can, therefore, study 

 the inheritance of phage mutants affecting 

 plaque type. Genetically different phages 

 also differ in the hosts which they are able to 

 infect, and mutants occur in phage which 

 change the range of hosts which may be 

 attacked. Accordingly, the inheritance of 

 phage mutants affecting host range may also 

 be studied. What kinds of results are ob- 

 tained when both types of mutants are in- 

 volved simultaneously? 



It is possible to obtain a strain of virulent 

 T phage which is mutant both for host range, 

 h, and plaque type, r. When sensitive bac- 

 teria are singly infected, the mutation rates to 

 the wild-type alleles (/?+ or r+) can be deter- 

 mined. It is also possible, using wild-type 

 phages, to determine the mutation rates to 

 the two kinds of mutant alleles. The sensi- 

 tive bacterial strain also may be exposed to a 

 high concentration of a mixture of the doubly 

 mutant (// r) and wild-type (//+ /•+) phages, as 

 a result of which some doubly infected cells 

 occur that carry both phage types. When the 

 daughter phages from such exposures are 

 harvested and tested, it is found that not 



only the parental types occur (/? r and /?+ /•+) 

 but that recombinational types (/?+ r and h /•+) 

 occur in such high frequency as to exclude a 

 mutational explanation for most of them 

 (Figure 42-3). Accordingly, such experi- 

 ments ^ prove that genetic recombination 

 occurs between phage particles in a multiply 

 infected cell. Using a variety of mutants, and 

 on the basis of the relative frequencies with 

 which different recombinants appear in phage 

 released from multiply infected cells (this 



^ Following the work of M. Delbruck and W. T. 

 Bailey, and of A. D. Hershey and R. Rotman. 



FIGURE 42-3. Plaques produced by parental and 

 recombinant phage types. Progeny phage of a 

 cross between h r+ andh^ r were tested on a mixture 

 of suitable indicator bacteria. The small clear and 

 the large turbid plaques are made by the parental 

 types of phage progeny (h r^ and h+ r, respectively). 

 The large clear and the small turbid plaques are pro- 

 duced by the recombinant types of progeny (h r and 

 h+ r+, respectively). {Courtesy of A. D. Hershey.) 



